tag:blogger.com,1999:blog-7147187096016208662024-03-18T13:55:52.907-07:00David Burch Navigation BlogNotes on marine navigation and weatherDavid Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.comBlogger377125tag:blogger.com,1999:blog-714718709601620866.post-48059562610559927422024-03-18T09:28:00.000-07:002024-03-18T13:55:20.456-07:00 A New Revolution in Barometers<p><span style="font-family: Helvetica; font-size: 18px;">We have worked for many years promoting the use of accurate pressure in marine navigation, which had literally fallen out of all standard texts on marine weather twenty years ago. The word "barometer" was barely mentioned. We would see occasionally that a falling barometer means bad weather, but nothing more, and certainly nothing about how fast it must fall for bad weather. And all of these books state—they are all still in print—that the value of the pressure does not matter; it is just a question of rising or falling, fast or slow, but never with any numerical values.</span></p><p><span style="font-family: Helvetica; font-size: 18px;">Accurate pressure was crucial in the late1700s and early 1800s when much of global marine weather was first learned and understood with the aid of accurate mercury barometers used at sea. But they were unwieldy and difficult to use and happily set aside with the development of aneroid barometers in the mid 1800s. That revolution took place without the full recognition that</span><span class="Apple-converted-space" style="font-family: Helvetica; font-size: 18px;"> with the great convenience of the aneroids came a notable loss of accuracy over the higher and lower ends of the dial, which typically matter the most in routing decisions—a fact that has followed aneroid use into modern times. Thus began the doctrine that only the change in the pressure matters, not its actual value.</span></p><p><span class="Apple-converted-space" style="font-family: Helvetica; font-size: 18px;">Now it remains as it was then: only the high-end, expensive aneroid units can be counted on for accurate pressures over the full range we care about in marine navigation. I would venture to guess that most barometers on vessels today are there primarily for traditional reasons, and not referred to for routing decisions.</span></p><p><span style="font-family: Helvetica; font-size: 18px;">We began our goal to change that with the first edition of </span><i style="font-family: Helvetica; font-size: 18px;"><a href="https://starpath.com/catalog/books/1886.htm" target="_blank">Modern Marine Weather</a>, </i><span style="font-family: Helvetica; font-size: 18px;"> and had gone into the interesting history of how this came about in </span><a href="https://www.starpath.com/catalog/books/1889.htm" style="font-family: Helvetica; font-size: 18px;" target="_blank"><i>The Barometer Handbook</i>.</a><span style="font-family: Helvetica; font-size: 18px;"> Both books show how important it is to know accurate pressure to evaluate numerical weather predictions that we ultimately rely on for routing. </span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDxaKxSS768BkXNcnExWDwcKzqbSYQMGLnEGGuOyEqgIUqYf_RaK5eEyDX-yWINbLsugd4dkntR651xvcwE9LB7O_7Ms0oQ-o61WN3yayR5xxmC-FQfjYiQFwdeejUwb4eL8tkwuBfilnsTYsx7Hq3KJ9jPj1zwFuX2RpaMBalIPp4F-ahtaKhjWUQLe8/s590/2%20books.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="382" data-original-width="590" height="207" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiDxaKxSS768BkXNcnExWDwcKzqbSYQMGLnEGGuOyEqgIUqYf_RaK5eEyDX-yWINbLsugd4dkntR651xvcwE9LB7O_7Ms0oQ-o61WN3yayR5xxmC-FQfjYiQFwdeejUwb4eL8tkwuBfilnsTYsx7Hq3KJ9jPj1zwFuX2RpaMBalIPp4F-ahtaKhjWUQLe8/s320/2%20books.png" width="320" /></a><br /><br /></div><span style="font-family: Helvetica; font-size: 18px;"><p>Accurate pressure is also often the fastest way to detect a change in the weather or the movement of a High pressure system we are carefully navigating around. Responding to the motion of a High is often a key decision for sailors in an ocean crossing.</p></span><p></p><p><span style="font-family: Helvetica; font-size: 18px;">I</span><span style="font-family: Helvetica; font-size: 18px;">n the tropics, where the standard deviation of the seasonal pressure is just a couple millibars (mb), we can know from accurate pressure alone whether or not a tropical storm is approaching—and we can know this before we see notable changes in the clouds or wind. Needless to say, we navigate in such waters primarily based on official forecasts and tropical cyclone advisories, but an accurate barometer gives us early notification that forecasted storm motions are on time, early, or late. On the other hand, any loss of wireless communications makes the barometer even more important.</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjFoinIsqwsc0bJVC9S1mxaht5Z1YUJ1M8yh3qrW2WElElDd-5uyBYXnqPIkc3l52gPtLRkvJ8Kq2Ng3J79AiyQ4byeq6tcG_u_WEYwgYZv7FQE2PltOjQOWQlr5oxo43XL58ulNf-IKPEV8svKUlfiIXPXoFX9mMc7XCQ5HevAUpiCP35EWsFrrfkf3s/s1144/sd%20vs%20mslp.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="342" data-original-width="1144" height="192" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhjFoinIsqwsc0bJVC9S1mxaht5Z1YUJ1M8yh3qrW2WElElDd-5uyBYXnqPIkc3l52gPtLRkvJ8Kq2Ng3J79AiyQ4byeq6tcG_u_WEYwgYZv7FQE2PltOjQOWQlr5oxo43XL58ulNf-IKPEV8svKUlfiIXPXoFX9mMc7XCQ5HevAUpiCP35EWsFrrfkf3s/w640-h192/sd%20vs%20mslp.png" width="640" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><i>In the hurricane zone between Panama and Hawaii, we would expect a July pressure of about 1012 mb, with a standard deviation of 2 to 2.8 mb. A measured pressure of 1007 mb (2.5 standard deviations below normal) has only a 0.6% chance of being a statistical variation and a 99.4% chance of being an early tropical storm warning. This type of analysis does not work at higher latitudes because the standard deviations are much larger.</i></div><span style="font-family: Helvetica; font-size: 18px;"><br />Pressure statistics needed for this type of analysis are included in our <i><a href="https://www.starpath.com/catalog/books/1995.htm" target="_blank">Mariners Pressure Atlas</a></i>.</span><p></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsWD7DHL6iiI1kNu_ZFscndkbzLJK_JRTiRvZ6d4voCahTTvXveKDld6hZAHjhyphenhyphenqS0kZxUTv0xaj_aVVk5dKTX-ne2flTTw08IZh5lAY-e7ix66xKanttmwzdY964TnBi90q0_3GERMMUuCFCCjvnnoHk_Ty5o8ktQ3jShFJiecHgyjWcK_UzxBT_e5fc/s331/MSLP-book%20cover.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="331" data-original-width="250" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjsWD7DHL6iiI1kNu_ZFscndkbzLJK_JRTiRvZ6d4voCahTTvXveKDld6hZAHjhyphenhyphenqS0kZxUTv0xaj_aVVk5dKTX-ne2flTTw08IZh5lAY-e7ix66xKanttmwzdY964TnBi90q0_3GERMMUuCFCCjvnnoHk_Ty5o8ktQ3jShFJiecHgyjWcK_UzxBT_e5fc/s320/MSLP-book%20cover.jpg" width="242" /></a></div><br /><p></p><p><span style="font-family: Helvetica; font-size: 18px;">We developed a sophisticated electronic barograph that was quickly adopted by the NWS for use on the voluntary observing ships (VOS). We later sold that product to another company.</span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAzWi-Y3HJIhlvDSWXQKojvDAjKzn5xd_KUXeJT7AIgLqr45jEp6n-tQLQonUAL5OR0xF2LLmhX3aSK4tjKNJMLfcIW3pIJ9-sYLRGbyOuIoKIwOHlfhqEeUWLO111wzQxFXhdL4XHW6HRk4I7tsPLtOn2r-uAUTTPRBE7d0EQ3scLpKx6AXh9FkqKuYE/s2934/vos%20baro.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1812" data-original-width="2934" height="198" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAzWi-Y3HJIhlvDSWXQKojvDAjKzn5xd_KUXeJT7AIgLqr45jEp6n-tQLQonUAL5OR0xF2LLmhX3aSK4tjKNJMLfcIW3pIJ9-sYLRGbyOuIoKIwOHlfhqEeUWLO111wzQxFXhdL4XHW6HRk4I7tsPLtOn2r-uAUTTPRBE7d0EQ3scLpKx6AXh9FkqKuYE/s320/vos%20baro.JPG" width="320" /></a></div><br /><p><span style="font-family: Helvetica; font-size: 18px;">To further support the use of accurate pressure, we became the US distributor for the state of the art </span><a href="https://www.starpath.com/catalog/accessories/103PMB.htm" style="font-family: Helvetica; font-size: 18px;" target="_blank">Fischer Precision Aneroid Barometer</a><span style="font-family: Helvetica; font-size: 18px;">, used by those who want the best of the best in a mechanical unit, including the Navies, Coast Guards, and Weather Service vessels around the world, including the US. Fischer is one of the last sources for accurate, hand-made aneroid barometers.</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJQhhu8mWvp_Iv96VWVezk0H6VLvJU0C5DRgH380FFWGFo14xOSiyfMsM5HLBZRsdiNYVKz91ivCe2gNwkV8ZyS9uMp1daWSGo4sPVxKkfKRfbCN8i9Zx_a2GXkApFZqZHpQGDcvUUfUZLJh4BgISfyy9pVkSMb4oqOhyQJRGVhTcmp7aFSRctDceyWhs/s356/PAB103.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="356" data-original-width="350" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgJQhhu8mWvp_Iv96VWVezk0H6VLvJU0C5DRgH380FFWGFo14xOSiyfMsM5HLBZRsdiNYVKz91ivCe2gNwkV8ZyS9uMp1daWSGo4sPVxKkfKRfbCN8i9Zx_a2GXkApFZqZHpQGDcvUUfUZLJh4BgISfyy9pVkSMb4oqOhyQJRGVhTcmp7aFSRctDceyWhs/s320/PAB103.jpg" width="315" /></a></div><p></p><p><span style="font-family: Helvetica; font-size: 18px;">To follow up on that, we developed both a free <a href="https://www.starpath.com/marinebarometer/about.htm" target="_blank">Marine Barometer app and low-cost Marine Barograph app</a> for iOS and Android mobile devices. </span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFEi3Z62MqcOUWpO6ol03qvM1bJT90n5wKerych0kJ78CzEbs6teTzf3rAdMgbNcY1qvvKZsKdo7fOD55H46p-A7CCH4zsKoqwFd3PV4WXwMH-4IBwCh1jJB-wjWpUmoqdg8WJVDg-Xl5NBYCNqqbpgLECzQnNyoCIoYFTqPPJLaslUCL5zAf2cLzpl-o/s975/phone%20apps.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="627" data-original-width="975" height="412" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgFEi3Z62MqcOUWpO6ol03qvM1bJT90n5wKerych0kJ78CzEbs6teTzf3rAdMgbNcY1qvvKZsKdo7fOD55H46p-A7CCH4zsKoqwFd3PV4WXwMH-4IBwCh1jJB-wjWpUmoqdg8WJVDg-Xl5NBYCNqqbpgLECzQnNyoCIoYFTqPPJLaslUCL5zAf2cLzpl-o/w640-h412/phone%20apps.png" width="640" /></a></div><p style="text-align: center;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></p><p style="text-align: left;"><span style="font-family: Helvetica; font-size: 18px;">In short, we have worked on barometers for over 20 years now, but I felt we still did not have the unit that could have the biggest impact on marine navigation, which is what lead to the development of the Starpath USB Baro.</span></p><p><span style="font-family: Helvetica; font-size: 18px;">Not all vessels can invest in the high-end units. The mobile apps, while providing a convenient backup that can indeed broadcast pressure data to a navigation program, still rely on a device that must be charged and protected. Also running it full time does put a strain on the phone's battery life.</span></p><p align="center"><b style="font-family: Helvetica; font-size: 18px; text-align: center;">The New Revolution</b></p><p><span style="font-family: Helvetica; font-size: 18px;">Our goal was to develop a barometer that was first and foremost highly accurate and dependable, plus we wanted it to be easily portable. Finally, we wanted to produce it at a low enough cost to be attractive to all mariners, even those using it as a backup. For mariners we also need the output signals to be in the NMEA standard to match navigation electronics and software.</span></p><p><span style="font-family: Helvetica; font-size: 18px;">The result is the <a href="https://www.starpath.com/catalog/accessories/103USB.htm" target="_blank">Starpath USB Baro</a> for $49, which includes a metal transport case. It can be read in any Navigation program, or use our free <a href="https://www.starpath.com/marinebarometer/about_computer.htm" target="_blank">USB Baro app for Mac or PC</a>.</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhym4rTyEn_hzIegPq1hQqirwyFlsqB6eh90THB2vrGc5NCn2YmCoxjzD-inHL5zQ622ZSAsNu4SEKHJdglF5-7pgt_FtxTsj-dISbG_QVMqdzBgHB3bNiE97nEUdFK8HTn_HSexm1hsFgDfzc6G9gT6Ll_2zegBu02vzgaKwlxkxS1XFfG-ExZwPFf8Fw/s1042/baro%20and%20can.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="471" data-original-width="1042" height="290" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhym4rTyEn_hzIegPq1hQqirwyFlsqB6eh90THB2vrGc5NCn2YmCoxjzD-inHL5zQ622ZSAsNu4SEKHJdglF5-7pgt_FtxTsj-dISbG_QVMqdzBgHB3bNiE97nEUdFK8HTn_HSexm1hsFgDfzc6G9gT6Ll_2zegBu02vzgaKwlxkxS1XFfG-ExZwPFf8Fw/w640-h290/baro%20and%20can.png" width="640" /></a></div><i>In stock and ready to ship from the link above.</i></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><div style="text-align: left;"><span style="font-family: Helvetica; font-size: 18px;">Below shows how the pressure appears in three popular navigation programs. Video setup procedures for each are shown in the link above.</span></div><div style="text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0E1FlRaFYlNj3G-4ogCVEm60aE7gXa2DXBsekUHVvU-Xt-qc1p2wbQQVMuYGVAzND2G4dnMj_fZmQbmBQKA6J1QhI_MfKr6BhMntVBVhyfyjrGGjO5dzSPRb6m-mFnJ93MgjnaLaRGuhPFB6EbT-BiI9QDNf_wr-aX8EjX4tnDU_F2O8XolbeYP1MbpI/s803/3%20apps%20no%20overlays.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="803" data-original-width="464" height="767" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0E1FlRaFYlNj3G-4ogCVEm60aE7gXa2DXBsekUHVvU-Xt-qc1p2wbQQVMuYGVAzND2G4dnMj_fZmQbmBQKA6J1QhI_MfKr6BhMntVBVhyfyjrGGjO5dzSPRb6m-mFnJ93MgjnaLaRGuhPFB6EbT-BiI9QDNf_wr-aX8EjX4tnDU_F2O8XolbeYP1MbpI/w443-h767/3%20apps%20no%20overlays.png" width="443" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;">We can compare this with official pressure data from the West Point Lighthouse (<a href="https://www.ndbc.noaa.gov/station_page.php?station=wpow1&unit=M&tz=STN" target="_blank">NDBC WPOW1</a>), which is 1.6 nmi from where the USB Baro data were accumulated.</span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgyHshoE6uKlh33ot5qfW3abuSse1mJpi888vnnaawbb2KpRY58bAvISGfyHkWyLrRfvzHa1194sbUe7AVOkyLPbx7VkG8L0-PTJIeOhyphenhyphengX2X4ZmybHPd117XeED8-7FdEUJYrXOonWz7epA8GbzLHa9TRhEsWhXFxYky-rzJkxgFyDFk1qmhBWPc7dvEw/s991/overlay%20data%20source.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="749" data-original-width="991" height="484" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgyHshoE6uKlh33ot5qfW3abuSse1mJpi888vnnaawbb2KpRY58bAvISGfyHkWyLrRfvzHa1194sbUe7AVOkyLPbx7VkG8L0-PTJIeOhyphenhyphengX2X4ZmybHPd117XeED8-7FdEUJYrXOonWz7epA8GbzLHa9TRhEsWhXFxYky-rzJkxgFyDFk1qmhBWPc7dvEw/w640-h484/overlay%20data%20source.png" width="640" /></a></div><br /><span style="font-family: Helvetica; font-size: 18px;">The red square marks the data corresponding to our measurements with the USB Baro. We can now overlay that data with what we measured, as shown below.</span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-g4f6ygVVP0WxJceAK8Vv1ATqPfFe397eo2LcUK3M4r7YmM9o1-Gd4os0vs7cME4kfoF8RYmAtZ-S3K9hkX_uKejdBpvchtAn9Y5OUXG-m3PO6thrjIXq-7OPDGSD6DULj1QZf-lmLQ6aW2JvaUPJYKlH3friWSCUCqUT5PJvjE9ejvPJM73b0p_U4Hc/s798/3%20apps%20with%20overlays.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="798" data-original-width="458" height="801" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi-g4f6ygVVP0WxJceAK8Vv1ATqPfFe397eo2LcUK3M4r7YmM9o1-Gd4os0vs7cME4kfoF8RYmAtZ-S3K9hkX_uKejdBpvchtAn9Y5OUXG-m3PO6thrjIXq-7OPDGSD6DULj1QZf-lmLQ6aW2JvaUPJYKlH3friWSCUCqUT5PJvjE9ejvPJM73b0p_U4Hc/w461-h801/3%20apps%20with%20overlays.png" width="461" /></a></div><br /><span style="font-family: Helvetica; font-size: 18px;">So, we see that with this simple device we have access to the same pressure data that NOAA relies on to make their official forecasts and numerical weather predictions. </span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;">The difference between1023.0 mb indicated in the Lighthouse value and the 1017.2 mb observed in our office can be accounted for to the tenth of a mb, because of the elevation of the USB Baros compared to the sea level data from NOAA. All of the Nav apps used offer the option to incorporate this offset so the instrument reads sea level pressure directly. Our <a href="https://www.starpath.com/marinebarometer/about_computer.htm" target="_blank">free Marine Barograph apps made for the USB Baro</a> also have that option.</span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><p align="center" style="font-family: Times; font-size: medium;"><b style="font-family: Helvetica; font-size: 18px;">Our Guarantee</b></p><div>If you have now a common aneroid barometer and then compare what it reads with the known accuracy of the USB Baro over a pressure variation of 30 mb or so, you will be very pleased to own the USB Baro. </div><div><br /></div><div>You will either show that your aneroid is accurate, effectively calibrating it, which <a href="https://www.starpath.com/catalog/accessories/1872cal.htm" target="_blank">otherwise costs $195</a>, or you will learn that you did indeed need a more accurate source of pressure for your boat or home.</div></span></div><br /></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-26265638003239742042024-03-14T20:27:00.000-07:002024-03-14T20:27:11.716-07:00Special Uses of the Star Finder and Sight Reduction Tables<p>The <a href="https://www.starpath.com/catalog/accessories/1841.htm" target="_blank">2102-D Star Finder</a> is essentially a hand-held planetarium designed for mariners to assist with celestial navigation. It can be used to plan the best sights as well as its main function which is to identify stars or planets whose sights have already been taken. We have devoted a short book to the many uses of this powerful tool called <a href="https://www.starpath.com/catalog/books/1831.htm" target="_blank"><i>The Star Finder Book</i></a>.</p><p>Sight reduction tables are permanent mathematical solutions to the <a href="https://www.starpath.com/cgi-bin/web_card/courses/glossary.pl?show_def=105&cat=Celestial_Navigation" target="_blank">Navigational Triangle</a> that form the backbone of celestial navigation carried out in the traditional manner using books and manual plotting — as opposed to modern solutions using computers or calculators with dedicated cel nav apps.</p><p>There are several styles of these tables, popular versions are called <i>Pub 229</i>, <i>Pub 249</i>, and the <i>NAO Tables</i>, a copy of which is included in every <i>Nautical Almanac</i>. Complete free copies are <a href="https://www.starpath.com/celnavbook/" target="_blank">available online</a> as free downloads, which are good for practice, but do not make sense for use underway because any device that can read the files can also support a cel nav app that does the full process, sights to fix.</p><p>All sight reduction tables, regardless of format, do the same thing. You enter the tables with three angles and come out with two angles. We enter with the declination (dec) and local hour angle (LHA) of the object sighted and the assumed latitude (a-Lat) of the observer, and we come out with the angular height of the object (Hc) and its direction (Zn) as seen from the assumed position.</p><p>Put in plainer terms, the <i>Almanac</i> tells us where the sun and moon, stars and planets are located at any time of the year, and the <i>sight reduction tables</i> tells us what the height and bearing of any one would be as seen from any latitude and longitude.... or it tells us the object is below the horizon at that time and place.</p><p>The Star Finder does exactly the same thing, but with less accuracy. We look up in the <i>Almanac</i> a number that tells us how to set up the disks for the time and latitude we care about, and then we read the Hc and Zn of the celestial objects from the blue templates.</p><p>With that background, I want to point out that either of these tools can also be used to answer more non-conventional cel nav questions such as one that is part of our <a href="https://www.starpath.com/catalog/courses/1924.htm." target="_blank">Emergency Navigation course</a>. Part B of question 6 on quiz 5, asks us what are the conditions that lead to the sun's bearing changing with time at a rate of 45º per hour or faster?</p><p>This comes up in the context of using the "Eskimo Clock Method" to get bearings from the sun based on the local time of day, which makes the assumption that the sun's bearing moves along the horizon at the rate of 15º per hour. That condition, we show in the course, requires the peak height of the sun at noon (Hc) to be less than 45º, which leads to the nick name "Eskimo clock," because at high latitudes the sun is always low.</p><p>Here we have a more specific related question, but it can be solved with the Star Finder or with sight reduction tables. </p><p>We know that fast bearing changes means the object is very high and the fastest change will occur when the object passes overhead or near so. Consider sailing at lat 15º N during a time when the declination of the sun is also about N 15º (first few days of May). [Note latitudes get the label following the value; declinations get the label preceding the value.] In this example, the sun will bear near due east (090) all morning and then change to near due west (270) in a matter of minutes as it passes overhead. The question we have is, how do we specify the conditions that will lead to this bearing change being ≥ 45º/hr? It will have to be high, but it won't have to cross over head.</p><p>This could be worked from any latitude in the tropics, but we stick with 15º N, and look at the star Alnilam (declination about S 1º, which corresponds to the sun's declination in Sept, 19th-21st). Below is the Star Finder set up for the time Alnilam crosses our meridian bearing due south.</p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGh8X9vP6JaHoFAqKo64yM1P5EbtW745aqF9y7cPZxBEXykytLih_0_FKLtY3bRjc4Chc6oqiR0ub1PQPeMcRzPRfs9OI9Ta0VVbZSA901oqL-sSIUbtuZPfgcCAN3_eMen0gC6r4DMKz5zlo4zB_kPr_nQ-okechkSR3AQZxaWhx0LMUHdWHJPok8yQM/s4032/alnihlin%2083.5.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiGh8X9vP6JaHoFAqKo64yM1P5EbtW745aqF9y7cPZxBEXykytLih_0_FKLtY3bRjc4Chc6oqiR0ub1PQPeMcRzPRfs9OI9Ta0VVbZSA901oqL-sSIUbtuZPfgcCAN3_eMen0gC6r4DMKz5zlo4zB_kPr_nQ-okechkSR3AQZxaWhx0LMUHdWHJPok8yQM/w480-h640/alnihlin%2083.5.JPG" width="480" /></a></div><br /><p>Alnilam crosses the meridian bearing due south (180) at a local hour angle of Aries equal to 84.5º. We see that the height of the star as it crosses is 74º, which we would expect in that we are at 15N and the star is at S1 so the zenith distance (z) is 15+1 = 16º which makes the Hc (90-z) = 74º. </p><p>The rim scale corresponds to time at the rate of 15º/hr, so 30 min later (LHA Aries = 91.0º (84.5+7.5), we see that the star has descended very slightly but now has moved west.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZwdAc9q7gFjSLB0pbnQ1bbNlrgXpxumdFKw1E-9Mxr63MO5N5147HtW1PkH5W9av6iQr7X4XHltaRH8AjZHJebIlRhtvlraNlFYV0zOLbd2SsMeLLeVyqNAVTsIWIN9yoreLB3Gza0i8UzKWZSQN1ib9sw4jLy5JerfVakUda2A4__rqP6k2LdvQ78NM/s4032/alnihlin%2091.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZwdAc9q7gFjSLB0pbnQ1bbNlrgXpxumdFKw1E-9Mxr63MO5N5147HtW1PkH5W9av6iQr7X4XHltaRH8AjZHJebIlRhtvlraNlFYV0zOLbd2SsMeLLeVyqNAVTsIWIN9yoreLB3Gza0i8UzKWZSQN1ib9sw4jLy5JerfVakUda2A4__rqP6k2LdvQ78NM/w480-h640/alnihlin%2091.JPG" width="480" /></a></div><br /><p>Thirty minutes later the bearing is 205º, or 25º to the west of 180º. Thus if we imagine this star to be the sun in late Sept, viewed from 15º N, we would see its hourly change in bearing at midday to be about 50º per hour. This is a bit faster than the exercise asked for, but we could experiment around for a closer answer.</p><p>We can also do such studies with sight reduction tables, such as Pub 249. We enter the tables with a-Lat = 15º and dec = 1º. With these tables we do not use North or South labels but just specify if they are both north or both south or is one north and one south. The former condition is called Same Name; the latter is called Contrary Name. We have Same Name in this example.</p><p>We will also start with LHA = 0º, which means the sun is crossing our meridian (bearing 180), and like wise look at 30 min later with LHA = 7.5º. We could look at LHA = 15º, exactly one hour later, but the rate of bearing change at LHA 352.5º to 007.5º, as it crosses our meridian, is a bit faster than the full hour on either side.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhN5CMWGsmjQoT_Q4rwL2QbZIKadrrv9b2csfbiLcVnSclwFbiiT8fsnAEho13G6hcOih7i94nG4trMo13Sfy7rspsXJQC4DvGEhDhm-bs506_Gs4tzBrkmqa7Ca22lEGmmhaXc2UpK9TTPmIt5AeZ2x8xy_Qp4s4HybuEwl7feTmxXvRn7Td-5rC1L9G8/s745/SR%20tables%20anihlin%20from%2015N.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="416" data-original-width="745" height="358" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhN5CMWGsmjQoT_Q4rwL2QbZIKadrrv9b2csfbiLcVnSclwFbiiT8fsnAEho13G6hcOih7i94nG4trMo13Sfy7rspsXJQC4DvGEhDhm-bs506_Gs4tzBrkmqa7Ca22lEGmmhaXc2UpK9TTPmIt5AeZ2x8xy_Qp4s4HybuEwl7feTmxXvRn7Td-5rC1L9G8/w640-h358/SR%20tables%20anihlin%20from%2015N.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div>In Pub 249, each Lat has a set of pages, LHA is on the side of the page, and declination is across the page. The tabulated values are Hc, d, and Z. The d-value is how much the Hc changes with 1º of declination — for Alnihlin, dec = S 1º 12', so we would reduce the tabulated Hc by 12/60 x 60 = 12' for precise values of Hc, but we can neglect Hc for present study.)<div><br /></div><div>At meridian passage the bearing is 180º, then 30 min later (LHA=7.5), we see the body dropped from 74º high at the meridian to about 72º 20' at which time the relative bearing (Z) is about 154º, from which using the rule provided (Zn=360-Z) to find the new bearing of 206º, which agrees with what we found from the Star Finder.</div><div><br /></div><div>These terms and procedures become more familiar with a full study of cel nav, but we hope the brief discussion of the principles show how these tools might be used for other questions. Note that LHA is defined as how far west of you the body is, so as it approaches from the east it has large, increasing LHA, which goes from 358, 359, 360, 1, 2, 3 as it crosses the meridian.<br /><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p>***</p><p><br /></p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-1899320337012601442023-12-13T11:26:00.000-08:002023-12-13T11:29:09.049-08:00 How to Remember the Equation of Time<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZGiSIwBX_okzcTbnuSWW5AEw64fpK9zshgc9FjmFTiO1bUtoKR0rrmU9eStyi0m_3s83cCfeYPsF26Ov_hFJn9yBO7x-eegMLHni5Ihuf874qlfj5Jhft4EqB4td9fDUsZrAYRM869P-r8Ownw9zvROsRQuqWjN8BUrHYUGsb1b0dAVL5WUb6EmQgPwc/s1576/12-6.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1058" data-original-width="1576" height="430" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgZGiSIwBX_okzcTbnuSWW5AEw64fpK9zshgc9FjmFTiO1bUtoKR0rrmU9eStyi0m_3s83cCfeYPsF26Ov_hFJn9yBO7x-eegMLHni5Ihuf874qlfj5Jhft4EqB4td9fDUsZrAYRM869P-r8Ownw9zvROsRQuqWjN8BUrHYUGsb1b0dAVL5WUb6EmQgPwc/w640-h430/12-6.png" width="640" /></a></div><br /><p>On Valentine’s Day, February 14, the sun is late on the meridian by 14 minutes (LAN at 1214); three months later, it is early by 4 minutes (LAN at 1156). On Halloween, October 31, the sun is early on the meridian by 16 minutes (LAN at 1144); three months earlier, it is late by 6 minutes (LAN at 1206).</p><p>These four dates mark the turning points in the Equation of Time. You can assume that the values at the turning points remain constant for two weeks on either side of the turn, as shown in Figure 12-7. Between these dates, assume the variation is proportional to the date.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhX-AiH5aCSVDJEHJwQCY3g7ooMg5FBPQ5InhoOBsyLa4w2V78AxWqCkEw1f6zAZm9_jUiGRix2SOWJmgjUIdIPNjvMJo56oV9xw7BKx932gMLmjoD4nS5jQXgc0wO0CY162xmKs-9epWMcojxl7-3ZNPRxmzzltoFzccdhWfNFb4N7QrRVtEt3U_z5OFw/s1616/12-7.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1616" data-original-width="1336" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhX-AiH5aCSVDJEHJwQCY3g7ooMg5FBPQ5InhoOBsyLa4w2V78AxWqCkEw1f6zAZm9_jUiGRix2SOWJmgjUIdIPNjvMJo56oV9xw7BKx932gMLmjoD4nS5jQXgc0wO0CY162xmKs-9epWMcojxl7-3ZNPRxmzzltoFzccdhWfNFb4N7QrRVtEt3U_z5OFw/w530-h640/12-7.png" width="530" /></a></div><br /><p>There is some symmetry to this prescription, which may help you remember it:</p><p>14 late<span class="Apple-tab-span" style="white-space: pre;"> </span>three months later goes to<span class="Apple-tab-span" style="white-space: pre;"> </span>4 early</p><p>16 early<span class="Apple-tab-span" style="white-space: pre;"> </span>three months earlier goes to<span class="Apple-tab-span" style="white-space: pre;"> </span>6 late</p><p>but I admit it is no catchy jingle. Knowing the general shape of the curve and the form of the prescription, however, has been enough to help me remember it for some years now. It also helps to have been late sometimes on Valentine’s Day! An example of its use when interpolation is required is shown in Figure 12-7.</p><p>The accuracy of the prescription is shown in Figure 12-8. It is generally accurate to within a minute or so, which means that longitude figured from it will generally be accurate to within 15′ or so.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUm0SlE77W3ejgqhwVDZVRfxRQeFlh0HWDqsj0kXaOByO_ryu2h86PFy7YFuAhaRg03OfGlPzE81cqJZCiE03AYEyecI2a2mkSKMQPsYdgNkII4kvvp3Zx5zRQkBkIN4lVWU_VjrSeoIutrcoaNxWQ1pw0T_kYwOZEmKQbDYbCRx7e6diGylvRQYUnENU/s1652/12-8.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="908" data-original-width="1652" height="352" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUm0SlE77W3ejgqhwVDZVRfxRQeFlh0HWDqsj0kXaOByO_ryu2h86PFy7YFuAhaRg03OfGlPzE81cqJZCiE03AYEyecI2a2mkSKMQPsYdgNkII4kvvp3Zx5zRQkBkIN4lVWU_VjrSeoIutrcoaNxWQ1pw0T_kYwOZEmKQbDYbCRx7e6diGylvRQYUnENU/w640-h352/12-8.png" width="640" /></a></div><br /><p>This process for figuring the Equation of Time may appear involved at first, but if you work out a few examples and check yourself with the almanac, it should fall into place. If you are going to memorize something that could be of great value, this is it. When you know this and have an accurate watch, you will always be able to find your longitude; you don’t need anything else. With this point in mind, it is worth the trouble to learn it.</p><p>Also remember that the LAN method tells you what your longitude was at LAN, even though it may have taken all day to find it. To figure your present longitude, you must dead reckon from LAN to the present. Procedures for converting between distance intervals and longitude intervals are covered in the Keeping Track of Longitude section below.</p><p>For completeness, we should add that, strictly speaking, this method assumes your latitude does not change much between the morning and afternoon sights used to find the time of LAN. A latitude change distorts the path of the sun so that the time halfway between equal sun heights is no longer precisely equal to LAN. Consider an extreme example of LAN determined from sunrise and sunset when these times are changing by 4 minutes per 1° of latitude (above latitude 44° near the solstices). If you sail due south 2° between sunrise and sunset, the sunset time will be wrong by 8 minutes, which makes the halfway time of LAN wrong by 4 minutes. The longitude error would be 60′, or 1°. But it is only a rare situation like this that would lead to so large an error. It is not easy to correct for this when using low sights to determine the time of LAN. For emergency longitude, you can overlook this problem.</p><p>In preparing for emergency navigation before a long voyage, it is clearly useful to know the Equation of Time. Generally, it will change little during a typical ocean passage. Preparing for emergency longitude calculations from the sun involves the same sort of memorization required for emergency latitude calculations. For example, departing on a planned thirty-day passage starting on July 1, you might remember that the sun’s declination varies from N 23° 0′ to N 18° 17′ and the time of LAN at Greenwich varies from 1204 to 1206. Then, knowing the emergency prescriptions for figuring latitude and longitude, you can derive accurate values for any date during this period.</p><div style="text-align: center;">This article is taken from <i><a href="https://www.starpath.com/catalog/books/1830.htm" target="_blank">Emergency Navigation</a></i> by David Burch</div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com4tag:blogger.com,1999:blog-714718709601620866.post-49370547143538434672023-12-04T17:02:00.000-08:002023-12-19T19:27:32.147-08:00Great Circle Distance — The Three Options<p><span style="font-family: verdana;">The great circle (GC) route is the shortest distance between two points on the globe, so we must always keep it in mind when planning an ocean crossing, even if we do not end up following that route. </span></p><p><span style="font-family: verdana;">The GC route is defined by cutting the earth with a plane that goes through the departure (A), the destination (B), and the center of the earth (C). That plane cuts the earth in half, and the points A and B lie along a circle (a great circle) whose circumference is the circumference of the earth, and the track along that line from A to B is called the great circle route. If the plane does not go through the center of the earth, you also get a circle where it intersects the earth, but its circumference will be smaller than that of a great circle.</span></p><p><span style="font-family: verdana;"><br /></span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVMcIoK3IakO_Cr8STjQlos8ITS2ubSQEDrBV3IpiiKW9ni0pz69dXxB2QrFSDxev8CJxCGAD8siGc4pMBcqzaoxKYricor2of-2PAPMKQDiVW3iu6ke_lifYZPbl1IW-ENbb13iIshG_qAhRR3qxp19uydoLNGZol_THx4jItXjhBXzr6inA5jNhQrnk/s4032/ball%20and%20plane.JPG" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: verdana;"><img border="0" data-original-height="4032" data-original-width="3024" height="200" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVMcIoK3IakO_Cr8STjQlos8ITS2ubSQEDrBV3IpiiKW9ni0pz69dXxB2QrFSDxev8CJxCGAD8siGc4pMBcqzaoxKYricor2of-2PAPMKQDiVW3iu6ke_lifYZPbl1IW-ENbb13iIshG_qAhRR3qxp19uydoLNGZol_THx4jItXjhBXzr6inA5jNhQrnk/w150-h200/ball%20and%20plane.JPG" width="150" /></span></a></div><span style="font-family: verdana;"><br /></span><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgh-1rDjBNi5OSvG5PYa6A3B2DAEBjF9il4CzrQnhpVpAKuCdS0Pbr46OwRkvqzaioUi6h9XgWjFAJuUJgaRjNVWY7mPQg_l5VE0WgxoXmJojCC2EbKIS4jUhcAWbLiqVzXa1cLXRWzK78SKryZy00Xkpm46mzoHJDmIDNo_W3mDDvl0QFBTckn6uwkE2M/s3019/ball%20and%20plane.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: verdana;"><img border="0" data-original-height="1525" data-original-width="3019" height="203" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgh-1rDjBNi5OSvG5PYa6A3B2DAEBjF9il4CzrQnhpVpAKuCdS0Pbr46OwRkvqzaioUi6h9XgWjFAJuUJgaRjNVWY7mPQg_l5VE0WgxoXmJojCC2EbKIS4jUhcAWbLiqVzXa1cLXRWzK78SKryZy00Xkpm46mzoHJDmIDNo_W3mDDvl0QFBTckn6uwkE2M/w400-h203/ball%20and%20plane.png" width="400" /></span></a></div><span style="font-family: verdana;"><br /></span><p><span style="font-family: verdana;">Distance along a great circle is measured in nautical miles, which is a unit that was invented for just this purpose. Namely, the full great circle spans 360º, and each degree is 60', so a nautical mile (nmi) is defined as the length of 1 arc minute (1') along the circumference of a great circle of the earth. </span></p><p><span style="font-family: verdana;">This is very convenient for navigation if we consider the great circle between the north pole, earth center, and south pole, which is a meridian of longitude. Arc minutes along this great circle are minutes of latitude. Thus a navigator knows immediately if they are to sail from Cape Flattery, WA at about Lat 48 N to San Francisco at about Lat 38 N, they must go 10º of Lat or 600 nmi. Every 1' of Lat = 1 nmi.</span></p><p><span style="font-family: verdana;">There are other implications of this definition that are integrally related to the topic at hand. For one, this assumes the earth is a sphere... which is not too radical an idea, having been known — or believed to be true — by every educated person on earth except Christopher Columbus for <a href="https://davidburchnavigation.blogspot.com/2018/05/christopher-columbus-and-spherical-earth.html" target="_blank">over a thousand years</a>. </span></p><p><span style="font-family: verdana;">As it turns out, the earth is not a perfect sphere, it is squashed a bit at the poles, as we might slightly compress a beach ball into more of a doorknob shape. Consequently a nautical mile cannot be simply defined as 1' of Lat, because the length of 1' of Lat changes slightly with latitude on this non-spherical shape. That simple definition is reserved for the less precise term <i>sea mile</i>, which is defined as 1' of Lat at a constant Lon. But nautical mile is the official international unit of global navigation so it has to have a definition, and that was given to it 1929: 1 nmi = 1852 meters, exactly.</span></p><p><span style="font-family: verdana;">That definition then tells us what we mean by <i>spherical earth</i>, based on the geometry of a circle. Namely, the circumference (c) of a circle = 2 </span><span style="font-family: Helvetica; font-size: large;">𝜋</span><span style="font-family: verdana;"> x radius (r) of the circle. Thus we have for spherical earth, c = 2 </span><span style="font-family: Helvetica;"><span style="font-size: medium;">𝜋</span></span><span style="font-family: verdana;"> r = 360 x 60 x 1.852 km, or solving for r:</span></p><p style="text-align: center;"><span style="font-family: verdana;">r (spherical earth) = 360 x 60 x 1.852 /(2 x 3.141) = 6,367.9 km.</span></p><p><span style="font-family: verdana;">Thus we are at the first of three types of great circle distance computation, which is assume the earth is spherical with a radius of 6,367.9 km, which makes 1' on the circle = 1 nmi and we can use spherical trigonometry to compute the great circle distance (d) between point 1 and point 2, namely:</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-variation-settings: normal; line-height: normal; margin: 0px; text-align: center;"><span style="font-family: verdana;">Cos(d) = </span><span style="font-family: verdana;">Sin(Lat1) x Sin(Lat2) + Cos(Lat1) x Cos(Lat2) x Cos(Lon2 – Lon1).</span></p><p style="text-align: left;"><span style="font-family: verdana;">This formula can be solved with an inexpensive trig calculator, and indeed this is the solution we would see in many calculators or apps, especially those that are largely celestial navigation oriented, because cel nav assumes the earth is a sphere as defined above.</span></p><p style="text-align: left;"><span style="font-family: verdana;">If we use this method to compute the GC distance between San Francisco (37.8N, 122.8W) and Tokyo (34.8N, 139.8E) we would get 4,473.61 nmi.</span></p><p style="text-align: left;"><span style="font-family: verdana;">But it is not just cel nav apps that use this equation. The <a href="https://msi.nga.mil/Calc" target="_blank">Bowditch computations</a> also assume this same 1' = 1 nmi spherical earth, and present the same value.</span></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjOBoJyt-9eCURxz9qWMgqRhExHOMh6tT681nQHrj_nLBJxyWop59cS0It6e3PCsdRyu6fJeOE_l6zyBS5BUJX80bwjgpmcUFgvVQk8iVyk_cLGaKllym3chd8TbFpvM09rje4LY4Pte18ZJOPlDjptbUxFF5XL9yiHh0QzRDe43B4S8xG2cs253sa3Q2o/s634/bowditch%20spherical%20earth.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="421" data-original-width="634" height="424" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjOBoJyt-9eCURxz9qWMgqRhExHOMh6tT681nQHrj_nLBJxyWop59cS0It6e3PCsdRyu6fJeOE_l6zyBS5BUJX80bwjgpmcUFgvVQk8iVyk_cLGaKllym3chd8TbFpvM09rje4LY4Pte18ZJOPlDjptbUxFF5XL9yiHh0QzRDe43B4S8xG2cs253sa3Q2o/w640-h424/bowditch%20spherical%20earth.png" width="640" /></a></div><span style="font-family: verdana;"><p style="text-align: left;"><span style="font-family: verdana;">Besides cel nav focused apps, some chart navigation apps, officially referred to as electronic charting systems (ECS), also use this spherical earth solution, such as Rose Point's Coastal Explorer. We might call this traditional radius, the <i>cel nav radius (</i></span><span style="text-align: center;">6,367.9 km).</span></p><p style="text-align: left;"><span style="font-family: verdana;"><br /></span></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghP6CikUwx_Zncrad2LerYnvU6uZKDC1IEus3UdFICqYyuap4gv-3768X3XfsIpm5rQF3akE-UrCPjFHlKVEpqfAxZU1e0gl1LVtAxQ1oqMNluQY5eitQbZMAYOCNMwBVuojYSj_5hdk96PI6UPyoCILsw5-IWIsD_r6cAvFNPI5loPPc5eq_z4LSp_O0/s1821/RosePoiint%20uses%20GC.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1074" data-original-width="1821" height="378" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghP6CikUwx_Zncrad2LerYnvU6uZKDC1IEus3UdFICqYyuap4gv-3768X3XfsIpm5rQF3akE-UrCPjFHlKVEpqfAxZU1e0gl1LVtAxQ1oqMNluQY5eitQbZMAYOCNMwBVuojYSj_5hdk96PI6UPyoCILsw5-IWIsD_r6cAvFNPI5loPPc5eq_z4LSp_O0/w640-h378/RosePoiint%20uses%20GC.png" width="640" /></a></div><br /><p></p>But if we open another popular ECS like qtVlm, and ask for the GC distance between these two points we get a different answer, </span><span style="font-family: verdana;">namely 4,476.62 nmi. </span><p></p><p style="text-align: left;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0FAj0YuRpU5JwZdL8rcRcvHKKbBiacll4hukL-xESqm_XnWB4ZJumaFrASHGsicMgv-9inNAbbLtwxn7Pm_EseBybbqPTmE3Jxekftul5or9-_k_dXbMLy_uoENKOn08lUbCf_69D0PC0kqiEMuF0fLrBQVWK-pZLBBFyzLog9npf-oM0MxFItkgk8E0/s1278/qtVlm%20solution.png" style="margin-left: 1em; margin-right: 1em; text-align: center;"><img border="0" data-original-height="546" data-original-width="1278" height="274" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi0FAj0YuRpU5JwZdL8rcRcvHKKbBiacll4hukL-xESqm_XnWB4ZJumaFrASHGsicMgv-9inNAbbLtwxn7Pm_EseBybbqPTmE3Jxekftul5or9-_k_dXbMLy_uoENKOn08lUbCf_69D0PC0kqiEMuF0fLrBQVWK-pZLBBFyzLog9npf-oM0MxFItkgk8E0/w640-h274/qtVlm%20solution.png" width="640" /></a></p><p style="text-align: left;"><span style="font-family: verdana;"><br /></span></p><p style="text-align: left;"><span style="font-family: verdana;">We see essentially the same answer in OpenCPN.</span></p><p style="text-align: left;"><span style="font-family: verdana;"><br /></span></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjz2LlkHwJ00DXU30QwXEqOYpd60Z6-EvKABvUGtPBZo8Oog7Kmet66B6pCnHfbdM2Mt9aaPNdVs6a7yT7fZJu1nv-WnX2eKnw_2ynVz1tvDoTQ_5jD5fhTfrjE6ABgSxA4nUl5hFrs7QEXYeCGlvUfEGr-jTIdhxyi-_Eg9Q-Olwi7S6yGnGRHp5fCUgE/s1785/openCPN%20avg%20R.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1662" data-original-width="1785" height="596" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjz2LlkHwJ00DXU30QwXEqOYpd60Z6-EvKABvUGtPBZo8Oog7Kmet66B6pCnHfbdM2Mt9aaPNdVs6a7yT7fZJu1nv-WnX2eKnw_2ynVz1tvDoTQ_5jD5fhTfrjE6ABgSxA4nUl5hFrs7QEXYeCGlvUfEGr-jTIdhxyi-_Eg9Q-Olwi7S6yGnGRHp5fCUgE/w640-h596/openCPN%20avg%20R.png" width="640" /></a></div><br /><span style="font-family: verdana;"><br /></span><p></p><p style="text-align: left;"><span style="font-family: verdana;">It is not just qtVlm and OpenCPN (two popular free ECS), other computer or mobile nav apps might show this answer for these two points.</span></p><p style="text-align: left;"><span style="font-family: verdana;">...that is, unless we are looking at a GPS chart plotter app or a handheld GPS unit with routing options, such as the Garmin GPSmap 78 shown below.</span><span style="font-family: verdana;"> </span></p><p style="text-align: left;"><span style="font-family: verdana;"><br /></span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmibrVc38o4rCicGI_rXCmjT_teMcIPGtgRMgFB6ObK9q5SAaZrusIXHAMyxIeMhn0RBaYpO6U6gps08VfHFHAaezHFRlU0NFsjBGwRciE8hbCV3IsMLQ4LyQChXScQojTlLYuNiORxgB-APMTbdQ2WToR8JVTKn5-B3nVXwVBek_A4pnblETo2udb-uk/s1099/Grmin%20WPs%20and%20distance.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="770" data-original-width="1099" height="448" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmibrVc38o4rCicGI_rXCmjT_teMcIPGtgRMgFB6ObK9q5SAaZrusIXHAMyxIeMhn0RBaYpO6U6gps08VfHFHAaezHFRlU0NFsjBGwRciE8hbCV3IsMLQ4LyQChXScQojTlLYuNiORxgB-APMTbdQ2WToR8JVTKn5-B3nVXwVBek_A4pnblETo2udb-uk/w640-h448/Grmin%20WPs%20and%20distance.png" width="640" /></a></div><br /><span style="font-family: verdana;">In this case, we get a still different value of this same "great circle distance," namely </span><span style="font-family: verdana;">4,486.7 nmi. </span><div><span style="font-family: verdana;"><br /></span></div><div><span style="font-family: verdana;">We also see this value in the ECS TimeZero.</span></div><div><span style="font-family: verdana;"><br /></span></div><div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi62alCeKe2UAPHtT_X3z9zk5mgLkNHBxlFtPObiWLrcBt5AACt2pYJa6_zO4H6IrN6ejZY4Xcy4Iee3USQTIlJyL_a5wABsdiGYds3guneOtoUAG2B9EFo5DMQOg9mIElNBXrsWwhoD15mPlbNFZD3H7ZtuU53yUo7bw0EVshseva9pkMH6_yi1yNNZ24/s1642/TZ%20uses%20geodetic.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1022" data-original-width="1642" height="398" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi62alCeKe2UAPHtT_X3z9zk5mgLkNHBxlFtPObiWLrcBt5AACt2pYJa6_zO4H6IrN6ejZY4Xcy4Iee3USQTIlJyL_a5wABsdiGYds3guneOtoUAG2B9EFo5DMQOg9mIElNBXrsWwhoD15mPlbNFZD3H7ZtuU53yUo7bw0EVshseva9pkMH6_yi1yNNZ24/w640-h398/TZ%20uses%20geodetic.png" width="640" /></a></div></div><div><span style="font-family: verdana;"><br /></span><div><span style="font-family: verdana;"><br /></span></div><div><span style="font-family: verdana;">In short, we have three values for the "great circle" distance between SF and TKY, and the one we get depends on how or who we ask. The differences in these example spans 13.1 nmi — and this, in an age where we pride ourselves with a GPS that gives our position accuracy to about a boat length or two (± 0.01 nmi).<br /></span><div><br /></div><div><span style="font-family: verdana;">Navigator's do not like inconsistent information, and will usually stop to figure out the source of the discrepancy. This note is intended to help with that.</span></div><div><br /><p></p><p style="text-align: left;"><span style="font-family: verdana;">The three values we noted were presented in increasing accuracy, which is tied to the shape of the earth that was used to compute the value. In most cases, these differences do not have a practical affect on navigation, but it is good to know if something is working right or not, and to understand what we see.</span></p><p style="text-align: left;"><span style="font-family: verdana;"><b>Type 1.</b> SF to TKY = </span><span style="font-family: verdana;">4,473.61 nmi. </span><span style="font-family: verdana;">Spherical earth with 1' = 1 nmi. This solution is used in cel nav and other apps, as noted. Earth radius used is </span><span style="font-family: verdana; text-align: center;">6,367.9 km. The <i>cel nav radius</i>.</span></p><p style="text-align: left;"><span style="font-family: verdana;"><b>Type 2.</b> </span><span style="font-family: verdana;">SF to TKY = </span><span style="font-family: verdana;">4,476.62 nmi. This is what we would see in selected ECS that want to improve on the accuracy by using an improved earth radius. </span></p><p style="text-align: left;"><span style="font-family: verdana;">An improved earth shape is more of an oblate ellipsoid (doorknob), which can be approximated with a new spherical earth, but now using the average of the polar and equatorial radii, as shown. This improved method still computes the distance as a spherical earth, but uses this slightly smaller average radius of 6,371.0 km. This can be called the <i>WGS84 average radius</i>.</span></p><p style="text-align: left;"><br /></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjp77Mmx-c7LLHBmn8hp89UktBteHJFO0MMvbhw3JGJ_vVgOI6OI6h1C3vO-IwWODTlnY2YxMdf0pHnZOBO9KcNgKuYLA6FHOO2IjepYmjo_EIn0pd2KHgfzQkGpPHqYue0yc62lIDAgFfASLeu5wTWshKoAVo8xWDt9Eb1alieqfbjhIjlIfcCqllnGr4/s1024/WGS84_mean_Earth_radius.svg.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1024" data-original-width="1024" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjp77Mmx-c7LLHBmn8hp89UktBteHJFO0MMvbhw3JGJ_vVgOI6OI6h1C3vO-IwWODTlnY2YxMdf0pHnZOBO9KcNgKuYLA6FHOO2IjepYmjo_EIn0pd2KHgfzQkGpPHqYue0yc62lIDAgFfASLeu5wTWshKoAVo8xWDt9Eb1alieqfbjhIjlIfcCqllnGr4/s320/WGS84_mean_Earth_radius.svg.png" width="320" /></a></div><i>WGS84 earth dimensions. Keep in mind the scale. The equatorial bulge (7 km) is just 0.1% of the radius; the depression of the poles (15 km), just 0.2%. The earth is actually pretty spherical.</i><br /><div style="text-align: left;"><br /></div><span style="font-family: verdana;"><div style="text-align: center;"><br /></div></span><p></p><p style="text-align: left;"><span style="font-family: verdana;"><b>Type 3.</b> </span><span style="font-family: verdana;">SF to TKY = </span><span style="font-family: verdana;">4,486.7 nmi. Is in principle the most accurate solution as it uses not assume a spherical earth shape, but computes the distance along the surface of an oblate ellipsoid, the size and shape of which we get from the geodedic datum we have selected, such as <a href="https://ahrs.readthedocs.io/en/stable/wgs84.html" target="_blank">WGS84</a>. We will get this (Type 3) solution in most apps or hardware that lets us choose the horizontal datum, such as any GPS unit, hand-held or console chart plotter. This choice is actually an important thing to check in your GPS to be sure it matches your nautical charts; most should default to WGS84.</span></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjvwoPQy9kQWkb-a0-fRXRnu3iH9t9GrIHuN7PiJWDVn-7LybpTlFs4gcko1U1UMLoYbjXNzRSGr_HTE9IMyhlSoi76nxF5Q3r1-aLz-GGa-3NKR7DNnpNhn2g2TnI2s9mxXDvIS359NpjzABzduuV3bA1Xs8HD2lnibm2QJKy6DWkUQb2ETUBYlG72Z1U/s3791/Garmin%20datum.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="3791" data-original-width="1737" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjvwoPQy9kQWkb-a0-fRXRnu3iH9t9GrIHuN7PiJWDVn-7LybpTlFs4gcko1U1UMLoYbjXNzRSGr_HTE9IMyhlSoi76nxF5Q3r1-aLz-GGa-3NKR7DNnpNhn2g2TnI2s9mxXDvIS359NpjzABzduuV3bA1Xs8HD2lnibm2QJKy6DWkUQb2ETUBYlG72Z1U/w184-h400/Garmin%20datum.JPG" width="184" /></a></div><br /><p style="text-align: left;"><span style="font-family: verdana;">We also get this geodetic or ellipsoidal solution for "great circle" distances in several popular computer based ECS, such as TimeZero.</span></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUC3srHmmYJJjzGYDtcEu-06wZRiMHgEwaLijbLTMxuDsRb0ERtu1de00o01YCOOZkORHzdmCFBp-c7L1bW_8HyawUzjZpfLC9NSoqjlgceK-_3Xe-KN-FsSzVi12RfPw9e2lQKo238XbjnU_YJrhGqmavXTTr0SU13aJhmEIouu6o9h0SguUFxfWW9ck/s1642/TZ%20uses%20geodetic.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1022" data-original-width="1642" height="398" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiUC3srHmmYJJjzGYDtcEu-06wZRiMHgEwaLijbLTMxuDsRb0ERtu1de00o01YCOOZkORHzdmCFBp-c7L1bW_8HyawUzjZpfLC9NSoqjlgceK-_3Xe-KN-FsSzVi12RfPw9e2lQKo238XbjnU_YJrhGqmavXTTr0SU13aJhmEIouu6o9h0SguUFxfWW9ck/w640-h398/TZ%20uses%20geodetic.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: verdana;">Google Earth will also give this value, but for other locations you may get different results as they may use different datums for different locations, which we do not seem to have control over. (The same is true, by the way, for the elevation data set or model it uses for different parts of the world. It is likely the best we can conveniently come by, but we will not know the details.)</span></div><span style="font-family: helvetica;"><br /></span><p></p><p style="text-align: left;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgbAROonsMR0hT_Bf-8LXU-7AYxmeGZ8iyycF-IiM0uMWhGy1sRLo_AGIqCuSu8sT361jU2S_x292SjjoKCUX22uQGQmVe-S6jJsM2tGCXO92KysMoVdCc5I8uZMR1fCyz0BR1Q_V4zvYa_Ff-zXvpWWBJVEIAMIB24ligE-CNy3DkUPHfcaTliarlsm7A/s869/GE%20distance.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="302" data-original-width="869" height="222" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgbAROonsMR0hT_Bf-8LXU-7AYxmeGZ8iyycF-IiM0uMWhGy1sRLo_AGIqCuSu8sT361jU2S_x292SjjoKCUX22uQGQmVe-S6jJsM2tGCXO92KysMoVdCc5I8uZMR1fCyz0BR1Q_V4zvYa_Ff-zXvpWWBJVEIAMIB24ligE-CNy3DkUPHfcaTliarlsm7A/w640-h222/GE%20distance.png" width="640" /></a></div><span style="font-family: helvetica;"><p style="text-align: left;"><span style="font-family: helvetica;"><br /></span></p>Numerical values of these distances can be checked online with the </span><a href="https://edwilliams.org/gccalc.htm" style="font-family: helvetica;" target="_blank">Jack Williams calculators</a><span style="font-family: helvetica;">.</span><p></p><p style="text-align: center;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj33pcHoUPHs_xjjqMJqvJHef2GMxIoB92ArpBQwPEx3mhkbTGMs7yfTldMhnck8mDWAAso0GS2K6Zcr0VWq8hOfP5q0SYWnnxpKm71E-9QCCKIGr5V-mqKqkjIWJzjUZbsJ_-D08D9MfzudzRVarDbIlelZvdsiB1T3QDK9aHCGuY1vdcHUU9EeH8ij5M/s1788/jack%20williams%20results.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1119" data-original-width="1788" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj33pcHoUPHs_xjjqMJqvJHef2GMxIoB92ArpBQwPEx3mhkbTGMs7yfTldMhnck8mDWAAso0GS2K6Zcr0VWq8hOfP5q0SYWnnxpKm71E-9QCCKIGr5V-mqKqkjIWJzjUZbsJ_-D08D9MfzudzRVarDbIlelZvdsiB1T3QDK9aHCGuY1vdcHUU9EeH8ij5M/w640-h400/jack%20williams%20results.png" width="640" /></a></div><br /><div style="text-align: left;"><span style="font-family: verdana;"><span>These values can be used to determine what type of computation your device is doing. Use </span><span>Departure = (37.8, -122.8); Destination = 34.8, 139.8). </span></span></div><div style="text-align: left;"><span style="font-family: verdana;">Then check for the GC distance between them.</span></div><div style="text-align: left;"><span style="font-family: verdana;"><br /></span></div><div style="text-align: left;"><span style="font-family: verdana;">4473.6 means spherical earth using the <i>cel nav radius</i> (<span style="text-align: center;">6,367.9 km</span>)</span></div><div style="text-align: left;"><span style="font-family: verdana;">4476.6 means spherical earth using the <i>WGS84 average radius</i> (6,371.0 km)</span></div><div style="text-align: left;"><span style="font-family: verdana;">4486.7 means a WGS84 <i>ellipsoidal computation</i></span></div><div style="text-align: left;"><span style="font-family: verdana;"><br /></span></div><div style="text-align: left;"><span style="font-family: verdana;">A consequence of a true ellipsoidal computation means a nominal, long-distance great circle estimated position depends on which way you are headed. Consider starting from the equator at 130 W and traveling 50º N versus 50º E. Sailing along the surface of a spherical earth, the distance you travel would be the same in both directions, namely 3,000 nmi. But sailing on the surface of an oblate ellipsoid, this is not the case. You have a smaller radius going toward the pole than you do going along the equator. Going north you sail 2,991.8 nmi but sailing east you go 3,005.4 nmi.</span></div><div style="text-align: left;"><span style="font-family: verdana;"><br /></span></div><div style="text-align: left;"><br /></div><div style="text-align: center;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiYeXC_DfJndE5TvxvFH9BWvuvumINEA9Ejw4ZPAPde0niFpznzUZbEClhrA5pqeBjcK1_IcNXeqsQDsFdxqqFyr1ZwUCvQySNxydwSM9XDwWWQ-QghlPKtfO4_vWIYZau4MJSjm2Wde9sYB8Ir1O7UDYf8PBdrjCnQ-gzUA5jAabghoXsCMnx8fZBXwEo/s1260/50%20deg%20dr.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1260" data-original-width="1253" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiYeXC_DfJndE5TvxvFH9BWvuvumINEA9Ejw4ZPAPde0niFpznzUZbEClhrA5pqeBjcK1_IcNXeqsQDsFdxqqFyr1ZwUCvQySNxydwSM9XDwWWQ-QghlPKtfO4_vWIYZau4MJSjm2Wde9sYB8Ir1O7UDYf8PBdrjCnQ-gzUA5jAabghoXsCMnx8fZBXwEo/w636-h640/50%20deg%20dr.png" width="636" /></a></div><br /><span style="font-family: verdana;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghlfbqWaIa6Jg5_fUiHHyg8dofKPs7eIw1UuE9jXmOVxuybhzT7b5D0as31ndN_JahNM8P40i1x92r_lEsZnEhtdMkfrj6Tk-zTXYqj-MEHkXwDzhId_fuaZL-6fs-0ZP3zn4VKk8cQwEt6ty5n6LnauzIsx66tRF_EfY9zHGGwoQuUUZU5ucyojTORSo/s1784/sphere%20vs%20ellipsoid.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="872" data-original-width="1784" height="312" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghlfbqWaIa6Jg5_fUiHHyg8dofKPs7eIw1UuE9jXmOVxuybhzT7b5D0as31ndN_JahNM8P40i1x92r_lEsZnEhtdMkfrj6Tk-zTXYqj-MEHkXwDzhId_fuaZL-6fs-0ZP3zn4VKk8cQwEt6ty5n6LnauzIsx66tRF_EfY9zHGGwoQuUUZU5ucyojTORSo/w640-h312/sphere%20vs%20ellipsoid.png" width="640" /></a></div><div style="text-align: center;"><br /></div><p></p><p><span style="font-family: verdana;">For completeness, let me add a 4th solution! One that goes in the other direction: not striving for high precision, but looking for a solution that can be done with a plastic device that still works if soaking wet, after falling off the nav station and getting stomped on by numerous crew members' wet boots.</span></p><p style="text-align: center;"><span style="font-family: verdana;"><a href="https://davidburchnavigation.blogspot.com/2019/08/great-circle-sailing-with-2102-d-star.html" target="_blank">Great Circle Solutions with the 2102-D Star Finder</a></span></p><p><span style="font-family: verdana;"><br /></span></p><p><br /></p></div></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-49379425197972072082023-10-24T11:50:00.007-07:002023-10-24T13:11:19.140-07:00Reducing Station Pressure to Sea level pressure<p style="font-family: Arial; font-stretch: normal; line-height: normal; margin: 0px 0px 5px; text-align: right;"> </p>
<p style="font-family: Arial; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><i>The following is taken from <a href="https://www.starpath.com/catalog/books/1889.htm" target="_blank">The Barometer Handbook By David Burch</a>. All references are to that text.</i></p><p style="font-family: Arial; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Recalling that the vertical rate of pressure change is always thousands of times higher than the horizontal rate that creates the wind and weather we care about, it is easy to see that observed pressures at various elevations must be carefully normalized to sea level if we are to learn about the true pressure pattern at hand.</p><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">In this section we outline how meteorologists determine sea level pressures from the reports they receive from varying elevations. We do not have call to do this ourselves very often, but the procedures are here if you care to. To be more precise, this is how meteorologists used to do it, based on procedures specified in detail in the Manual of Barometry (WBAN). These procedures give some insight into the physical factors that contribute to the reduction, but in practice today they use a much more empirical method, covered at the end of this section.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Step one is to clarify the concept of sea level pressure at, for example, a high plateau located inland, far from the sea—or even far from anywhere whose elevation might be near sea level. This is certainly an abstract concept, but one that is needed to normalize the observations.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The procedure is to imagine a large hole in the ground at the elevated station that reaches down to sea level. Then the question reduces to estimating what the pressure would be at the bottom of this hole based on the pressure we read at the elevated station level, along with the temperature and dew point of the air at the station level.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">We know the weight of the air from the station level on up to the top of the atmosphere. That is just the station pressure we observe. So the problem reduces to figuring out how much the fictitious air column weighs in the fictitious hole.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">An easy way to approximate the answer is to assume the air in the hole behaves exactly like the International Standard Atmosphere (ISA). Then we can just go to Table A2 and look up the answer. For example, consider being at an elevation of 1,200 feet above sea level. From Table A2 we see that this elevation corresponds to a pressure drop of 43.2 mb in the standard atmosphere. So if our actual station pressure were 985.5 mb, we would estimate that the pressure at sea level was 985.5 + 43.2 = 1028.7 mb.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">This approximation assumes the air in the hole has exactly the average properties of the standard atmosphere. This is unlikely to be true, and we could even know this ahead of time by comparing the station pressure and temperature with the standard atmosphere values at our elevation. We can improve on this ISA approximation significantly, but it takes some number crunching to do so.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The weight of the air in the hole depends on the density of the air, which in turn depends on the average temperature of the air column as well as the moisture content—the ISA assumes dry air (relative humidity = 0%). For a better estimate of the weight of the air column, we need a better estimate of the average temperature of the air column. A complicating factor is the amount of water vapor in the air. This not only changes the density of the air directly, it also affects how the temperature changes with increasing elevation.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The standard way to simplify these calculations is to define the “virtual temperature” (T<span style="vertical-align: -2px;">v</span>) of moist air as the temperature that dry air must have in order to produce the same pressure and density the moist air has. The definition is illustrated in Figure A3-1.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhtJgLxxlFlKTkgBH-pdrTP8IonYkWrAnEz_rUobFMU8pN1cj50x9O2VMe124kDgQDGP3XsQWauFOfG-tb5gtX-23uCuaPqKX5a1CKzAqi2p1A4hpepzGVofuKIJkaN7YBqrf0jlsAeFrgUoCp_mnErgD0Hrx5pWCnQcaplNW6jWB3cCuAS1IYbfuP6bo/s802/Figure%20A3-1.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="802" data-original-width="754" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhtJgLxxlFlKTkgBH-pdrTP8IonYkWrAnEz_rUobFMU8pN1cj50x9O2VMe124kDgQDGP3XsQWauFOfG-tb5gtX-23uCuaPqKX5a1CKzAqi2p1A4hpepzGVofuKIJkaN7YBqrf0jlsAeFrgUoCp_mnErgD0Hrx5pWCnQcaplNW6jWB3cCuAS1IYbfuP6bo/w376-h400/Figure%20A3-1.png" width="376" /></a></div><br /><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">We can then study the properties of a column of moist air as if it were dry air by replacing the average temperature with an average T<span style="vertical-align: -2px;">v</span>. The formula for Tv depends on the station temperature, pressure, and dew point. In principle, each equation in Chapter 9 on altimetry that contains a T, should have that T replaced with T<span style="vertical-align: -2px;">v </span>for the most accurate results. We will calculate this T<span style="vertical-align: -2px;">v</span> in a moment, but first a more basic practical matter.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">We will need a measurement of the station pressure if we are to find the sea level pressure. If you have actually measured the station pressure yourself, then you are done. That is the one you will use. But if you are testing this procedure of reducing station pressure to sea level pressure by analyzing data from another location, you still need the station pressure at that location, but you will soon learn that information may not be available. With the exception mentioned at the end of this section, station pressures are rarely reported. What they do, instead, is automatically reduce the station pressures to sea level pressures and report those. All airport reports, however, always compute the altimeter setting, discussed in Chapter 9. The reports are called “Metars,” derived from a French phrase meaning weather reports from airports.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 7.2px; text-align: justify; text-indent: 14.4px;">Altimeter setting, by definition, depends only on the station pressure and elevation of the station, so we can unfold the altimeter setting (AS) to get the station pressure (Ps) we need from the equation:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 7.2px; text-align: center;">Ps =[AS<span style="vertical-align: 4px;">0.1903</span> - (1.313 x 10<span style="vertical-align: 3px;">-5</span>) x H]<span style="vertical-align: 4px;">5.255</span>,</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">where H is the station elevation in feet. This is the hypsometric equation with the temperature replaced with the ISA lapse rate. AS is given in inches of mercury, so Ps will be inches of mercury as well, but we can convert to mb as:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">Ps (mb) = 33.864 x Ps (inches)</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">The above two equations are not from the WBAN procedures, but taken directly from NWS computer code. I apologize for the mixed units necessary if we use the exact equations presented in both methods.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"> Once we have the station pressure, we can proceed with the WBAN procedure by computing the virtual temperature of the air. Start with finding the vapor pressure of the air (e) in mb from:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">e = 6.11 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> 10<span style="vertical-align: 3px;">E</span></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">where e is in mb, </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">E = 7.5 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> T<span style="vertical-align: -2px;">d</span>/(237.7 + T<span style="vertical-align: -2px;">d</span>), </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px;">and T<span style="vertical-align: -2px;">d</span> is the dew point of the station air in °C. Then we can find T<span style="vertical-align: -2px;">v</span> in °K from:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">T<span style="vertical-align: -2px;">v </span>= (Ts + 273.15)/[1 - 0.379 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> (e/Ps)]</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">where e and Ps are in mb, and Ts is the station air temperature in °C. The factor of 0.379 is the ratio of molecular weights of water to air. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The Ts, as always, takes special care. It is the temperature of the air at the station elevation, but not at the time the station pressure was measured. This Ts should be the average of the temperature at the time of the pressure measurement and the temperature at the station 12 hours earlier. Add the two and divide by 2. It has been found over the years that this accounts for the small, but detectable diurnal variation of the pressure (Table 5.6-1). This whole process is an attempt to do the best at a difficult task, so every factor counts. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 7.2px; text-align: justify; text-indent: 14.4px;">Once we have T<span style="vertical-align: -2px;">v</span> at the station level, we need to figure the average T<span style="vertical-align: -2px;">v</span> in the fictitious air column. At this point we fall back on the ISA for an estimate of how the temperature changes in the fictitious air column. To find the <i>mean</i> virtual temperature (T<span style="vertical-align: -2px;">mv</span>) in °K use the ISA lapse rate to get:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 7.2px; text-align: center;">T<span style="vertical-align: -2px;">mv</span> = T<span style="vertical-align: -2px;">v</span> + [273.15 + 0.0065 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> (H/2)].</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Now we rewrite the hypsometric equation from Chapter 9 for the sea level pressure P1 = Psl, P2 = Ps, with Z1=0 and Z2 = H = height of the station in meters as:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">Psl = r <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> Ps, </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">where</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 7.2px; text-align: center;">r = exp[ H / (29.28980 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> T<span style="vertical-align: -2px;">mv</span>)].</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">r is a fraction with no units, called the “pressure reduction ratio.” H must be in meters and T<span style="vertical-align: -2px;">mv</span> in °K. Recall °K = °C + 273.15°.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">This can be thought of as the basic solution. As an example, check data from Table A2, such as H = 600 m, Ts = 11°C (in dry air T<span style="vertical-align: -2px;">v</span>=Ts), with Ps = 942.1 mb. Then you should find that Psl = 1013.25 mb, since we used the ISA values. Change Tv to 2°C to get 1015.6 mb or use 20°C to get 1011.0 mb. If you assume the relative humidity of that 20°C air is 75%, then the dew point is 15.4°C, and this will yield T<span style="vertical-align: -2px;">v</span> = 22.1°C, which in turn would imply Psl = 1010.5 mb. The humidity correction is more important in warm air than in cold.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">This basic solution is the one generally used for stations below 50 meters elevation in the WBAN procedure. For higher stations two more corrections are made. First the height H is converted to a <i>geopotential height</i> (H<span style="vertical-align: -2px;">gp</span>), because the weight of the air depends on gravity, and the strength of the gravitational force varies with latitude and with elevation. This is a very small effect, but it can adjust a high elevation by several meters, which could have an effect on the pressure that is larger than what the humidity does. Samples of geopotential corrections are given in Table A3-1. It is made up of two terms. The latitude factor increase H with increasing latitude, whereas the elevation factor decreases it with increasing elevation. </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6__NTi5GNRfGeG09MbRvMj5LJyke1ILwRCWySwtqsamvcewre_s1fY__df1oJ1J4azALzCLQMJvwyQyTO7sTprEh-iwpaGFLIBDNme8NID9IwA8muPzOuDjm4ppcx1TkL1i31XMsTn1PBnITX2tlGFpGk_l3Z8ioISqLEh0xsEOWb87KCUPJwyxqueR8/s1030/Table%20A3-1.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="602" data-original-width="1030" height="234" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6__NTi5GNRfGeG09MbRvMj5LJyke1ILwRCWySwtqsamvcewre_s1fY__df1oJ1J4azALzCLQMJvwyQyTO7sTprEh-iwpaGFLIBDNme8NID9IwA8muPzOuDjm4ppcx1TkL1i31XMsTn1PBnITX2tlGFpGk_l3Z8ioISqLEh0xsEOWb87KCUPJwyxqueR8/w400-h234/Table%20A3-1.png" width="400" /></a></div><br /><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Finally there is what is called the “Plateau Correction” to the temperature, which can be a significant correction of up to 10°C or more to T<span style="vertical-align: -2px;">v,</span> leading to large changes in Psl for high elevations in extreme temperatures. The correction was first proposed by William Ferrel in 1886, which is more evidence of his genius. His reasoning and reckoning still apply today, though there have been improvements to this overall process since then. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Ferrel noted that average summertime sea level pressures deduced at high elevations were too low, and average wintertime sea level pressures were too high, compared to averages from around the country determined at lower elevations. When deduced at high elevations, the summer-winter difference in average sea level pressures was about 10 mb higher than from stations closer to sea level. In other words, he noted an effect that was obviously caused by the land within a process that was supposed to remove the effects of land. And so a correction was called for.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">He concluded that the effective lapse rate must be different when the high land is present from what it would be if the land were removed. In short, the practice of using the ISA lapse rate for the fictitious air column was not right, and the seasonal average sea level pressure differences gave him a way to estimate a correction.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">He formulated his correction to be applied to the sea level pressure itself as:</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 7.2px 0px; text-align: center;">Correction (mb) = 0.064 (Ts-Tn) ( H/1,000),</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">where H is elevation in feet, Ts is the station temperature, and Tn is the annual average temperature at the station, both in °C. Thus an air temperature that is 20°C higher than the average temperature at an elevation of 5,000 ft would add 6.4 mb to the sea level pressures. This correction smooths out the seasonal differences seen in average sea level pressures across the land.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">By 1900 it was recognized that this correction could be improved by reformulating it in terms of adjustments to the lapse rate itself, yielding a more accurate mean virtual temperature. In modern times, each weather station over 50 meters high reporting sea level pressures has its own Plateau Correction factor it uses to optimize the reduction to sea level. Samples are presented in Table A3-2 for stations above and below 1,000 ft elevation.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjpX8VcpWPBMJJ2kHPEbhjxONCUtoW0Tar26LPr9K7rBGfVHts95Aso_R3WCF9SQIOfl7RrJbtF2sNAO5Dmfb56UTWb3TGhVuiPkbQ_2VZmwdxqB5vOOAFOoTDJk-4_Ymo_29Btinu0jeVjOsQqWph1bkEu4y2bQ5glGlbB2TGFy36EfZgMjl8Qh-2zXZ0/s1458/Table%20A3-2.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="972" data-original-width="1458" height="426" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjpX8VcpWPBMJJ2kHPEbhjxONCUtoW0Tar26LPr9K7rBGfVHts95Aso_R3WCF9SQIOfl7RrJbtF2sNAO5Dmfb56UTWb3TGhVuiPkbQ_2VZmwdxqB5vOOAFOoTDJk-4_Ymo_29Btinu0jeVjOsQqWph1bkEu4y2bQ5glGlbB2TGFy36EfZgMjl8Qh-2zXZ0/w640-h426/Table%20A3-2.png" width="640" /></a></div><br /><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px; text-align: justify; text-indent: 14.4px;">The Plateau Correction is called F(s) as a reminder that it depends on the station. It is applied to T<span style="vertical-align: -2px;">mv as:</span></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 7.2px 0px; text-align: center;">T<span style="vertical-align: -2px;">mv </span>—><span style="vertical-align: -2px;"> </span>T<span style="vertical-align: -2px;">mv </span>+ F(s).</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Ferrel had developed one of the first ways to decide if the “sea level pressures” over elevated lands were correct. He also looked, as others did and still do, at neighboring stations that might be at lower elevations to compare their sea-level results to seek a uniform flow of the sea-level isobars.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"> Another evaluation used today is to plot out the sea level isobars predicted by the sum of all the station reports, and then compare the wind speeds and directions they predict with what is actually observed. In one sense, this is the ultimate test. We want the isobars so we can predict the wind, and if we do get isobars that predict the wind properly then we are doing a good job of measuring and deducing the isobars.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">In modern meteorology there is still another crucial way to evaluate the reduction process and that is to compare the measured isobars with those predicted by any of several computerized atmospheric models. The models predict many properties of the atmosphere, at many levels of the atmosphere, not just at sea level. To the extent these other predicted properties agree with the observations, we want the predicted isobars to agree with observations as well. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">If a model, for example, reproduced the isobars and other properties of the atmosphere over low lands very well, but over high lands or steep slopes the predicted isobars did not agree, but still other predicted properties of the atmosphere did agree, then we could consider that maybe the model is right and the way we are deducing the isobars in these difficult regions is not yet optimized. In short, the interplay between model predictions and deduced sea level pressures is yet another way to evaluate the process, and one that is actively pursued at present. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Figure A3-2 shows samples of how the station pressure reduction constants might be evaluated with model computations to get the most useful set of sea level isobars.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZSvHPbYjjBMkwROLvHB1zxhI1DbTHEXJau2JFuI603kfyUfdwIsNK-JBmnBMphN9Zc2_7dzDBg6U32ST68_49yIjMlclhQFbR6coB523rJJLxow4oUksXEIT7bMrLyt3jEufdpN-fzOgDTWeGY2MuYMgmKgDQGr6OVG99_SseXfZqisnJKAcVTe7ZJ-w/s1740/Figure%20A3-2.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1740" data-original-width="934" height="768" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiZSvHPbYjjBMkwROLvHB1zxhI1DbTHEXJau2JFuI603kfyUfdwIsNK-JBmnBMphN9Zc2_7dzDBg6U32ST68_49yIjMlclhQFbR6coB523rJJLxow4oUksXEIT7bMrLyt3jEufdpN-fzOgDTWeGY2MuYMgmKgDQGr6OVG99_SseXfZqisnJKAcVTe7ZJ-w/w413-h768/Figure%20A3-2.png" width="413" /></a></div><br /><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 3.6px 0px 7.2px; text-align: center;"><b>Sample Pressure Reduction</b></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">KCOS is Colorado Springs, CO, station elevation 6171 ft (1880.9 m), latitude = 38.8°N gave this Metar report: “101554Z AUTO 05005KT 10SM SCT020 OVC029 08/03 A3017 RMK AO2 SLP194 T00830028 TSNO. Observed 1554 UTC 10 May 2009, Temperature: 8.3°C (47°F), Dewpoint: 2.8°C (37°F) [RH = 68%], Pressure (altimeter): 30.17 inches Hg (1021.8 mb) [Sea level pressure: 1019.4 mb]”</p><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 3.6px 0px; text-align: justify;">The question is, how did they get the reported sea- level pressure of 1019.4? </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;"><span style="text-decoration: underline;">WBAN Procedure</span></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (1).</b> Find the reported station temperature from 12h earlier, which is: Observed 0354 UTC 10 May 2009, Temperature: 9.4°C (49°F), and from this figure the average station temperature. Ts = (9.4+8.3)/2 = 8.9°C = 48°F.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (2).</b> From the altimeter setting (30.17) and elevation (6171 ft), find the station pressure Ps = 24.03” = 813.8 mb.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (3).</b> From Ps (813.8 mb), Ts (8.9°C), and Td (2.8°C), find virtual temperature T<span style="vertical-align: -2px;">v</span> = 9.8°C = 283.0°K</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (4).</b> From H = 1880.9 m (6171 ft) at Lat = 38.8 N and Table A3-1, find geopotential height Hgpm = 1880.5 m.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (5).</b> From H<span style="vertical-align: -2px;">gpm</span> (1880.5 m), T<span style="vertical-align: -2px;">v</span> (9.8°C) find mean virtual temperature T<span style="vertical-align: -2px;">mv</span> = 15.9°C = 289.1°K</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (6)</b>. From Ts (8.9°C = 48°F) and interpolation of Table A3-2, find Plateau Correction F(s) = -7.4 F° = -7.4 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x </span>(5/9) = -4.1 C°. Note the correction is a temperature interval, not a temperature.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><b>Step (7).</b> From corrected T<span style="vertical-align: -2px;">mv</span> (15.9 - 4.1 = 11.8°C) and H<span style="vertical-align: -2px;">gpm</span> (1880.5m) find r = 1.2527, and using Ps = 813.8 we find Psl = 1019.4 mb. </p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">This agrees with the Metar report, but the result is very sensitive to which values are rounded at which stage of the computation. Changes could lead to variations of ±0.2 mb. Multiple tests from various stations would have to be done to see how well this historic method compares to the modern method used in the U.S. NWS. Other nations use other procedures.</p><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 3.6px 0px; text-align: center;"><b>ASOS Procedure</b></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Starting sometime around 1992, the NWS in collaboration with the Federal Aviation Administration and the Department of Defence initiated an Automated Surface Observations System (ASOS) to collect and distribute weather data around the country. The data are collected by high precision sensors and then evaluated and analyzed by software at the stations, which are then transmitted to the various agencies and made available to the public.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Atmospheric pressure measurements are of course a crucial part of the program. Each station includes multiple electronic pressure sensors, which are compared to each other continuously. From the measured pressure at known elevation, along with the temperature and dew point, the ASOS software computes: station pressure, pressure tendency, altimeter setting, sea level pressure, density altitude, and pressure altitude.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The station pressure and altimeter setting are determined from the sensor pressures independently, but they are related as mentioned earlier. Since they are computed independently, you will find times when the equation given does not relate them exactly as they are published. You can find station pressures, altimeter settings, and sea level pressures to practice with and compare at this link</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;"><a href="http://www.wrh.noaa.gov/mesowest/getobext.php?table=1&wfo=lox&sid=KCOS" target="_blank">http://www.wrh.noaa.gov/mesowest/getobext.php?table=1&wfo=lox&sid=KCOS</a></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">by changing the last 4 letters to the Metar of interest.To find the closest Metar to your location you can use <a href="http://www.starpath.com/barometers" target="_blank">www.starpath.com/barometers</a>. To find the specifications of the station (elevation, location, ID, even accuracy!) go to (with correct metar):</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;"><a href="http://www.weather.gov/tg/siteloc.shtml" target="_blank">https://weather.gladstonefamily.net/site/KCMI</a>.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; min-height: 11px; text-align: justify; text-indent: 14.4px;"><br /></p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">The ASOS procedures have simplified the WBAN procedure significantly, and after crunching numbers with the latter procedure for some hours it is easy to appreciate the virtue of this approach. They no longer use mean virtual temperatures or plateau corrections, but instead simply define the sea level pressure as</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center; text-indent: 14.4px;">Psl = Ps <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> r + C,</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">where r is the pressure reduction ratio and C is the pressure reduction constant. A station will use either r or C, but not both. Typically stations below 100 ft would use C, in which case r = 1. C is then basically the ISA correction, perhaps adjusted to some extent for the location. It does not depend on temperature.</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify; text-indent: 14.4px;">Higher stations use r values (C = 0) from a table of values stored in the local ASOS computer that are unique to that station. A sample for KCOS is shown in Table A3-3. Using this table, and Ts = 48°F,</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: center;">Psl = 813.8 <span style="font-family: Arial; font-stretch: normal; line-height: normal;">x</span> 1.2526 = 1019.4 mb,</p>
<p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;">which is obviously easier to obtain than using the WBAN procedure—if we happen to know the official r factors. At least for now, these do not seem to be public information, so the WBAN method is the only guideline for making these reductions at arbitrary locations. Even with that, we must make some estimate of the Plateau Correction based on WBAN values. </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAFit8P8fZTngvcvbOI12ZFLCN79PRNiHRD1A7PK6BIgB0Galv5sydG0d19Us7RSAQEoMDwaWsatzxD3ormfguoqzKaHLZ3D4N1wNkdwjYebAMLALtUYvJ9GzgI9jHgiCIYvjGH3V1v2ReW5Rnbf2iah_mCy98rQwB_dM-nZ1WH1bxLDhYqWydL5UeQow/s922/Table%20A3-3.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="922" data-original-width="758" height="473" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAFit8P8fZTngvcvbOI12ZFLCN79PRNiHRD1A7PK6BIgB0Galv5sydG0d19Us7RSAQEoMDwaWsatzxD3ormfguoqzKaHLZ3D4N1wNkdwjYebAMLALtUYvJ9GzgI9jHgiCIYvjGH3V1v2ReW5Rnbf2iah_mCy98rQwB_dM-nZ1WH1bxLDhYqWydL5UeQow/w389-h473/Table%20A3-3.png" width="389" /></a></div><br /><p style="font-family: Georgia; font-stretch: normal; line-height: normal; margin: 0px 0px 3.6px; text-align: justify;"><br /></p><div>See also our related note where we show it is important to use the 12-hr average temperature.</div><div><br /></div><div style="text-align: center;"><a href="https://davidburchnavigation.blogspot.com/2013/10/air-temperature-dependence-of-sea-level.html" target="_blank">Air temperature dependence of sea level pressure conversions.</a></div><div style="text-align: center;"><br /></div><div style="text-align: center;"><br /></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-8403709180147146052023-10-02T15:27:00.003-07:002024-02-16T12:26:23.045-08:00Measure the Eye Relief of a Sextant Scope<p>The question came up today of the <i>eye relief</i> of the 7 x 35 monocular we sell for taking sun sights with more precision—it is also valuable for more accurate index correction measurements. Eye relief is the <br />distance from the eye to the front face of the lens in the sextant eye piece. With the eye at that distance from the lens, we see the full view of the telescope without distortion. Closer or farther offers some distortion around the fringe of the view. An adequate eye relief is valuable for mariners who must wear glasses when taking a sight. </p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/J0RCqpajMwA" width="320" youtube-src-id="J0RCqpajMwA"></iframe></div><p></p><p><br /></p><p>In principle the manufacturers of the instrument should provide this spec, but we notice that most do not for sextant scopes. Therefore we looked into the procedure for measuring this and report it here. If the published procedure is correct, then this is a fairly easy measurement that can be made to within ± a couple mm.</p><p>Procedure: </p><p>1) Find the <i>exit pupil </i>diameter by dividing the objective lens diameter by the magnification. We have a 7 x 35 scope, so this is 35/7 = 5 mm exit pupil.</p><p>2) draw a circle of this diameter on a paper and shine a light into the objective lens to view it on the paper below, as shown.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEixjv4r04B_s8WXSAVsa7HW14OyJHAk-1BE82Udz9SIO_xvWcTAaTcqMX7ceKpNJRcb8BiZT1qTHlkFSWmXOBe6KtJa1Z6Z0NEIhs0vwhJisynL6dvQ2xgrJvOmm6Tyxx95dfgZAtkAt7Bz7WG2JZH-LdH0Cy6dtJ30CRNadfpl48jEyG10MAiqf2mXCLI/s2146/exit%20pupil%20dia%20of%205%20mm%20.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1938" data-original-width="2146" height="361" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEixjv4r04B_s8WXSAVsa7HW14OyJHAk-1BE82Udz9SIO_xvWcTAaTcqMX7ceKpNJRcb8BiZT1qTHlkFSWmXOBe6KtJa1Z6Z0NEIhs0vwhJisynL6dvQ2xgrJvOmm6Tyxx95dfgZAtkAt7Bz7WG2JZH-LdH0Cy6dtJ30CRNadfpl48jEyG10MAiqf2mXCLI/w400-h361/exit%20pupil%20dia%20of%205%20mm%20.JPG" width="400" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;">There is some distortion in this image which looks like 5.5 mm but it was drawn with needle tip dividers to more exactly 5.</div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiHSeNkUVcSlGOLcBt7F_jMBFPhRW5fRBPoEmpGekIk2dGChTA7wKAGhO4nUsvtmoczsW807bbWGTtGjyxU2M8AJ41HtdnIhyphenhyphenMPzeamg9h8cqvJre3RWcnydqrgSlcxz49mvQvBgCZDz5H5tkSiFruCrw06DKX8gXWVUzMNMta2rwaS6Ee27Ad-yh_L1lU/s4032/ER-setup.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiHSeNkUVcSlGOLcBt7F_jMBFPhRW5fRBPoEmpGekIk2dGChTA7wKAGhO4nUsvtmoczsW807bbWGTtGjyxU2M8AJ41HtdnIhyphenhyphenMPzeamg9h8cqvJre3RWcnydqrgSlcxz49mvQvBgCZDz5H5tkSiFruCrw06DKX8gXWVUzMNMta2rwaS6Ee27Ad-yh_L1lU/w480-h640/ER-setup.JPG" width="480" /></a></div></div><br /><p>Here a light shines down through the scope making a bright pattern, then the scope is moved up and down until the image just matches the exit pupil.</p><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhWoYHKeigrOBD3KmCzWgvy7GaDfJlFT9bV8yJO83ZCRurb9OhUZpyfEllQxU3PkN8D_XIpdafJnAX4XNTZo9_SVQVi9UDGl8HkJQ3K29PcvHSBqnSSxCrdbHimR1D4s8GPYBy4alCU0gznpXT21mvxL14XCEA1hS1ZtG-ZBEqvcmLFnllrwwLcblz0WXI/s4032/ER-just%20before%20alignment.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="3024" data-original-width="4032" height="480" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhWoYHKeigrOBD3KmCzWgvy7GaDfJlFT9bV8yJO83ZCRurb9OhUZpyfEllQxU3PkN8D_XIpdafJnAX4XNTZo9_SVQVi9UDGl8HkJQ3K29PcvHSBqnSSxCrdbHimR1D4s8GPYBy4alCU0gznpXT21mvxL14XCEA1hS1ZtG-ZBEqvcmLFnllrwwLcblz0WXI/w640-h480/ER-just%20before%20alignment.JPG" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;">Here is the view just before alignment. The scope has to go down slightly to make the light pattern match the exit pupil diameter ring.</div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3CDNuPuv_svQY80BcJRriwAoOFt24S_DNPr1QK4ooq_GR152M45aHIPDmvz05q1zxMIPRItnBmH7jrkEM-f_mR88zLYx22KiQ5BNEZbeExoAKxON2Ompiq5TZeygofXk9AWmLwWmDzH0rHalr4NlaFEQ0ofkTDgC2UcXmxIt4gVnF5KBr0MkflpEChXw/s4032/ER-%20height%20above%20eye%20piece.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi3CDNuPuv_svQY80BcJRriwAoOFt24S_DNPr1QK4ooq_GR152M45aHIPDmvz05q1zxMIPRItnBmH7jrkEM-f_mR88zLYx22KiQ5BNEZbeExoAKxON2Ompiq5TZeygofXk9AWmLwWmDzH0rHalr4NlaFEQ0ofkTDgC2UcXmxIt4gVnF5KBr0MkflpEChXw/w480-h640/ER-%20height%20above%20eye%20piece.JPG" width="480" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;">Once the light pattern is aligned with the ring, we measure the distance from eye piece to paper</div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjyYax69dcKDgtb3z5heo9uGBpm2lIEIzdpgcTnjmjQZhyphenhyphensqA3eOa09ZVwhyO177YK6m1IBtSyOFi-GgwpWMupr_AcwRoRCRj54CVVtKFRKMG07rBsjQmhdpiYrwCr7lIf0rYMoRKpxhAjq9fynwVg0_FU-XP9QDAe56y0TXDNTpzXkuAri12qDoWU12Ao/s4032/ER%20height%20measured.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjyYax69dcKDgtb3z5heo9uGBpm2lIEIzdpgcTnjmjQZhyphenhyphensqA3eOa09ZVwhyO177YK6m1IBtSyOFi-GgwpWMupr_AcwRoRCRj54CVVtKFRKMG07rBsjQmhdpiYrwCr7lIf0rYMoRKpxhAjq9fynwVg0_FU-XP9QDAe56y0TXDNTpzXkuAri12qDoWU12Ao/w480-h640/ER%20height%20measured.JPG" width="480" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;">This turned out to be just under 10 mm. </div></div><br /><p>Next we measure the depth of the lens inside the eye piece using two crossed tongue depressors.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizsob2st7bwjVL83-eIPAan0neTOYdhMInujfbzWhLxXEHNtEAVtLrAYb6FmcVcmApCDrskXumcA9BC3gn35v7N7ilUNmCcPmH9TsSnmUbuzotiUWUMEDGVjwb50TPQ4vLw4ygi1ft4QLcL72jufEQ29Gjrt7OZrG0vjz44PatA9yJNt2ww6vrP2FZABE/s4032/lens%20depth%20inside%20eye%20piece.JPG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4032" data-original-width="3024" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEizsob2st7bwjVL83-eIPAan0neTOYdhMInujfbzWhLxXEHNtEAVtLrAYb6FmcVcmApCDrskXumcA9BC3gn35v7N7ilUNmCcPmH9TsSnmUbuzotiUWUMEDGVjwb50TPQ4vLw4ygi1ft4QLcL72jufEQ29Gjrt7OZrG0vjz44PatA9yJNt2ww6vrP2FZABE/w480-h640/lens%20depth%20inside%20eye%20piece.JPG" width="480" /></a></div><br /><p>This is very close to 7 mm.</p><p>Thus the total distance from eye piece lens to eye is 10 = 7 = 17 mm, which is the eye relief of this instrument.</p><p>In short when using this monocular, you would want the surface of your eye to be about 1 cm away from the lip of the eye piece, which seems about what it is when using this scope, which typically calls for pressing the eye against a large eyecup placed on the eyepiece. At sea we need that extra point of stability plus no light in from the side.</p><p>This might be as good a place as any to note that we have known mariners to make a custom set of glasses for taking sights. They remove the lens and frame on the sextant eye so it can press up against the cut and leave a lens on the other side for seeing around them.</p><p><br /></p><p><br /></p><p><br /></p>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-14188307597166553552023-10-02T13:11:00.001-07:002023-10-04T13:19:27.654-07:00Loading Surface Analysis Graphic Maps into qtVlm<p><span style="font-family: georgia;"> After spending some time trying to build a KML file that would reproduce OPC graphic maps in qtVlm, I have had to give up for a while on this. We used it successfully for the small (~150 nmi square) regions for presenting ASCAT data, but for large areas, it seems the KML format will not inherently reproduce a mercator projection. So that explains why, for now, we are not doing this with KML files which would in principle bring the map link and the georeferencing info into qtVlm in one easy step.</span></p><p><span style="font-family: georgia;">Instead, we have a couple of relatively easy steps that must be done just once, and then they are saved for later use.</span></p><p><span style="font-family: georgia;">Here we want to automatically load the latest Atlantic surface analysis georeferenced into qtVlm, so it looks like this:</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCFziw67IIkTQEAxp4XH88H6BJFSwTqbv0OYVk5zwmLD4cB88qRq0Hsl4EqQM3JQdpLjnakq5kPdSYKhGtuZWrvp4XupX5t4IAUqRJeXQEe83MFoCScK0HmsDwacPnmUeujCTpMDMv-KykN9Y9Cdd4-PC13-FwMp0DJq_dh3M-H2Y2mpFX_Bz1U1PG2zI/s2056/atlanic%20map.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: georgia;"><img border="0" data-original-height="1396" data-original-width="2056" height="434" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiCFziw67IIkTQEAxp4XH88H6BJFSwTqbv0OYVk5zwmLD4cB88qRq0Hsl4EqQM3JQdpLjnakq5kPdSYKhGtuZWrvp4XupX5t4IAUqRJeXQEe83MFoCScK0HmsDwacPnmUeujCTpMDMv-KykN9Y9Cdd4-PC13-FwMp0DJq_dh3M-H2Y2mpFX_Bz1U1PG2zI/w640-h434/atlanic%20map.png" width="640" /></span></a></div><p><span style="font-family: georgia;">To do this use menu Gribs/Weather images/Open a weather image and then choose a tab (1 to 8) that looks like this.</span></p><p><span style="font-family: georgia;">In the "File name" field type this link: https://ocean.weather.gov/A_sfc_full_ocean_color.png</span></p><p></p><p></p><p><span style="font-family: georgia;">and then set the other parameters as shown below.</span></p><span style="font-family: georgia;"><br /></span><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSlhbKbScqUa7FzG4Uc9dwAkSaQLqYqajOMx0Y12p28Kn_VxAHHsR2TDnIylqND1cHwFtWB1P6FOvZoHIdJwCBMUixOSFbCVuHqEb0-mP2vIUk7Amy6yyP_Qw6AWHbgXOd-JjZuhoA2ktEQqQhI_vGopktG4r1FKQXHOQhyphenhyphenf98lwf5bxChPhVqrt7k_aE/s926/atlantic%20set%20up.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: georgia;"><img border="0" data-original-height="926" data-original-width="884" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiSlhbKbScqUa7FzG4Uc9dwAkSaQLqYqajOMx0Y12p28Kn_VxAHHsR2TDnIylqND1cHwFtWB1P6FOvZoHIdJwCBMUixOSFbCVuHqEb0-mP2vIUk7Amy6yyP_Qw6AWHbgXOd-JjZuhoA2ktEQqQhI_vGopktG4r1FKQXHOQhyphenhyphenf98lwf5bxChPhVqrt7k_aE/w381-h400/atlantic%20set%20up.png" width="381" /></span></a></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;">_____________________________________________________</span></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: georgia;">To do this for the Pacific to look like this:</span></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPseOfIUc2Ky4zrVqt9pimSN7bCMmvbaSpkwOcwQfHprQRB_EsTB1_G-36XuoohEaOckKGTwSgImIYAeXiw-0yWfSV6Y7vxXPyGY-BAOc5wry4aaZI2jPUl3AVow7XeJQWgxVTBjbOFk8ZD0MU2Oh0_5hQXGzPUyjRO-scPjjIQs_Lft7sDdtnvoG0iLU/s2478/pacific%20map.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: georgia;"><img border="0" data-original-height="1598" data-original-width="2478" height="412" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiPseOfIUc2Ky4zrVqt9pimSN7bCMmvbaSpkwOcwQfHprQRB_EsTB1_G-36XuoohEaOckKGTwSgImIYAeXiw-0yWfSV6Y7vxXPyGY-BAOc5wry4aaZI2jPUl3AVow7XeJQWgxVTBjbOFk8ZD0MU2Oh0_5hQXGzPUyjRO-scPjjIQs_Lft7sDdtnvoG0iLU/w640-h412/pacific%20map.png" width="640" /></span></a></div><p><span style="font-family: georgia;">Choose another tab (2 in this example)</span></p><p></p><p><span style="font-family: georgia;">In the "File name" field type this link: https://ocean.weather.gov/P_sfc_full_ocean_color.png</span></p><p><span style="font-family: georgia;">Note the file name is the same except for P for Pacific vs A for Atlantic </span></p><p></p><p></p><p></p><p></p><p></p><p></p><p><span style="font-family: georgia;">and then set the other parameters as shown below, which are also the same except for the location of the top left corner coordinates.</span></p><p></p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqstXdAg4LZDY6n0-cV37p0gxxJpZ0FROwEFCL8OD_ihga13HzW_nQf6bUE_LGMVhUN_c-lxklC9CPJEFAtmhvv7LCdtfavd2bJUYiKru6ahpRVSFHWbGKo-jHP8hIHrQXxdeb2ea6HQQ-apRguM-jJrxRCTQlfbfHxuqeYoJ4gO7I7yfybAFkMbtDgwE/s974/pacific%20set%20up.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: georgia;"><img border="0" data-original-height="974" data-original-width="886" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqstXdAg4LZDY6n0-cV37p0gxxJpZ0FROwEFCL8OD_ihga13HzW_nQf6bUE_LGMVhUN_c-lxklC9CPJEFAtmhvv7LCdtfavd2bJUYiKru6ahpRVSFHWbGKo-jHP8hIHrQXxdeb2ea6HQQ-apRguM-jJrxRCTQlfbfHxuqeYoJ4gO7I7yfybAFkMbtDgwE/w364-h400/pacific%20set%20up.png" width="364" /></span></a></div><span style="font-family: georgia;"><br /></span><div class="separator" style="clear: both; text-align: center;"><span style="font-family: georgia;">_____________________________________________________</span></div><span style="font-family: georgia;"><br /></span>With one of these images in view, which are vetted by professional human meteorologists, you can over lay a numerical weather model forecast at the same time to evaluate it since it has not been vetted by humans.<p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzWWlIvch51ZM3UMyzBKPyZ0mhpeF7LRx4OtZcDxc3QDtIENBvsPDWdAVsfGpB3NnB5rOXNhqPWZR-4PGviEaQOqnDBkW4tAZPElHEQi1UL9LdDww0YM07Y0KJHo66TNxL17HrkB4lNOXK-nOWHNolWBOq-p-NMGGFsK7llBm-LQDlDyMNIU0VloWa-7w/s2858/gfs%20overlay.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1652" data-original-width="2858" height="370" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjzWWlIvch51ZM3UMyzBKPyZ0mhpeF7LRx4OtZcDxc3QDtIENBvsPDWdAVsfGpB3NnB5rOXNhqPWZR-4PGviEaQOqnDBkW4tAZPElHEQi1UL9LdDww0YM07Y0KJHo66TNxL17HrkB4lNOXK-nOWHNolWBOq-p-NMGGFsK7llBm-LQDlDyMNIU0VloWa-7w/w640-h370/gfs%20overlay.png" width="640" /></a></div><br /><p>In this case we see near perfect agreement that the OPC maps are more and more looking just like the 00h forecast of the GFS, meaning in large part that the GFS is pretty good. But despite not learning more about the lay of the isobars and hence winds, we do see from the maps where the fronts are located; fronts are not shown in the GFS model outputs we get in grib format.</p><p><br /></p><p><br /></p>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-62296194711454314162023-08-20T13:01:00.004-07:002023-08-20T15:10:50.010-07:00Viewing ASCAT Wind Measurements in OpenCPN<p><i><span style="font-family: inherit;">This article outlines the value of ASCAT wind data, and shows how work we have done at Starpath to make these files available on Google Earth can be transcribed to the format needed to be viewed in OpenCPN. This is effectively a request to OpenCPN developers to automate this conversion process and make the full set of ASCAT data files and then incorporate them into the standard weatherfax plugin.</span></i></p><p><i><span style="font-family: inherit;">_______</span></i></p><p><span style="font-family: inherit;">ASCAT is the name of the scatterometer on two EUMETSAT satellites, Metop-B and Metop-C. They circle the earth in sun-synchronous polar orbits every 1h 41m (101.3m) measuring ocean surface wind speed and direction. They are in the same orbit, but on opposite sides, so they are about 50 min apart, during which the earth rotates 50 min x (15º lon/60 min), or 12.5º of Lon. Thus the data from C is about 50 min later and covers a swath of the earth that is 12.5º of Lon farther west—or vice versa, thinking of B following C. We have extended background on ASCAT at </span><a href="https://www.starpath.com/ascat" target="_blank">starpath.com/ascat.</a></p><p><span style="font-family: inherit;">These direct wind measurements are the key information needed for weather routing, with a primary use being to evaluate the numerical weather model forecasts. The data are like having thousands of buoys at sea measuring the wind speed and direction, and if a model is to be dependable it should closely match what we see in the buoy measurements. Also we will periodically see real holes in the wind, or wind shear lines in the ASCAT measurements that are not forecasted in any model. The resolution of the ASCAT data (25 km) is about the same as that of the GFS model (27 km). The expected accuracy of the satellite measurements and that of the model forecasts is about the same, so very roughly ± 2 kts in wind speed and ± 20º in wind direction has to be considered in agreement. </span></p><p><span style="font-family: inherit;">Remember that the final goal of any numerical weather model is not to reproduce all of the specific observations that went into the computation, but rather to create the best overall forecast at all levels of the atmosphere, which almost always involves some compromise in matching the surface data. Thus, even though the ASCAT measurements are key data assimilated into any global model computation, we should not be surprised that we can learn real and significant discrepancies in the model forecast by looking at the same ASCAT data it was looking at, and when these do disagree, it is the measurements that are of course the correct answer.</span></p><p><span style="font-family: inherit;">ASCAT wind data are available in grib format, but only from two commercial sources, <a href="http://www.luckgrib.com" target="_blank">LuckGrib</a> and <a href="http://www.expeditionmarine.com" target="_blank">Expedition</a>, and their data cannot be viewed in other nav programs. This important data, however, are also readily available in graphic format, and OpenCPN is well suite to displaying this data using the powerful WeatherFax plugin—and the process of setting that up is the topic at hand.</span></p><p><span style="font-family: inherit;">We have an important background article on this process <a href="http://davidburchnavigation.blogspot.com/2021/01/updating-internet-file-source-for.html" target="_blank">Updating Internet File Source for OpenCPN WeatherFax Plugin</a>, which pretty much describes the process in general, but here we need to be more specific on how we generalize the ASCAT data.</span></p><p><span style="font-family: inherit;">We have created two graphic indexes of the files available, one for adjacent US waters the other for adjacent European waters. </span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEivpVJhP1N1faa4bu2a9PsrwPHJ2S_KVBHY3U8AIsb5XZ9ljtWoTSNx2VFQMldF5MUhOM-m_Obnq09jsHE7TAr9toAYWzLOtTAanvrLvuqHtnPyaOt2Y0AaBRzMTtfwZiEES9LDJsMcEH9Iv9kPiVMgcHnIGILgi26ulNSSGtXFwexFT--n1DNL15YoezU/s1465/North%20Ameria%20ASCAT%20Index.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"><img border="0" data-original-height="754" data-original-width="1465" height="330" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEivpVJhP1N1faa4bu2a9PsrwPHJ2S_KVBHY3U8AIsb5XZ9ljtWoTSNx2VFQMldF5MUhOM-m_Obnq09jsHE7TAr9toAYWzLOtTAanvrLvuqHtnPyaOt2Y0AaBRzMTtfwZiEES9LDJsMcEH9Iv9kPiVMgcHnIGILgi26ulNSSGtXFwexFT--n1DNL15YoezU/w640-h330/North%20Ameria%20ASCAT%20Index.png" width="640" /></span></a></div><span style="font-family: inherit;"><br /></span><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAa-PEGqby96K9PiuryVmzHEzLNlIo0TUQnCI2wHIjTBaqXYSCOtLIAKGt33c30h_hYSM3r88NvKg5sP_Tv4CN22--NdRv8oq2HR3MciIiXDwWMQDvOfSm5Y78CvyFeKAYyVOi6RSr8Md2n-5XEMr3amfzy0JtHbLPl__rIoyPxrXaXvXnFSUF_tvJChA/s1375/Europe%20ASCAT%20Index.jpg" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"><img border="0" data-original-height="1375" data-original-width="1276" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjAa-PEGqby96K9PiuryVmzHEzLNlIo0TUQnCI2wHIjTBaqXYSCOtLIAKGt33c30h_hYSM3r88NvKg5sP_Tv4CN22--NdRv8oq2HR3MciIiXDwWMQDvOfSm5Y78CvyFeKAYyVOi6RSr8Md2n-5XEMr3amfzy0JtHbLPl__rIoyPxrXaXvXnFSUF_tvJChA/w594-h640/Europe%20ASCAT%20Index.jpg" width="594" /></span></a></div><span style="font-family: inherit;"><br /></span><div class="separator" style="clear: both; text-align: center;"><span style="font-family: inherit;"><br /></span></div><span style="font-family: inherit;">Each of these regions that we have named have four ASCAT files, ascending (satellite moving north, data swaths tilt to the east and descending (satellite moving south, data swaths tilt to the west), one for Metop-B and one for Metop-C.</span><div><span style="font-family: inherit;"><br /></span></div><div><span style="font-family: inherit;">Here are the 4 examples for what we call the San Francisco region.</span></div><div><blockquote style="background-color: #ccffff;"><blockquote><p><span style="font-family: inherit;"><a href="https://www.starpath.com/ascat/San%20Francisco%20ASCAT%20B%20-%20Ascending.kml" target="_blank">San Francisco ASCAT B - Ascending.kml</a> </span></p><p><a href="https://www.starpath.com/ascat/San%20Francisco%20ASCAT%20B%20-%20Descending.kml" target="_blank"><span style="font-family: inherit;">San Francisco ASCAT B - Descending.kml</span></a></p><p><span style="font-family: inherit;"><a href="https://www.starpath.com/ascat/San%20Francisco%20ASCAT%20C%20-%20Ascending.kml" target="_blank">San Francisco ASCAT C - Ascending.kml</a> </span></p><p><span style="font-family: inherit;"><a href="https://www.starpath.com/ascat/San%20Francisco%20ASCAT%20C%20-%20Descending.kml" target="_blank">San Francisco ASCAT C - Descending.kml</a> </span></p></blockquote></blockquote><p><span style="font-family: inherit;">You can download anyone or all and drag onto Google Earth (desktop version) to see the latest ASCAT data in that region, defined above. Samples are below (<i>click it, then right-click, open in new tab, and zoom for detailed view</i>).</span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgIRcI7gNzjfPlCK1oO5ZSb8XdtskRo8Cz16aeeLiG78X8fT4vaOJM-xdhl7Dg42XggLhcPWZMVdh8thVqmybsaeMoYbdYB1nfdRth0-WLZ2PDNIqMjC_6eAh5kkNMYhircFrjmnyqRLG9HYYgKDUnFWADWS4cQ5X18vj5IEl0Uf2-UzblwO171flExAag/s3160/SF-combo.png" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"><img border="0" data-original-height="728" data-original-width="3160" height="148" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgIRcI7gNzjfPlCK1oO5ZSb8XdtskRo8Cz16aeeLiG78X8fT4vaOJM-xdhl7Dg42XggLhcPWZMVdh8thVqmybsaeMoYbdYB1nfdRth0-WLZ2PDNIqMjC_6eAh5kkNMYhircFrjmnyqRLG9HYYgKDUnFWADWS4cQ5X18vj5IEl0Uf2-UzblwO171flExAag/w640-h148/SF-combo.png" width="640" /></span></a></div><div><span style="font-family: inherit;"><br /></span></div><span style="font-family: inherit;">The times shown in our indexes tell us when new data are expected, which will come in pairs about 50 min apart, with the pairs separated aby about 13 hr. The times are the valid times of satellite passage, ± 1 hr, but we must wait about 2 hr for the data to be analyzed and made available. Thus in Biscay, we would expect see new data at about 1230 and 2320 UTC, adding say 30 min to save on air time by asking too early. Once you have this set up in OpenCPN or Google Earth you will quickly learn how it works. Video examples are listed at <a href="http://starpath.com/ascat.">starpath.com/ascat.</a></span></div><div><span style="font-family: inherit;"><br /></span></div><div><span style="font-family: inherit;">With that background, we now get to the process of how to convert what we have provided to work with OpenCPN using the weatherfax plugin, which has one of the most convenient displays of graphic weather data of any navigation program.</span></div><div><span style="font-family: inherit;"><br /></span></div><div><span style="font-family: inherit;">We have videos on how images are displayed in the weathefax plugin, and as the link above explains OpenCPN stores the data needed for quick display of any image in two files. One provides the link info stating where the data is online and the other specifies the georeferencing coordinate data so the images are displayed in the right place.</span></div><div><span style="font-family: inherit;"><br /></span></div><div><span style="font-family: inherit;">The data links are provided in a series of about a dozen XML files called, for example, </span></div><div><span style="font-family: inherit;"><br /></span></div><div><span style="font-family: inherit;">WeatherFaxInternetRetrieval_NAVY.xml, which looks like</span></div><div><span style="font-family: inherit;"><br /></span></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><?xml version=</b><span class="s1" style="color: #c41a16;">"1.0"</span><b> encoding=</b><span class="s1" style="color: #c41a16;">"utf-8"</span><b> ?></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><OCPNWeatherFaxInternetRetrieval></b></p><p class="p2" style="background-color: white; color: #0e0eff; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Server </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"NAVY"</span><span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>https://www.ncei.noaa.gov/jag/navy/data/satellite_analysis/<span class="s1" style="color: #c41a16;">"</span><span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Region </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Gulf Stream"<span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p4" style="background-color: white; color: #5d6c79; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><!-- Gulf Stream charts --></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"gsncofa.gif"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"North Altantic"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS1"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"gsscofa.gif"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Golf of Mexico"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS2"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"gsneofa.gif"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Coast Guard North Atlantic"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS3"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS1"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"30N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"53N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"80W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"45W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS2"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"17N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"40N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"98W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"65W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"GS3"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"30N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"60N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"80W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"35W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Region></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Server></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></OCPNWeatherFaxInternetRetrieval></b></p></blockquote><div><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">These files are located on a PC in:</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">C:\Users\username\AppData\Local\opencpn\plugins\weatherfax_pi\data</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">on a Mac, they are located in:</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;">HD\Users\username\Library\Application Support\OpenCPN\Contents\SharedSupport\plugins\weatherfax_pi\data\</span></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;"><br /></span></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;">We also need to work on the coordinates file which has to include an element for each "server name."</span></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;">This file is called CoordinateSets.xml Below we see the section that covers the NAVY server.</span></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;"><br /></span></span></p><p class="p1" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s1" color="rgba(0, 0, 0, 0.85)"> </span><span class="s2" style="color: #9b2393;"><b><Coordinate </b></span><span class="s3" style="color: #815f03;">Name</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"NAVY - Gulf Stream - GS3"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"304"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"60"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"59.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-76.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"2122"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1285"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"32.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-40.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Mapping</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"FixedFlat"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputPoleY</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-1515"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputEquator</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"3031.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputTrueRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingMultiplier</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s1" color="rgba(0, 0, 0, 0.85)"> </span><span class="s2" style="color: #9b2393;"><b><Coordinate </b></span><span class="s3" style="color: #815f03;">Name</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"NAVY - Gulf Stream - GS1"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"101"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"28"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"53.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-80.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"2375"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"927"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"32.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-45.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Mapping</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"FixedFlat"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputPoleY</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-2365"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputEquator</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"3475.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputTrueRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingMultiplier</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s1" color="rgba(0, 0, 0, 0.85)"> </span><span class="s2" style="color: #9b2393;"><b><Coordinate </b></span><span class="s3" style="color: #815f03;">Name</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"NAVY - Gulf Stream - GS2"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"376"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"82"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"38.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon1</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-94.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">X2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"2236"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Y2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"746"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lat2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"20.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Lon2</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-67.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">Mapping</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"FixedFlat"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputPoleY</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"-3444"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputEquator</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"2755.00000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">InputTrueRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingMultiplier</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">MappingRatio</span><span class="s2" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s2" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="background-color: white; color: #333333;"><span style="font-family: inherit;"></span></span></p><p class="p3" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><br /></p></div><div>So what we need for OpenCPN is a new set of Retrieval servers and we need new Coordinate Name entries for each of the custom regions we have defined, and these are likely best duplicated, one for Metop-B and one for Metop-C, which makes access to the data a bit easier. </div><div><br /></div><div>The new WeatherFaxInternetRetrieval servers would look something like this which covers just two regions. The other 46 would have to be entered into </div><div><br /></div><div><br /></div><div><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><?xml version=</b><span class="s1" style="color: #c41a16;">"1.0"</span><b> encoding=</b><span class="s1" style="color: #c41a16;">"utf-8"</span><b> ?></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><OCPNWeatherFaxInternetRetrieval></b></p><p class="p2" style="background-color: white; color: #0e0eff; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s3" style="color: #9b2393;"><b><Server </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"ASCAT B"</span><span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_METB/zooms/<span class="s1" style="color: #c41a16;">"</span><span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Region </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Cape Cod ASCAT B"<span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-converted-space"> </span></span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBas85.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-B ascending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-converted-space"> </span></span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBds85.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-B descending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"38.889N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"51.082N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"75.183W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"59.822W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Region></b></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Region </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Bermuda ASCAT B"<span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBas86.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-B ascending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBds86.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-B descending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"28.9871N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"40.810N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"75.1843W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"59.8349W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Region></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Server></b></p><p class="p2" style="background-color: white; color: #0e0eff; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Server </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"ASCAT C"</span><span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_METC/zooms/<span class="s1" style="color: #c41a16;">"</span><span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Region </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Cape Cod ASCAT C"<span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBas85.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-C ascending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBds85.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-C descending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"85"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"38.889N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"51.082N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"75.183W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"59.822W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Region></b></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Region </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Bermuda ASCAT C"<span class="s3" style="color: #9b2393;"><b>></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBas86.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-C ascending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Map </b></span><span class="s4" style="color: #815f03;">Url</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"WMBds86.png"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Contents</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"Metop-C descending"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">Area</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p3" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><span class="s3" style="color: #9b2393;"><b><Area </b></span><span class="s4" style="color: #815f03;">Name</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"86"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"28.9871N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lat2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"40.810N"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon1</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"75.1843W"<span class="s3" style="color: #9b2393;"><b> </b></span><span class="s4" style="color: #815f03;">lon2</span><span class="s3" style="color: #9b2393;"><b>=</b></span>"59.8349W"<span class="s3" style="color: #9b2393;"><b> /></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Region></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" color="rgba(0, 0, 0, 0.85)"> </span><b></Server></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b></OCPNWeatherFaxInternetRetrieval></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><br /></b></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">and here are the coordinates we need to add to the coordinates file:</span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; text-align: left;"><b><br /></b></p></div><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b><Coordinate </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Name</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"ASCAT B - Cape Cod ASCAT B - 85"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"650"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"38.889"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-75.183"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"740"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"51.083"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-59.822"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Mapping</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"Mercator"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingMultiplier</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.0000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingRatio</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> /></b></span><br /><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b><br /></b></span><div><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b><Coordinate </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Name</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"ASCAT B - Bermuda ASCAT B - 86"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"650"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"28.987"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-75.184"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"740"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"40.810"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-59.835"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Mapping</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"Mercator"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingMultiplier</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.0000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingRatio</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.0000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> /></b></span><br /><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b><br /></b></span></div><div><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b><Coordinate </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Name</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"ASCAT C - Cape Cod ASCAT C - 85"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"650"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"38.889"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon1</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-75.183"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">X2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"740"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Y2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"0"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lat2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"51.082"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Lon2</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"-59.822"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">Mapping</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"Mercator"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingMultiplier</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.0000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> </b></span><span class="s2" style="color: #815f03; font-family: Menlo; font-size: 12px;">MappingRatio</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b>=</b></span><span style="background-color: white; color: #c41a16; font-family: Menlo; font-size: 12px;">"1.000"</span><span class="s1" style="color: #9b2393; font-family: Menlo; font-size: 12px;"><b> /></b></span><br /><div style="text-align: left;"><p class="p1" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s1" style="color: #9b2393;"><b><br /></b></span></p><p class="p1" style="background-color: white; color: #c41a16; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s1" style="color: #9b2393;"><b><Coordinate </b></span><span class="s2" style="color: #815f03;">Name</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"ASCAT C - Bermuda ASCAT C - 86"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">X1</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"0"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Y1</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"650"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Lat1</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"28.987"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Lon1</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"-75.184"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">X2</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"740"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Y2</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"0"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Lat2</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"40.810"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Lon2</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"-59.835"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">Mapping</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"Mercator"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">MappingMultiplier</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s1" style="color: #9b2393;"><b> </b></span><span class="s2" style="color: #815f03;">MappingRatio</span><span class="s1" style="color: #9b2393;"><b>=</b></span>"1.0000"<span class="s1" style="color: #9b2393;"><b> /></b></span></p></div><div><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><br /></b></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">The programmer can get these values from the dimensions of our file size, which is always the same at 640 height and 740 width, using the data we provide in our KML files, which in these two cases are: </span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><br /></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><?xml version=</b><span class="s1" style="color: #c41a16;">"1.0"</span><b> encoding=</b><span class="s1" style="color: #c41a16;">"UTF-8"</span><b>?></b></p><p class="p2" style="background-color: white; color: #0e0eff; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" style="color: #9b2393;"><b><kml </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.opengis.net/kml/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">gx</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.google.com/kml/ext/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">kml</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.opengis.net/kml/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">atom</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.w3.org/2005/Atom<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b>></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><GroundOverlay></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><name></b></span>Bermuda ASCAT B Ascending<span class="s2" style="color: #9b2393;"><b></name></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><color></b><span class="s4" color="rgba(0, 0, 0, 0.85)">7affffff</span><b></color></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><Icon></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><href></b></span>https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_METB/zooms/WMBas86.png?<span class="s2" style="color: #9b2393;"><b></href></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><viewBoundScale></b><span class="s4" color="rgba(0, 0, 0, 0.85)">0.75</span><b></viewBoundScale></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b></Icon></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><LatLonBox></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><north></b></span>40.81048097146506<span class="s2" style="color: #9b2393;"><b></north></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><south></b></span>28.98713385708333<span class="s2" style="color: #9b2393;"><b></south></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><east></b></span>-59.83499451605183<span class="s2" style="color: #9b2393;"><b></east></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><west></b></span>-75.184380929696<span class="s2" style="color: #9b2393;"><b></west></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b></LatLonBox></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b></GroundOverlay></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b></kml></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><br /></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><br /></b></p></div><div><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><?xml version=</b><span class="s1" style="color: #c41a16;">"1.0"</span><b> encoding=</b><span class="s1" style="color: #c41a16;">"UTF-8"</span><b>?></b></p><p class="p2" style="background-color: white; color: #0e0eff; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s2" style="color: #9b2393;"><b><kml </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.opengis.net/kml/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">gx</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.google.com/kml/ext/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">kml</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.opengis.net/kml/2.2<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b> </b></span><span class="s3" style="color: #815f03;">xmlns</span><span class="s2" style="color: #9b2393;"><b>:</b></span><span class="s3" style="color: #815f03;">atom</span><span class="s2" style="color: #9b2393;"><b>=</b></span><span class="s1" style="color: #c41a16;">"</span>http://www.w3.org/2005/Atom<span class="s1" style="color: #c41a16;">"</span><span class="s2" style="color: #9b2393;"><b>></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b><GroundOverlay></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><name></b></span>Cape Cod ASCAT B Ascending<span class="s2" style="color: #9b2393;"><b></name></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><color></b><span class="s4" color="rgba(0, 0, 0, 0.85)">7affffff</span><b></color></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><Icon></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><href></b></span>https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_METB/zooms/WMBas85.png?<span class="s2" style="color: #9b2393;"><b></href></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><viewBoundScale></b><span class="s4" color="rgba(0, 0, 0, 0.85)">0.75</span><b></viewBoundScale></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b></Icon></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b><LatLonBox></b></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><north></b></span>51.08285287270985<span class="s2" style="color: #9b2393;"><b></north></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><south></b></span>38.88930541210142<span class="s2" style="color: #9b2393;"><b></south></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><east></b></span>-59.82222792454112<span class="s2" style="color: #9b2393;"><b></east></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><west></b></span>-75.18387376026712<span class="s2" style="color: #9b2393;"><b></west></b></span></p><p class="p3" style="background-color: white; color: rgba(0, 0, 0, 0.85); font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="s2" style="color: #9b2393;"><b><rotation></b></span>-0.5009419918060303<span class="s2" style="color: #9b2393;"><b></rotation></b></span></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span class="s4" color="rgba(0, 0, 0, 0.85)"><span class="Apple-tab-span" style="white-space: pre;"> </span></span><b></LatLonBox></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b></GroundOverlay></b></p><p class="p1" style="background-color: white; color: #9b2393; font-family: Menlo; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-size: 12px; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><b></kml></b></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Note that ascending and descending for both B and C are the same dimensions, although the files have different, systematic names. The file number changes, here 85 vs 86, and the term METB changes to METC, plus the ascending vs descending changes from WMBas85 to WMBds85.</span></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Here is what we see when we append the new ASCAT servers into the NAVY retrieval file and also append into the stock coordinates file the four listed above:</span></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/-T9W2VoSt3Q" width="320" youtube-src-id="-T9W2VoSt3Q"></iframe></div><div style="text-align: center;"><i>Viewing ASCAT winds in OpenCPN</i></div><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; text-align: center;"><br /></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;">My guess is that one of the talented developers that contribute to OpenCPN could <a href="https://www.starpath.com/ascat/Starpath_Google_Earth_ASCAT_Files.zip" target="_blank">download all 48 of our ASCAT regions</a> and then write custom code to open each one and generate the right retrieval and coordinate statements to make it work for all of this data. I also guess that this would not take too long. We have spent a large amount of time in creating what we have, with the hopes that the rest will go very fast.</p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="background-color: white; font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;">In the meantime, if someone could benefit from this data before that happens, the process can be created manually as we have done here. </p></div><div><br /></div><div><br /></div><div><br /></div><div><br /></div><div><br /><p><br /></p></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-90170875817323675352023-08-17T09:21:00.008-07:002023-08-21T20:52:47.552-07:00Practical Use of ASCAT Winds in Weather Routing<p>ASCAT winds are satellite scatterometer measurements of wind speed and direction on the surface of the ocean and other large bodies of water, normalized to a 10 m height. The data resolution is 25 km, which is comparable to the resolution of typical global weather models available to the public such as GFS (27.8 km) and ECMWF (44 km). New data are available typically four times a day, from various combinations of two satellites, Metop-B and Metop-C, tracking N-NE (ascending) or S-SW (descending), bringing us data from either the port or starboard side of their split data swaths. For background in ASCAT, see <a href="http://www.starpath.com/ascat">www.starpath.com/ascat</a>. There is a video at the end here demonstrating the use of the data.</p><p>Before presenting the specifics of how to use this data, let me back up a moment and put this in perspective. Wind speed and direction are the key factors in marine navigation, in large part because the wind makes the waves, which can be a hazard to any size vessel. For sailors, it is even more primary because wind is their engine. Thus this is the most fundamental information available. It is like having the ocean covered with thousands of buoys measuring wind speed and direction. But unlike isolated real ocean buoys who give us data every 10 min in some cases, hourly at worst, the ASCAT data covers large swaths of the ocean but only give us data 3 or 4 times a day. </p><p>In a sense, the ASCAT data have done a major job for us even if we do not look at it, because it is a crucial input to the global weather models whose forecasts we must rely on for routing. But even though the models have assimilated the latest ASCAT data, the model forecasts are not guaranteed to be correct. It is specifically not a goal of the models to reproduce the ASCAT and buoy observations. They have a broader goal to produce the best overall forecast at many levels of the atmosphere. In short, the model forecasts may in fact not be correct in some circumstances, which is why we must compare several models to decide which is likely the best.</p><p>And indeed, circling back to check a model forecast with actual ASCAT measurements at a specific place and time we care about is one of the primary reasons for us to access the ASCAT data ourselves. If one model forecast agrees with the ASCAT measurements and another does not, it is most likely the better one to use for our routing computations</p><p>Another reason to continually monitor the ASCAT wind measurements for our intended route is the occasional observation of localized anomalous flow. We might spot a notable hole in the wind or a shear line that is not apparent in the global model forecast. In these cases, we have to realize that the ASCAT are real data. That is what the wind was doing at that time, regardless of what the model might imply or not show, and we need to route with that in mind. </p><p>The other aspect of "perspective" is to recognize that even though these ASCAT wind measurements are indeed the most fundamental data we care about, the use of this data, which takes several easy, but non-standard procedures, is definitely in the realm of "going the extra mile" to learn the very most we can about the wind ahead of us. For a racing sailor, this is standard operating procedure, but when cruising it would be called on less often, unless we are negotiating a dangerous wind pattern or, more likely, some light air pattern in which we are just looking for enough wind to get there. </p><p>We have presented this perspective in the past, and in earlier editions of our text <a href="https://www.starpath.com/catalog/books/1886.htm" target="_blank"><i>Modern Marine Weather</i></a>, but then after outlining the basic guides to getting the data by standard procedures, we moved on. Based on discussions with practicing navigators, however, it seems that we needed a more specific procedure for accessing this crucial data, and that is what we have created.</p><p>We have semi-empirically compiled graphic indexes of the available data and pass times for the typical cruising and racing waters around North America and Europe, presented below, as well as ways to automatically access the latest available data of interest. (Note that we are not covering here the even more convenient means of obtaining this data in grib format, which can be achieved with <a href="http://www.luckgrib.com" target="_blank">LuckGrib</a> or <a href="http://www.expeditionmarine.com" target="_blank">Expedition</a>. Users of those two fine apps, might still find some benefit from the timing structure we present here.)</p><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhHfmpbvjzdv5FmldtrylaWR8dEOT9XlbMbaHQjbRgXcwJC0wC8GkgmLXPkK3L46N-aAuzvJ7pWGdljsVeiRiV0A8hT6rMnFNzyxhruqlzufcXIe_wOHZNijx0NZVTpHO685WelnaGMDb_h2SqHVUNbDuPW7ZgmF2mHRwypPggnGixag0-JO6tbDaZx_nc/s1465/NA%20ASCAT%20Index.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="754" data-original-width="1465" height="330" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhHfmpbvjzdv5FmldtrylaWR8dEOT9XlbMbaHQjbRgXcwJC0wC8GkgmLXPkK3L46N-aAuzvJ7pWGdljsVeiRiV0A8hT6rMnFNzyxhruqlzufcXIe_wOHZNijx0NZVTpHO685WelnaGMDb_h2SqHVUNbDuPW7ZgmF2mHRwypPggnGixag0-JO6tbDaZx_nc/w640-h330/NA%20ASCAT%20Index.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;">We assigned the names to these regions; they are not official.</div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRoB1oiyuHg2MSt8FONPySRZyr656VpcXiWVQk3ZeEob--4zgNzcJT5JuUr9ipgyWTGD2qlxEPvhwXpe2iE9pEghcbKhNmAB29hMKuR5eImdE7nyNApRGWME5BBji5HTIxVUF2NdqOl21eR9NwodpULT3g6__Bi7PGOkDjdb6uSJSKCZh9hZGMtF7-5A0/s1375/EU%20ASCAT%20Index%20sm.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1375" data-original-width="1276" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRoB1oiyuHg2MSt8FONPySRZyr656VpcXiWVQk3ZeEob--4zgNzcJT5JuUr9ipgyWTGD2qlxEPvhwXpe2iE9pEghcbKhNmAB29hMKuR5eImdE7nyNApRGWME5BBji5HTIxVUF2NdqOl21eR9NwodpULT3g6__Bi7PGOkDjdb6uSJSKCZh9hZGMtF7-5A0/w371-h400/EU%20ASCAT%20Index%20sm.png" width="371" /></a></div><br /><div>Each of these named regions provide a potential of 4 data opportunities in each 24 hr period, and the only way to see which might be latest data available is to download all 4 images. The file size varies from 20 to 40 kb, depending on how much data it includes. The UTC times shown (±1 hr) tell us satellite passage times of the 4 opportunities for new data. They occur in pairs, 13 hours apart, where the two passes of each pair are about an hour apart.</div><div><br /></div><div>For example, in Biscay, we have data passes at 10:30 and 21:30, ± 1 hr. If I go to the ASCAT web site now and look at what passes took place yesterday, I get the files shown below. </div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJEA_uClTPA9GcIBBneauMqZrX1UHH0GAU26UnxsZXOjTT66KqtFLODTNMhW0Zp2mIbLTDjW5RRr7afpuu1G-FeiANBUxbJRfoVls7lJz0upQzDCkQ2F7rete4iCIAA-5pRZjWiON2y2H7ReP8OKvrJ5O8XRfKbR0Oj23EEunicHpp9IJoKCQ0HK7Jjgg/s1412/Biscay%204%20samples.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1246" data-original-width="1412" height="564" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJEA_uClTPA9GcIBBneauMqZrX1UHH0GAU26UnxsZXOjTT66KqtFLODTNMhW0Zp2mIbLTDjW5RRr7afpuu1G-FeiANBUxbJRfoVls7lJz0upQzDCkQ2F7rete4iCIAA-5pRZjWiON2y2H7ReP8OKvrJ5O8XRfKbR0Oj23EEunicHpp9IJoKCQ0HK7Jjgg/w640-h564/Biscay%204%20samples.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div>In the top we see the two passes at 21:30 ± 1h and on the bottom the two at 10:30 ± 1hr. These are the actual live transit times of the satellites, representing the <i>valid times</i> of the wind measurements. On other days, these times will be different, but remain within these windows. </div><div><br /></div><div>But we do not get this data instantly. It takes 2 to 3 hr to process the data, so for the Biscay region, we would only look for new data at about 00:30 and 1430. Note that this delay time or latency is about the same as it is with the model forecasts, which also take 2 to 3 hr to process for our consumption in grib format. </div><div><br /></div><div><div>To clarify the concept that there will be new data about 4 times every 24hr: consider the example of San Francisco with valid times of 0525 and 1825 UTC, ± 1 hr, noting that it takes about 2 hr to process the data.</div><div><br /></div><div>Thus while underway or planning a route across these waters, we would go to Google Earth (or wherever we are looking at the data) at about 0725 or later UTC and download all four passes given. We do not know ahead of time which of the four will bring the new data but most likely two or them will have a swath of new data that will be valid at 0525 ± 1hr. Then again at 2025 or later we would again download all four of the passes and among those will likely be two new data swaths valid at 1825 ± 1hr.</div><div><br /></div><div>That is in effect a cookbook approach to the data, using our indexes as guides for when to look. We will not get new data between these two periods. We then have to correlate the valid times with the model times and forecasts at hand.</div></div><div><br /></div><div>There are several ways to get the data underway. You can request the images from <a href="http://www.saildocs.com" target="_blank">Saildocs</a> or you can use the custom KML files we made to use in either <a href="https://www.meltemus.com/index.php/en/" target="_blank">qtVlm</a> or the desktop version of Google Earth, which is a free download for Mac or PC. The KLM files can also be adapted for use in <a href="http://www.opencpn.org" target="_blank">OpenCPN</a>. For the last two methods at sea, we need to link our internet connection to a satphone or Iridium Go type device. With those procedures the links can be stored in the apps and accessed with a button click.</div><div><br /></div><div>To request the files from Saildocs, use this request structure for Biscay (green number from our index), meaning send this text in the body to query@saildocs.com (colors added to show what changes for each file, but using this method, you just need to change the 133 for Biscay to say 86 for Bermuda.</div><div><br /></div><div><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">Send https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_MET</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; font-size: 14.85px;"><span style="color: red;">B</span></span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">/zooms/WMB</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: red; font-size: 14.85px;">as</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #6aa84f; font-size: 14.85px;">133</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">.png</span></div><div><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">Send https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_MET</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; font-size: 14.85px;"><span style="color: red;">B</span></span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">/zooms/WMB</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: red; font-size: 14.85px;">ds</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #6aa84f; font-size: 14.85px;">133</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">.png</span></div><div><div><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">Send https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_MET</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; font-size: 14.85px;"><span style="color: red;">C</span></span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">/zooms/WMB</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: red; font-size: 14.85px;">as</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #6aa84f; font-size: 14.85px;">133</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">.png</span></div></div><div><div><div><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">Send https://manati.star.nesdis.noaa.gov/ascat_images/cur_25km_MET</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; font-size: 14.85px;"><span style="color: red;">C</span></span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">/zooms/WMB</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: red; font-size: 14.85px;">ds</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #6aa84f; font-size: 14.85px;">133</span><span face="Arial, Tahoma, Helvetica, FreeSans, sans-serif" style="background-color: white; color: #333333; font-size: 14.85px;">.png</span></div><div><span style="background-color: white; color: #333333; font-size: 14.85px;"><span style="font-family: inherit;"><br /></span></span></div><div><span style="background-color: white; color: #333333;"><span style="font-family: inherit;">This will get you the 4 images of the data that you can then analyze manually from the graphics alone. They have a Lat-Lon grid as shown above but will not be georeferenced into a nav program. But the main point is you can get the data that easily and our indexes show what to ask for and when.</span></span></div><div><span style="background-color: white; color: #333333;"><span style="font-family: inherit;"><br /></span></span></div><div><span style="background-color: white; color: #333333;"><span style="font-family: inherit;">Our <a href="https://www.starpath.com/ascat" target="_blank">ASCAT page</a> links to articles and demos with details, plus has a link to get all of the KML files and the graphic indexes. We will add more examples as soon as possible, and in particular the process of comparing with model forecasts.</span></span></div><div><span style="font-family: inherit;"><br /></span></div></div></div><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/XPl7TGLxGnQ" width="320" youtube-src-id="XPl7TGLxGnQ"></iframe></div><br /><div style="text-align: center;">Viewing ASCAT winds in Google Earth desktop version (using the Starpath indexes above.)</div><div><br /></div><div><br /></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com2tag:blogger.com,1999:blog-714718709601620866.post-69402251824423086102023-06-28T13:59:00.010-07:002023-06-28T22:27:18.823-07:00Speed and Heading — Basic, But Not Always Simple<p>Speed and heading are two fundamental parameters in marine navigation, but as instruments and navigation programs become more sophisticated, we can face challenges in keeping these basic parameters in order. I do not refer to speed over ground (SOG) or course over ground (COG), these one-time elusive parameters are now the simplest to understand and to measure. They come from the GPS instrumentation; meaning their abbreviations and place in NMEA sentences are all very clear.</p><p>The challenge comes with boat speed through the water (knotmeter speed) and more often with the heading of the boat, being the angle between the centerline and either true north or magnetic north. These are sensor measurements that get transmitted to the nav program via NMEA sentences or N2k parameters. </p><p>The issue is that these parameters can be transmitted in more than one NMEA sentence, and these days it is not uncommon to have more than one heading sensor, and if these might be from different companies they might be telling you the heading in two different sentences. At that point we need to learn if the s<span style="font-family: inherit;">ensor software will let us choose which sentence to use, some do, some don't, or if our navigation program lets us filter out the ones we do not want—again, some do, some don't. </span></p><p><span style="font-family: inherit;">That is in large part the issue. Seems simple enough, but I know from recent experience this can eat up hours of time sorting out, so these notes are an attempt to alleviate that issue—and to document the findings we needed to resolve a related case.</span></p><p><span style="font-family: inherit;">Heading is the more common issue, so we look at that first. Here are the primary NMEA sentences that involve the heading of the boat, keeping in mind that electronics can create proprietary sentences, plus some electronics still export old sentences that have been deprecated—a NMEA term meaning do not use these anymore. </span></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSiLmOt5UlF0BtvbzDmkyX61B8OXno5fjSC5Uwchp2SH20tQoec1p_NC6lZ_CYM-MoboJ4fhlPjE4Ky_b0odg7qJSBNUJ9HhBvFB8X6yg_6fsdhaf9PAJQKPmti7vKYgAwi_j1GUeOqfJathfAMEyMeZHf0aMYFf9T0zXRfznxfYebV-N3hlxDHb0XbG8/s924/NMEA_HDG_HDT_HMR.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="924" data-original-width="772" height="713" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgSiLmOt5UlF0BtvbzDmkyX61B8OXno5fjSC5Uwchp2SH20tQoec1p_NC6lZ_CYM-MoboJ4fhlPjE4Ky_b0odg7qJSBNUJ9HhBvFB8X6yg_6fsdhaf9PAJQKPmti7vKYgAwi_j1GUeOqfJathfAMEyMeZHf0aMYFf9T0zXRfznxfYebV-N3hlxDHb0XbG8/w596-h713/NMEA_HDG_HDT_HMR.png" width="596" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhyF5fa9gnw9W5wjE4E_jwlakmFxWiP9Q5SckkbehFpEgJ1o3WnKTh3czRyVVAAR1mniWuCACveZHdcSTeJjuU33hXhfL0V8PUqfpdHuyqST5F-2R6IxqAC-unt3IJnNpa_rgPCuA_b1nBX1t2amN4Q4rVec-2ykMg-Acv6JtwaTvLk-SLfTVfBg3glBxk/s779/NMEA_VHW.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="203" data-original-width="779" height="166" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhyF5fa9gnw9W5wjE4E_jwlakmFxWiP9Q5SckkbehFpEgJ1o3WnKTh3czRyVVAAR1mniWuCACveZHdcSTeJjuU33hXhfL0V8PUqfpdHuyqST5F-2R6IxqAC-unt3IJnNpa_rgPCuA_b1nBX1t2amN4Q4rVec-2ykMg-Acv6JtwaTvLk-SLfTVfBg3glBxk/w640-h166/NMEA_VHW.png" width="640" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgUaSRPkQ7f6pag-vQDV7mtLiLGmjBYqi77csHV3nPg4-i2OVIymTkgUQ3m9hZjwEXi9l2XdnEs6zynW-uDDBMqOd4TftJ14Cg-Tzkd_1S8ZNoA-Vo7gLlD6_cC9BayR9grop2OwA_yocRJb1eGtisRuWWlH1Nja73JiGrQ2kByF4PbL4wtQFJVgsz2peE/s796/NMEA_THS.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="377" data-original-width="796" height="304" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgUaSRPkQ7f6pag-vQDV7mtLiLGmjBYqi77csHV3nPg4-i2OVIymTkgUQ3m9hZjwEXi9l2XdnEs6zynW-uDDBMqOd4TftJ14Cg-Tzkd_1S8ZNoA-Vo7gLlD6_cC9BayR9grop2OwA_yocRJb1eGtisRuWWlH1Nja73JiGrQ2kByF4PbL4wtQFJVgsz2peE/w640-h304/NMEA_THS.png" width="640" /></a></div><p></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Old equipment still using HDT use it in the form:<span class="Apple-converted-space"> </span></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">$--HDT,x.x,T*hh</span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Likewise you will also sometimes see:</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">$--HDM,x.x,M*hh.<span class="Apple-converted-space"> </span>Both of these are old.</span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">When using N2k (See </span><a href="http://davidburchnavigation.blogspot.com/2020/02/N2k.html" style="font-family: inherit;" target="_blank">Introduction to NMEA 2000—with a Review of NMEA 0183</a><span style="font-family: inherit;">) there will be a conversion between PGN parameters and NMEA sentences. The correlation is shown below as presented in an <a href="https://actisense.com/" target="_blank">Actisense</a> gateway </span>manual<span style="font-family: inherit;">. </span><span style="font-family: inherit;">See a similar list at</span><span style="font-family: inherit;"> </span><a href="https://www.rosepoint.com/nemo-gateway/support/Nemo%20Gateway%20-%20Conversion%20Lists.pdf" style="font-family: inherit;" target="_blank">RosePoint Navigation</a><span style="font-family: inherit;">. </span></p><div><span style="font-family: inherit;"><br /></span></div><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNnrUSsuMEoD9uJfhGK_ZfUB4-hnLL-oM1J-XGzpXUUDD1-ajdDLAs0fKEE8QUKc6RxNON7mV7NRw-9MbQCL5PzbmEL4Tuz4Uez3GHzWwfQb79eqwt8vIcgNrooC_MTKEfqxXO1j6p4TFJckKN7L4ehZwjMMlHLmq3r1ybOwGVcIXuwE_x5Sg9BByO-8Y/s893/Actisense-1.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="873" data-original-width="893" height="626" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiNnrUSsuMEoD9uJfhGK_ZfUB4-hnLL-oM1J-XGzpXUUDD1-ajdDLAs0fKEE8QUKc6RxNON7mV7NRw-9MbQCL5PzbmEL4Tuz4Uez3GHzWwfQb79eqwt8vIcgNrooC_MTKEfqxXO1j6p4TFJckKN7L4ehZwjMMlHLmq3r1ybOwGVcIXuwE_x5Sg9BByO-8Y/w640-h626/Actisense-1.png" width="640" /></a></div><br /><span style="font-family: inherit;"><br /></span><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifXzKP3Nj9TiIbiqV_mO6xfhfiWh4TUtlSz7CMvm9CrGTl3ImzmaRZSXms8jtZVeC3lP2JQulUHfYoncppNBXcdb3TIoDT2HSSN0p1o4yjL-OGY11H9jq18L_LZLF9UvkXH9TXiquVxJR0SCfW1KBD65cxM7HAC14RpsjhXvZF4CC4SD2pf7FZfpCSNF4/s886/actisense%202.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="858" data-original-width="886" height="620" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifXzKP3Nj9TiIbiqV_mO6xfhfiWh4TUtlSz7CMvm9CrGTl3ImzmaRZSXms8jtZVeC3lP2JQulUHfYoncppNBXcdb3TIoDT2HSSN0p1o4yjL-OGY11H9jq18L_LZLF9UvkXH9TXiquVxJR0SCfW1KBD65cxM7HAC14RpsjhXvZF4CC4SD2pf7FZfpCSNF4/w640-h620/actisense%202.png" width="640" /></a></div><span style="font-family: inherit;"><br /></span><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Note that the vessel heading (PGN 127250) can be exported to several NMEA sentences, which can potentially cause a conflict if other devices are also exporting the heading. </span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; text-align: center;"><span style="font-family: inherit;"><b>Abbreviations in ECS</b></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">Now taking a look at how these parameters are presented in various ECS, with a note that electronic charting system (ECS) is the official name of any navigation program that is not a type-approved ECDIS. In short, any "nav program," "charting software," or console "chart plotter" interface to a GPS sensor. We cover this distinction in our text </span><a href="https://www.starpath.com/catalog/books/1996.htm" target="_blank"><i>Introduction</i></a><span style="font-family: inherit;"><a href="https://www.starpath.com/catalog/books/1996.htm" target="_blank"><i> to Electronic Chart Navigation</i></a>.</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjk4ME_BjiLRe9d5AeAgj81jprgEqqdB2ToYyrgPQNONf4NdG5pwxzTqWSBmU4Zj2TTogMO5I3n2cwsZjm0jWqnTcSWBVBihyLL06XCKD3PqG-Bvp3kdxRgogVgYIeROtmfIu1aS4flKkIHrfUPTGDi5ezheyrvdODNxn3IMZToZAkmjg4QAIBzQLHNwbU/s1560/abbreviations.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="530" data-original-width="1560" height="218" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjk4ME_BjiLRe9d5AeAgj81jprgEqqdB2ToYyrgPQNONf4NdG5pwxzTqWSBmU4Zj2TTogMO5I3n2cwsZjm0jWqnTcSWBVBihyLL06XCKD3PqG-Bvp3kdxRgogVgYIeROtmfIu1aS4flKkIHrfUPTGDi5ezheyrvdODNxn3IMZToZAkmjg4QAIBzQLHNwbU/w640-h218/abbreviations.png" width="640" /></a></div><br /><p></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">It is not easy to find official sources for recommended abbreviations. <i>Bowditch</i>, for example, 2021 edition, page 437, Glossary of Abbreviations, gives S for speed and Hdg for heading. STW and CTW are not listed. S is also used for set, south, slow, sand, and the sea-air temperature difference correction!</span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;">A more useful source is the <a href="https://std.iec.ch/terms/terms.nsf/ByPub?OpenView&Count=-1&RestrictToCategory=IEC%2062388 " target="_blank">International Electrotechnical Commission (IEC) Glossary online</a> </span><span style="font-family: inherit;">where we find these definitions:</span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"><span style="font-family: inherit;"><br /></span></p><p class="p1" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9_3sMC_r0N72ne1NXCJWfN63d_Y8VrM-3nIErorfVVsuWoSGKc0DeyZU9XJGpZjj02iwPPYgH3PgfoLTeIN4dEPNNpGaLiOQKielKxVZW3AXOe7YLFudiC-sjBnsRifFZ6QdLX2U0_jOZAVCpoIeOBMpmOQjvPAWL4hdJ73svUwUl3N4yv8ycWgJ8LIM/s1701/IEC%20definitions.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1544" data-original-width="1701" height="580" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg9_3sMC_r0N72ne1NXCJWfN63d_Y8VrM-3nIErorfVVsuWoSGKc0DeyZU9XJGpZjj02iwPPYgH3PgfoLTeIN4dEPNNpGaLiOQKielKxVZW3AXOe7YLFudiC-sjBnsRifFZ6QdLX2U0_jOZAVCpoIeOBMpmOQjvPAWL4hdJ73svUwUl3N4yv8ycWgJ8LIM/w640-h580/IEC%20definitions.png" width="640" /></a></div><br /><span style="font-family: inherit;">According to this important source, CTW and STW are well recognized abbreviations. Also it appears that CTW and "heading" could be used interchangeably. In principle we have a heading at the dock but not a CTW. ECS that use both (HDG and CTW), such as qtVlm and </span><span style="font-family: inherit;">Expedition (identifies CTW as "course"), are more general. Some consider that CTW should be HDG + leeway, but this is just the sort of thing that has to be sorted out within the ECS in use.</span><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;">Looking at the speed parameter, that can be made complex as well. Generally we think of STW as the speed in the direction we are pointed, our HDG, but if we have much leeway it will reduce the proper knotmeter reading, as shown below and discussed in <a href="http://davidburchnavigation.blogspot.com/2018/04/effect-of-leeway-on-knotmeter-speed.html" target="_blank">Leeway effect of knotmeter speed</a>.</span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEir3XaFPoEVEy_a0AXVzn3ryjAKrbZAh2W3aaOLYiDwgxXaMuBL9rcnzCRyFMo9yTajxMTfzg9b_Sq-dGphjQVZc3E1-HVYTsV2qaTidvwWdxnIeGbXnL5-atFzewaP9pGHBWlOQHP2Ot5_NIHmeRlHtL9H4bH2IsCcz8fjoV1uuBETCR17mev0u9mXy8E/s640/Paddle%20Wheel.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="640" data-original-width="435" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEir3XaFPoEVEy_a0AXVzn3ryjAKrbZAh2W3aaOLYiDwgxXaMuBL9rcnzCRyFMo9yTajxMTfzg9b_Sq-dGphjQVZc3E1-HVYTsV2qaTidvwWdxnIeGbXnL5-atFzewaP9pGHBWlOQHP2Ot5_NIHmeRlHtL9H4bH2IsCcz8fjoV1uuBETCR17mev0u9mXy8E/w437-h640/Paddle%20Wheel.png" width="437" /></a></div><br /><span style="font-family: inherit;"><br /></span><div><div style="text-align: center;"><b style="font-family: inherit;">New age instruments</b></div><div><span style="font-family: inherit;"><br /></span></div><div>There is every reason to be sure to have a traditional paddle wheel knotmeter as well as both a magnetic compass and a magnetic heading sensor on the boat, but with those in place we can improve on both with new GPS based sensors, often called "satellite compass" or "satellite speed log" and indeed some provide both data and more.</div><div><br /></div><div>A single GPS sensor can measure SOG and COG but it has no way to know your heading, but two GPS sensors in line with the centerline, say one at the bow and one at the stern, does know your heading, which is just the bearing from your stern location to your bow location. Two such sensors make up a "satellite compass" that can measure your true heading at any time, in addition to COG and SOG, being how the pair is moving relative to the fixed earth. The heading can also be exported in magnetic units using the USGS software program called <a href="https://www.ncei.noaa.gov/products/world-magnetic-model" target="_blank">GeoMag</a>. It computes accurate variation based on your Lat and Lon and the World Magnetic Model, which is updated every five years. (You can access GeoMag on your phone with the USGC app CrowdMag.)</div><div><br /></div><div>The technology for this type of instrument is well advanced; the two GPS units do not have to be on the bow and stern, they can be just a foot or so apart in a single unit. There are several models, in the $1,000 price range. There is a lot of math involved and they do require at least 5 satellites in view for best performance. Once the heading is known, the SOG in the direction of the COG can be projected onto the direction of the heading, called the <i>longitudinal velocity</i>, which is the speed you are moving in the direction you are headed, and projected onto the perpendicular direction, called the <i>transverse velocity</i>. The latter is the speed you are moving sideways. </div><div><br /></div><div><br /></div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj0UZXDJoIYWI4RLyvNfYXwRK_PATZeXFhomRyOWlNb2ipZ5Y_gCHGIa3zobg5G8Y-AybEnnPYoKprtkvJGHjUvXLjP_NNlM1dT0ZL75j-KhdAIXjWixjyn_4SpIwsdqdTxxy_nFdkEte5NF9lrKyUbbli1YWJvxhOBqhjeth7iJOZa3pjIIcduzyp26W0/s1269/Sat%20Com.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1269" data-original-width="1009" height="554" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj0UZXDJoIYWI4RLyvNfYXwRK_PATZeXFhomRyOWlNb2ipZ5Y_gCHGIa3zobg5G8Y-AybEnnPYoKprtkvJGHjUvXLjP_NNlM1dT0ZL75j-KhdAIXjWixjyn_4SpIwsdqdTxxy_nFdkEte5NF9lrKyUbbli1YWJvxhOBqhjeth7iJOZa3pjIIcduzyp26W0/w440-h554/Sat%20Com.png" width="440" /></a></div><br /><div><br /></div><div>Here we see that one GPS tells COG the course over ground, and we can imagine the computation of SOG as the distance between the locations at time T1 and T2 divided by the time interval (T2-T1). With two GPS sensors we can compute HDG and then project the SOG vector onto that heading and the transverse direction.</div><div><br /></div><div>Note that we are not measuring leeway. The instrument does not even know we are in water. When this data gets reported to us in a NMEA sentence it would likely be VBW.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9vOEHWD2732ORimjJoSvq2XN8CoFILZGyz7AihvOtWayiGFDzp8g2HECTXnhNHChDc0dMLewciAQ3V3jTSxHjbDV92sXTOVoAeUptbe2Xjt03BpRUsIg0yypS6xiKLz_oXODUdbwbl01Uc_YdiZFEFc7MZDmXj47SFerST1MAMCTZg3xTx41UYemUtHI/s1848/VBW.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="882" data-original-width="1848" height="306" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9vOEHWD2732ORimjJoSvq2XN8CoFILZGyz7AihvOtWayiGFDzp8g2HECTXnhNHChDc0dMLewciAQ3V3jTSxHjbDV92sXTOVoAeUptbe2Xjt03BpRUsIg0yypS6xiKLz_oXODUdbwbl01Uc_YdiZFEFc7MZDmXj47SFerST1MAMCTZg3xTx41UYemUtHI/w640-h306/VBW.png" width="640" /></a></div>We would be getting ground speeds, not water speeds. This does not help analyze leeway but could be helpful docking and it could help us interpret current sets and our thinking on the derived parameter course made good (VMC).</div><div><br /></div><div>Furuno has an attractive advancement of this concept in their <a href="https://www.furuno.com/en/products/compass/SCX-20" target="_blank">SCX 20/21 Satellite compass</a>. They use four GPS sensors so they get more reference lines to compute around, six in principle, two fore and aft, two abeam, and two diagonal. This way, with still more math, they can measure and account for all vessel motions (roll, yawl, pitch) and correct for them, in addition to reporting heel angle (roll). This allows them to get good results with fewer satellites. It is a relatively small box, with list price of about $1,400, and taking only about 2 watt of power. We see these on several <a href="https://www.teamzvi.com/" target="_blank">high-tech race boats</a>.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiIrZx18Rb57mFEPxx0DVW_PLWvctfo0RaBDY9G7PUE17zvqZdp_60e3bKB8GeJ5FFKoimPji8ULaqCykd1SqwS9vOO_rxeRq1EZOJIUQTSeiqs3OjHtZVcNai6zM2H-N37rVjbdK6H5zSzsImxmykh5WaFlXHR3imVQ4j8D1zhNoS12nzCd0xDjJha-4E/s2387/Furuno-SCX21-and-21-satelitte-compass-mounts.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1830" data-original-width="2387" height="490" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiIrZx18Rb57mFEPxx0DVW_PLWvctfo0RaBDY9G7PUE17zvqZdp_60e3bKB8GeJ5FFKoimPji8ULaqCykd1SqwS9vOO_rxeRq1EZOJIUQTSeiqs3OjHtZVcNai6zM2H-N37rVjbdK6H5zSzsImxmykh5WaFlXHR3imVQ4j8D1zhNoS12nzCd0xDjJha-4E/w640-h490/Furuno-SCX21-and-21-satelitte-compass-mounts.jpg" width="640" /></a></div><br /><div>On the left is the SCX 21 for NMEA 0183 and the one sitting loose on the deck is the SCX 20 for N2k. This image and the one below are from a 2020 <a href="https://panbo.com/furuno-scx20-scx21-satellite-compasses-the-impressive-demo-at-nmea/" target="_blank">Panbo review of the instrumen</a>t. The latest instrument includes 2023 improvements.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDNNIklq9jDHCIRzgIh301RVqoTKfqxpLGf7zQvHALLF1QrlhFRopnQuQ_fU3Pu5HikYE3UUFf8D9wiuYvm33kAcCs6P2Zdgu42cUrI81wfWiWff-ZAlEQsjHcmzowroftJoo1xcg7V7XiqgYvAD8YfRXA6Z8SJfuPZpygGCpLZoLrPgynzkg1oFDRY0I/s2548/Furuno-SCX20-21-satelitte-compass-scaled.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1053" data-original-width="2548" height="264" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDNNIklq9jDHCIRzgIh301RVqoTKfqxpLGf7zQvHALLF1QrlhFRopnQuQ_fU3Pu5HikYE3UUFf8D9wiuYvm33kAcCs6P2Zdgu42cUrI81wfWiWff-ZAlEQsjHcmzowroftJoo1xcg7V7XiqgYvAD8YfRXA6Z8SJfuPZpygGCpLZoLrPgynzkg1oFDRY0I/w640-h264/Furuno-SCX20-21-satelitte-compass-scaled.jpg" width="640" /></a></div><br /><div>The displays from the two units above. Note that the headings differ as neither one has been carefully aligned with the center line in the demo pictured here.</div><div><br /></div><div>I do not have direct experience with this unit, but the <a href="https://www.furuno.com/files/Brochure/484/upload/SCX_20_21_EN.pdf" target="_blank">specs and output</a> seem impressive:</div><div><br /></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div>— heading to ± 1º</div><div>— accurate, stable heading leads to clean radar trails</div><div>— very high GPS accuracy of ± 5m</div><div>— SOG to ± 0.02 kt with 5 satellites</div><div>— VBW speeds to ± 0.02 kts</div><div>— rate of turn (ROT) output for AIS broadcasts</div><div>— barometer ± 1mb with calibration offset option </div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div>— air temp ± 2ºC </div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div>— heel output for leeway computations and anemometer corrections</div><div>— data smoothing and offsets available on all outputs</div><div>— DR mode for lost GPS signals</div></blockquote><div><br /></div><div>_______________</div><div><br /></div><div>I have been reminded in a comment by navigator Mark D'Arcy of the related issue that ECS often have access to multiple GPS sensors, which in turn can present similar conflicts that might lead to the vessel position jumping around a bit on the screen. Most ECS and indeed all ECDIS have a means of selecting what will be the primary source of GPS and setting this will solve that problem.</div><div><br /></div><div><br /></div><div><div><div><div><div><div><div><div><div><div><p class="p2" style="font-feature-settings: normal; font-kerning: auto; font-optical-sizing: auto; font-stretch: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variation-settings: normal; line-height: normal; margin: 0px; min-height: 14px;"><span style="font-family: inherit;"><br /></span></p><p><span style="font-family: inherit;"><br /></span></p><p><span style="font-family: inherit;"><br /></span></p><p><br /></p></div></div></div></div></div></div></div></div></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-26987330384893091422023-05-30T16:45:00.000-07:002023-05-30T16:45:26.600-07:00Battery Life of Phones Running Location Services<p>Phones and tablets are often valuable backup navigation systems, with many excellent navigation and weather programs available such as <a href="https://www.meltemus.com/index.php/en/" target="_blank">qtVlm</a> and <a href="https://luckgrib.com/" target="_blank">LuckGrib</a>. It is known that location services, which we must turn on to access the internal GPS, cause an enhanced drain on the battery. Here we look into how much of a drain is this.</p><p>The topic came up again as we introduced the new low-cost miniature AIS called <a href="https://www.starpath.com/catalog/accessories/AIS-1.htm" target="_blank">dAISy</a> that will not only run in any computer-based nav program, but it will also work plugged into an Android phone or tablet running qtVlm. The battery-life question then extended to how much more of a drain will it be to run this AIS in a phone, which will be on top of the drain from the required location services. </p><p>Below we see the battery drain with with dAISy being fully powered by an Android phone along with qtVlm.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKqor_cPTZmn-3YhHfjazhbePf9Sgl39uLJuPf7YL4rQn7KWTIAHWHJ0vndYjZWbCViWhfqSB4t7lo9LuqGCGnpknFxrRxBv92vWRwctdzTaA7XO2-L0Wtt29qWVvp5oOCladYcQpltpswibOzrto85VWNLIT8Jizkv6I42gWKw9H5x1irO2RG7Hus/s1440/A03s%20with%20qtVlm%20and%20dAISy.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="888" data-original-width="1440" height="394" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKqor_cPTZmn-3YhHfjazhbePf9Sgl39uLJuPf7YL4rQn7KWTIAHWHJ0vndYjZWbCViWhfqSB4t7lo9LuqGCGnpknFxrRxBv92vWRwctdzTaA7XO2-L0Wtt29qWVvp5oOCladYcQpltpswibOzrto85VWNLIT8Jizkv6I42gWKw9H5x1irO2RG7Hus/w640-h394/A03s%20with%20qtVlm%20and%20dAISy.png" width="640" /></a></div><b>Figure 1.</b> <i>A new Galaxy A03s Android phone running both qtVlm and the dAISy plugged in and monitoring AIS Traffic without external power</i>.<div><br /></div><div>This test ended as shown when we noted that the dAISy had quit working at about 5.5 hr, although qtVlm continued to function. We suspect that the phone shut off the USB connection at some level to protect battery life — not realizing that in this case the dAISy had in fact almost negligible effect on the drain as we see below.</div><div><br /></div><div>Note too that this direct plugin arrangement of the dAISy is just for quick observations of traffic, up to, as we know now, four or five hours. For continuous operation, the dAISy should be <a href="https://www.starpath.com/catalog/accessories/dAISy_on_qtVlm_android.html" target="_blank">connected with an OTG adapter</a> which allows for external power application.</div><div><br /></div><div>To see how this differs with no dAISy, we have the data below for full navigation functions running in qtVlm but no USB attachments. </div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0LR6xR4BfZElaD2eYEQPTKlKSqw8gJDo8wsNoqJLEY1uwcCSu3Z4RHqK_SJ2CtPVKAdBUMB5VFVXXcIixYa2c8HVAGhJDsSsDqLgq9L-rHvifh1Oee3TLOu_AzL5YH1ZZuf9KqfsUl2LAlaRCVH_PH15DOP6Al7s0otNP73XEscegpexblvJMMsfx/s2166/A03s%20with%20qtVlm%20and%20continuous%20fix.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1062" data-original-width="2166" height="314" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0LR6xR4BfZElaD2eYEQPTKlKSqw8gJDo8wsNoqJLEY1uwcCSu3Z4RHqK_SJ2CtPVKAdBUMB5VFVXXcIixYa2c8HVAGhJDsSsDqLgq9L-rHvifh1Oee3TLOu_AzL5YH1ZZuf9KqfsUl2LAlaRCVH_PH15DOP6Al7s0otNP73XEscegpexblvJMMsfx/w640-h314/A03s%20with%20qtVlm%20and%20continuous%20fix.png" width="640" /></a></div><b>Figure 2.</b> <i>A new Galaxy A03s Android phone running qtVlm with a continuous GPS fix.</i><div><i><br /></i></div><div>We notice here that the rate of battery life drain is essentially the same as with the dAISy attached. In short, this phone running location services and a large nav app loses battery life at about 10% per hour, with or without the dAISy attached.</div><div><br /></div><div>This is a new phone with its new battery; older phones might not be as efficient. Also note that this <a href="https://www.amazon.com/Samsung-Unlocked-Smartphone-Expandable-Infinite/dp/B09SM24S8C/" target="_blank">Galaxy A03s</a> is an economical choice for a backup nav system at about $88 for refurbished unit with both GPS and a barometer... plus it will run an inexpensive AIS system.</div><div><br /></div><div>Below is the same measurement as Figure 2 using 18-month-old iPhone 11.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghrQy7Zqs5bKSnb0aLA_cV6sbWZreZP_Y21Irhrd1Tijv8wiLPbJw0pmzLWlIB7XbPpWMLAOak1RAO6yTFPeLAT99zMWu4yz-ADIyVRAesJTAR6-2LItamUXTihDdBe1WvkF7k170Zd_Dpdoc4UZCcoyWzdfeh9Dmax0DVLC92pJgCVni6j6xQTIGb/s1880/iPhone%2011%20qtVlm%20w%20continuous%20fix.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1048" data-original-width="1880" height="356" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEghrQy7Zqs5bKSnb0aLA_cV6sbWZreZP_Y21Irhrd1Tijv8wiLPbJw0pmzLWlIB7XbPpWMLAOak1RAO6yTFPeLAT99zMWu4yz-ADIyVRAesJTAR6-2LItamUXTihDdBe1WvkF7k170Zd_Dpdoc4UZCcoyWzdfeh9Dmax0DVLC92pJgCVni6j6xQTIGb/w640-h356/iPhone%2011%20qtVlm%20w%20continuous%20fix.png" width="640" /></a></div><b>Figure 3.</b> <i>An 18-mo-old iPhone 11 phone running qtVlm with a continuous GPS fix.</i><br /><div><br /></div><div>So we see a rough generality emerging, namely the battery drain from location services in action is about 10% per hour. This will of course have to be tested to learn what other factors may influence this observation, but at least we have a starting point.</div><div><br /></div><div><br /></div><div><div><br /></div><div><br /><p><br /></p></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-50760503743944081562023-05-21T16:24:00.005-07:002023-05-21T19:00:30.427-07:00Fast Barometer Setting in US Waters<p>Setting a barometer to the right pressure is not the same as calibrating it, but it is the certainly the first step, and might indeed meet many practical needs. In short, we assume that if we set it to be right at one pressure, we hope it is at least nearly right at other pressures.</p><p>Also, if the barometer is in the boat, then we know it is roughly at sea level so we do not have to worry about the corrections for elevation above sea level, which is usually a dominating factor. A barometer that is 6 ft above the water is only reading 0.2 mb lower than what it would read floating on the water.</p><p>What we need is just an accurate (official) value of the sea level pressure that we can set our own barometer to read, because our own yacht's barometer is also effectively at sea level. This is not true for a ship, were the instrument could be 80 feet above the waterline.</p><p>In the past we taught that you can get accurate pressure from various National Data Buoy Center (NDBC) or lighthouse reports, or from airports (METARS). That still works, but those data takes some steps to access and on top of that they are only updated every hour. Thus if we are to use one of them, we have to note the time and the given 3-hr pressure tendency to compute the correction to the official reading.</p><p>We now have a much faster and easier solution. I say "now," but this source has actually been available for probably over a year now, I just had not discovered it. We get the pressure data now from exactly the same place that all mariners now have to work with to get tide and current data, namely</p><p style="text-align: center;"><a href="http://www.tidesandcurrents.noaa.gov." target="_blank">www.tidesandcurrents.noaa.gov.</a></p><p>Go to that site, click the state you are in, and then on the top right, turn Legends on if needed, and then check Barometric Pressure and signs will pop up at the places were it is known. These data are updated every 6 minutes, which is ideal for this operation.</p><p>Then you can either set your barometer to that pressure, or maybe better still, start a note book with the time and date and the correction you observed. As you get more of this table filled in over various pressures, then indeed you are calibrating your barometer. If you just set it, and do not record the correction you do not learn about the pressure dependence of its errors.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgrRWJawizTCgBXMKdDYXSLvHZBxnH6AyvnAJk6Lk9wELJnR9V-3E5v5OZglbLum_WrNEx0RNb8IlK1NMVqu3eRp4McRImqjO_WkTfcPDItoQNeDpAwG-OndynQxWrqbkNliUZ2gXJDeQH344EUn9ABma1Jac2q5cvllB_zP7RrY2Lbo-qAxcN-ZWGX/s1321/baro%20tide%20FL.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="826" data-original-width="1321" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgrRWJawizTCgBXMKdDYXSLvHZBxnH6AyvnAJk6Lk9wELJnR9V-3E5v5OZglbLum_WrNEx0RNb8IlK1NMVqu3eRp4McRImqjO_WkTfcPDItoQNeDpAwG-OndynQxWrqbkNliUZ2gXJDeQH344EUn9ABma1Jac2q5cvllB_zP7RrY2Lbo-qAxcN-ZWGX/w640-h400/baro%20tide%20FL.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><i><div style="text-align: center;"><i>Sample barometer data display at tidesandcurrents.noaa.gov when clicking FL.</i></div></i><div style="text-align: center;"><br /></div><div>In this view there is good coverage on the Gulf Coast, but not much north of there. When this happens, try clicking another state. </div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAp0AGyrqlppmvwhzyIG-eUmjmO7DNMSHahMYjHoJKcfztcRHUGb7eu7pAFjrSRHx1318CcHhBX0eFsyr0wYgZupvOfZoPwipYELi81mSuvHyu1GYDtO_qOYeR6akudPBAzF9VKbfyah-99ZtQ_8J_Z7QeoXwi5iSw2kHHCFMGmp_o6sfhwAO9PEa2/s1421/barotides%203.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="838" data-original-width="1421" height="378" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAp0AGyrqlppmvwhzyIG-eUmjmO7DNMSHahMYjHoJKcfztcRHUGb7eu7pAFjrSRHx1318CcHhBX0eFsyr0wYgZupvOfZoPwipYELi81mSuvHyu1GYDtO_qOYeR6akudPBAzF9VKbfyah-99ZtQ_8J_Z7QeoXwi5iSw2kHHCFMGmp_o6sfhwAO9PEa2/w640-h378/barotides%203.png" width="640" /></a></div><div style="text-align: center;"><i><br /></i></div><i><div style="text-align: center;"><i>Sample barometer display clicking the state of SC</i></div></i><div><br /></div><div>When trying all near by states does not work, then you can turn to <a href="http://www.starpath.com/barometers" target="_blank">www.starpath.com/barometers</a>, which covers global sources of various kinds.</div><div><br /></div><div>This could be a great resource when sailing on other vessels and you want to check its barometer. You can read read this pressure data from your phone (tidesandcurrents.nooa.gov—we should know it by heart because it is now the only <i>official source</i> of tide and current data)... but if you have a phone, then you are better off using our <a href="https://www.starpath.com/marinebarometer/about.htm" target="_blank">barometer app</a>, which you could set using this method as well.</div><div><br /><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-9194125409062410962023-05-16T10:56:00.001-07:002023-05-18T08:47:14.289-07:00ECS Without GPSWe might assume an electronic charting system (ECS) is only useful if we have a GPS connected to it so we can track our boat across the chart. But the main components of the system are actually the echart navigation software and a set of echarts. The GPS is just a luxury. We can do a tremendous amount of sophisticated navigation, both easily and accurately, without the GPS.<br />
<br />In fact, the crucial part of a well-navigated voyage is done before the boat pulls away from the dock. We can do a rough layout of the route for quick time and distance estimates, and then zoom in along the route and fine tune each of the waypoints to optimize their use underway. This usually calls for choosing them such that they correlate with good visual or radar targets along the route. Then give each waypoint a name, not just a number. <br />
<br />These programs also have built-in tide and current predictions, so we can estimate anticipated speeds made good (SMG) along the route, and then print out a <i>route plan </i>with individual route-leg courses and distances and ETAs all tabulated. This is prudent preparation, with or without GPS to track us across the chart. All US echarts are free, so now there is no reason not to have all possible charts of interest at hand for any route. See chart download options at <a href="http://starpath.com/getcharts" target="_blank">starpath.com/getcharts</a>.<div><br /><div>In this note we look at a couple ways you can navigate with echarts without relying on GPS–or put another way, if you happen to lose your GPS, don’t shut off the ECS. It remains your most convenient and accurate means of chart navigation.</div><div><br /></div><div>Also keep in mind that essentially all ECS include some form of DR mode with which you can set the boat icon at your best estimated position on the chart and then turn on DR and set the speed to match your knot meter and heading to match what you are steering. Your ECS will then plot out your DR track. Your job is to then update heading and speed as they change. </div><div><br /></div><div>We look here at a couple samples of navigation that might occur on the chart in Figure 1 that we must solve using our ECS without GPS.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhfyNbUmYetUTv69eYmt85ZpEYxY3Cx_BLatkKNm74tFakcR5EBdVYKCXNJqKp6ulFulyzSzjcQlfAx5KDc_zUMXhM-ArwLMlVH03EWbjYrfnuhODo7xlXxlNbCHS5-3J6o7mOsbtiJkZSFqQMLJJpkd7j_Np4ERTOUEIwQHscSiV2d__KpdGJ-4kM3/s1446/ECS%20wo%20GPS%20route.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="458" data-original-width="1446" height="202" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhfyNbUmYetUTv69eYmt85ZpEYxY3Cx_BLatkKNm74tFakcR5EBdVYKCXNJqKp6ulFulyzSzjcQlfAx5KDc_zUMXhM-ArwLMlVH03EWbjYrfnuhODo7xlXxlNbCHS5-3J6o7mOsbtiJkZSFqQMLJJpkd7j_Np4ERTOUEIwQHscSiV2d__KpdGJ-4kM3/w640-h202/ECS%20wo%20GPS%20route.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div style="text-align: center;"><b style="font-weight: bold;">Figure 1</b><b>.</b><i> It is night, in light fog. The boat is near buoy VD and wishes to enter Baynes Channel, about 3 miles to the West. NOAA current predictions call for a current of 1.5 kts flowing toward 135T. Broadcast Notice to Mariners has just informed us that the light on buoy VK is not showing and the buoy may be off station. We have no valid GPS signals to rely on. We have a calibrated knotmeter, compass, and depth sounder. We are left to use traditional navigation to get into this channel, taking care not to get set down onto the rocks that buoy VK was supposed to mark. We are under power at 6 kts</i>. <i>The magenta shade on the chart means that area is in a separation zone between two vessel traffic lanes. </i></div><div style="text-align: center;"><i><span style="color: #990000;"><br /></span></i></div><div style="text-align: center;"><br /></div><div>First we look at what course to steer to make good a due West track correcting for the tidal current. We know we must point into the current, but how much? We do not have the benefit of a nice COG and past track from a working GPS, so we will rely on theoretical values... and then monitor the rocks by other means. The goal is to have more than one check on our navigation at all times if possible.</div><div><br />
<br /><div style="text-align: center;"><b>Course to Steer (CTS) to Correct for Current</b></div>
<br /><div>The solution is illustrated in Figure 2. We could do this plotting on a chart, or indeed on a blank paper using standard plotting tools, but this can be solved faster with electronic plotting tools available in any ECS. One procedure is outlined below.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhI_vXwGBfnHTw-hQ1I8CrJMmOyIZNEIFKCSo3fXkOpvp2gWk_tiXVhI6PsqI_78PSk-z8nUPUw4G3961eTT1kJtRCdwj8wqz3i0rDGHCWVhDv5tdKR0rtnftGEwzsS2LNWxMmmC3v2UxFI2x31iN9M0epJcOlWQofhjP3UenpM6mkMrtc-miI9V1gs/s1100/CTS%20w%20qtVlm.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="329" data-original-width="1100" height="192" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhI_vXwGBfnHTw-hQ1I8CrJMmOyIZNEIFKCSo3fXkOpvp2gWk_tiXVhI6PsqI_78PSk-z8nUPUw4G3961eTT1kJtRCdwj8wqz3i0rDGHCWVhDv5tdKR0rtnftGEwzsS2LNWxMmmC3v2UxFI2x31iN9M0epJcOlWQofhjP3UenpM6mkMrtc-miI9V1gs/w640-h192/CTS%20w%20qtVlm.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div style="text-align: center;"><b style="font-weight: bold;">Figure 2.</b> <i>Solution to the CTS problem, which can be worked on paper charts or on an ECS.</i></div></div><div><i><br /></i><div> <br /><div style="text-align: center;"><b>CTS Procedure</b></div><br />
</div></div></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><b>Step 1.</b> From your DR position, draw a line in the direction you want to make good (CMG) with a length longer than the knotmeter speed (boat speed) — 270 T in this example.</span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><b>Step 2. </b>From the DR position, imagine the boat drifting with the current for 1 hour. Mark the end point of that drift on the chart — 1.5 nmi in direction 135T in this example.</span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><b>Step 3. </b>From that drift position, set your dividers to the anticipated boat speed, and swing an arc to find where your speed for one hour will put you onto your desired track. Mark that point on your desired track — radius = 6.0 nmi in this example.</span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div></div><div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><b>Step 4. </b>The direction of that line, drift point to intersection point, is the CTS that will maintain a CMG of 270 at a boat speed of 6 kts in a current of 1.5 kts in direction 135 — CTS = 280T in this example.</span></span></div></div></div></blockquote><div><div style="text-align: left;"><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px;"><div><div style="text-align: left;"><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><b>Step 5. </b>Measure the length of the line from the DR position to the intersection point and that will be your speed made good (SMG) as you crab along the the desired track — 4.9 kts in this example.</span></span></div></div></blockquote><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">Solving this vector triangle following these steps can be done numerous ways with an ECS, depending on plotting options offered. It could all be done with one 3-leg route. </span></span></div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">Starting at the DR position, use the ruler tool to find a point that is farther than your boat speed in the desired direction, and drop a mark there. Then start the route at that mark with leg 1 back to the DR position; leg 2 from there to the drift location; and finally from there find the point on the first line that is your boat speed from the drift point; and end the route there. You know the final point has to have length equal boat speed, so just monitor that read-out as you move the cursor along the line. Then use the ruler tool or measurement tool to find the lengths and directions you need. Here is a </span></span>video example of this method.</div><div><br /><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/GGzMpHhM_Io" width="320" youtube-src-id="GGzMpHhM_Io"></iframe></div><div class="separator" style="clear: both; text-align: center;"><span face="Roboto, Noto, sans-serif" style="background-color: white; color: #0d0d0d; font-size: 15px; text-align: left; white-space: pre-wrap;"><i><br /></i></span></div><div class="separator" style="clear: both; text-align: center;"><span face="Roboto, Noto, sans-serif" style="background-color: white; color: #0d0d0d; font-size: 15px; text-align: left; white-space: pre-wrap;"><i>Course to Steer by Quick ECS Solution</i></span></div><br /></span></span></div><div><br /></div><div><br /></div><div><div style="text-align: center;"><b>Danger Bearings</b></div>
<br />In the above scenario, we cannot be certain that the current predictions are correct, so we need a way to monitor our progress to be sure we stay off of Fulford Reef, whose buoy (VK) is now missing. One way is to set up a danger bearing using Cadboro Point Light, which is clearly in view, near dead ahead. </div><div><br /></div><div>Using an ECS, we can, in seconds, measure an accurate danger bearing to the QG (Quick flashing Green) light that will keep us off the reef, as shown in Figure 3. We see that this bearing should remain less than 254 M (we are assuming a 20º E variation). We watch it with a bearing compass as we proceed, after first checking that it is now due west (250 M) as it should be. If the bearing gets smaller we are slipping north of our track; if it gets larger we are getting pushed south. If it gets larger than 254 M, we are headed for the reef, as shown in Figure 3.</div><div><br /></div><div>Note that this type of measurement requires a good bearing compass such as the hockey puck or equivalent.<br />
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<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiluCAi9BIJiJQjxP5gVxFcvijuQQrjmOSU1agiykqXzS8k-N8OVMLy6ofom910P__L5XgRIDZpqhNXDRhq7CQd0bb1VCDeDq3IEzP9q1mSen7hqtnqGSi_ruOhKtUCnl4qXz_zRZltahk/s1600/BWS_Danger+Bearing.jpg" style="margin-left: auto; margin-right: auto;"><img border="0" height="284" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiluCAi9BIJiJQjxP5gVxFcvijuQQrjmOSU1agiykqXzS8k-N8OVMLy6ofom910P__L5XgRIDZpqhNXDRhq7CQd0bb1VCDeDq3IEzP9q1mSen7hqtnqGSi_ruOhKtUCnl4qXz_zRZltahk/s1600/BWS_Danger+Bearing.jpg" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><b><br />Figure 3.</b> <i>Selecting a danger bearing to the QG light dead ahead at the start. The 0.5 nmi range rings mark distance from the starting point, which helps for quick DR chart plotting from log readings. Note on this older chart, there was a US buoy there with name FR for Fulford Reef, but that was later replaced by a Canadian buoy VK shown in Figure 1. The reef name is also different on newer charts.</i></td></tr>
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<br /><br /></div><div style="text-align: center;"><b>Line of Soundings Navigation</b></div><div><br /></div><div>In this hypothetical situation, the only thing we have left to use is our sounder. This is not an ideal route for depth sounding navigation, but even in this case there are still a few guidelines that can assist our navigation. The main point for now is to show how powerful the ECS is for setting up sounder navigation. To do this accurately we need tide height, which <a href="https://youtu.be/yafYQvOpgGg" target="_blank">we also get from the ECS program</a> with the click of a button.</div><div><br /></div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwPCGz_Rk22LLmiAt7-Bwl5oDBegkKygdwPZI4gd9mBckNo5NBOG2R6V0M1oTGiuS6pMSQxLr3ki9k2M5_Bz3HY3jvEqowOEMWwP8Vng0_x24ugB7egQ7LyjVCCfiugJXBTAbH58Pg0kP1nZBMxxlbLSmQrhnlvW5LeLojloz_lRwODfFijZ2Kd86p/s1569/sounder%20depths.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1499" data-original-width="1569" height="306" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwPCGz_Rk22LLmiAt7-Bwl5oDBegkKygdwPZI4gd9mBckNo5NBOG2R6V0M1oTGiuS6pMSQxLr3ki9k2M5_Bz3HY3jvEqowOEMWwP8Vng0_x24ugB7egQ7LyjVCCfiugJXBTAbH58Pg0kP1nZBMxxlbLSmQrhnlvW5LeLojloz_lRwODfFijZ2Kd86p/s320/sounder%20depths.png" width="320" /></a></div><div><br /></div><div style="text-align: center;"><b>Figure 4.</b><i style="font-style: italic;"> Corrections to be applied to the sounder readings so they can be compared to the charted depth contours. In the following, we assume these corrections were made so we are working with corrected depths.</i></div><div style="text-align: center;"><i style="font-style: italic;"><br /></i></div><div style="text-align: center;"><i style="font-style: italic;"><br /></i></div><div style="text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi_1ZQNM9em7BHJu1nviYK2YWZbWqriEUu7MpKlOMG0Sa3RfQfA_JiefZsGhdhh6qqXbvmljBKbO_CjlwlHOotvxkW-7XAA1fjOsGO_WHU4lEp7TXTga0qYqS_-6rdMBCnFzgXykZvBte4Raew0uYo_v9PX1qXGRNXxHa2Np8mbkD2KZGt4elfr-wBp/s6668/BWS_sounding%20log%20fathoms_with%20Plot.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="3118" data-original-width="6668" height="300" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi_1ZQNM9em7BHJu1nviYK2YWZbWqriEUu7MpKlOMG0Sa3RfQfA_JiefZsGhdhh6qqXbvmljBKbO_CjlwlHOotvxkW-7XAA1fjOsGO_WHU4lEp7TXTga0qYqS_-6rdMBCnFzgXykZvBte4Raew0uYo_v9PX1qXGRNXxHa2Np8mbkD2KZGt4elfr-wBp/w640-h300/BWS_sounding%20log%20fathoms_with%20Plot.png" width="640" /></a></div><br /><div style="text-align: center;"><b>Figure 5</b><i>. A line of soundings made from an ECS program. The data can be accumulated in an automated plot or recorded by hand. Range rings at 0.5 nmi set on the starting point helps monitor the correlation between depth and DR position. Though not the best bottom for sounding navigation, we still have a few things to note. First the depth should drop off rapidly, then carry on slowly deepening. If it should go up at about 0.5 - 0.7 nmi off (marked red in the plot), we know we are getting pushed south. Then if we are right on track, we should see the 19-fathom bump at about 1.7 nmi off (marked green in the plot) </i></div><div><br />
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<span style="font-size: small;"><b><span face="Arial,Helvetica,sans-serif"> ECS Line of Soundings Procedure</span></b></span></div>
<span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br />Whenever the water is not too deep (meaning our sounder works—often referred to as "within soundings") and not too flat, we can usually gain useful nav info from a line of soundings. Here is a fast and accurate way to set this up using ECS.</span></span><br />
<span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">(1) Look ahead to see if depths or contours are favorable and if so choose a couple to monitor, and make rough estimates of what you expect if you are where you think you are.</span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">(2) Check tide height to figure net correction to sounder reading, i.e., if sounder is 2 ft below water level with 5 ft of tide, then sounder reading in feet + 2 - 5 = charted depth expected.</span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">(3) Note time, speed, and heading, and log preferably, and start the plot of depth vs log. The goal is to have a log reading or time for each notable sounding change.</span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif"><br /></span></span></div></div><div><div><span style="font-size: small;"><span face="Arial,Helvetica,sans-serif">(4) After crossing the contours of interest, figure distances between key depths or contours and use ECS plotting tools to locate most likely track around the contours, as shown in the example below. Range rings on movable marks is one way to check for closest path across the contours.</span></span></div></div></blockquote><p> </p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/AcZmvpBVSYQ" width="320" youtube-src-id="AcZmvpBVSYQ"></iframe></div><div><br /></div><div style="text-align: center;"><i>Example of Line of Soundings Fix using ECS</i></div><p>With paper charts you can draw a line on transparent paper in the right orientation, and mark off the measured depths using the same miles scale as the chart. Then slide the transparency around till you match your measurements. <a href="https://youtu.be/BSKwmWmNYF8" target="_blank">Line of Soundings Fix 2,</a> is a video example of the paper solutrion.</p><div><div>
<br />These are just a couple chart navigation exercises that are nicely solved by ECS without GPS. There are many. Besides the range and bearing tool for standard plotting, a versatile option that is often overlooked is the ability to set multiple range rings on any mark or waypoint, or your vessel icon. This has numerous applications, we have shown only one.<br />
<br />The navigation procedures described here are adapted from our textbook <i><a href="http://www.starpath.com/catalog/books/1985.htm" target="_blank">Inland and Coastal Navigation, 2nd Edition</a></i>.<br />
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<br /><br /><a href="https://youtu.be/BSKwmWmNYF8" target="_blank">Line of Soundings Fix 2</a></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com2tag:blogger.com,1999:blog-714718709601620866.post-72520503094658202022023-04-17T15:40:00.009-07:002023-07-24T14:16:45.118-07:00ASCAT Data in Grib Format — Gone and Back Again<p><i>Update Apr 23, 2023: Check comments at the end. Since this article was first posted, this data has come back, and then gone, and come back again. </i></p><p>ASCAT is the name of a scatterometer aboard two European satellites, Metop A and B. They provide real near live surface wind observations several times a day to mariners located in any ocean worldwide. They provide the exact data we need as sailors, as if the ocean were covered with anemometers on buoys, but more important they also provide the exact data we need to verify numerical weather models such as GFS or ECMWF. The primary US source online is NOAA's <a href="https://manati.star.nesdis.noaa.gov/datasets/ASCATData.php" target="_blank">Center for Satellite Applications and Research</a> (STAR). Which shows global wind data, updated hourly in graphic format.</p><p>The introduction of scatterometer data to marine navigators in grib format came from an exclusive experimental arrangement between the Ocean Prediction Center (OPC) and the Pacific Northwest communications company <a href="https://www.ocens.com/" target="_blank">Ocens</a>, who was the sole source for this valuable data in grib format from about 2007 to 2009. In exchange for their unique access to the data, they agreed to do the custom programming to show this complex data set. At that time, they were distributing data from a specific US satellite scatterometer program, QuikSCAT, and the Ocens program ended when the QuikSCAT program ended in 2009. </p><p>There followed a period of time with no grib access to scatterometer data, until about 2015 when the OPC made data from the new European ASCAT program available to the public in grib format for the first time. At that time Ocens offered it again, but now there were other sources, namely <a href="http://www.luckgrib.com" target="_blank">LuckGrib</a> and <a href="http://expeditionmarine.com" target="_blank">Expedition</a>, who both offered data downloads and display. This did indeed take extensive special programming from both of these companies, because even in grib format this data set was not at all like showing a standard wind forecast from, say, GFS.</p><p>Then, completely unannounced, in 2018 OPC discontinued the distribution of ASCAT data in grib format. Reasons for this were not clear, but we had informed hopes that the problem would be resolved. But as time passed, it seems with COVID limiting the workforce and likely other reasons, this simply did not happen. No more ASCAT in grib format.</p><p>The digital data were still available, but no longer in grib format. It was available in a more complex format called netCDF. Scientists have viewers that show this data, but mariners typically do not. We posted an <a href="http://davidburchnavigation.blogspot.com/2020/01/viewing-netcdf-weather-files-in-panoply.html" target="_blank">article and video on how to use Panoply</a>, one of those programs, but this was not a practical solution. Thus we were again with no ASCAT data in grib format.</p><p>Then in early 2022 LuckGrib took up the challenge to convert the NetCDF to grib format so it could be displayed in the LuckGrib viewer—or once downloaded from LuckGrib it could also be viewed in Expedition. So we were back with access to ASCAT in grib format thanks to LuckGrib.</p><p>But in March of 2023, NOAA removed its traditional (OpenDAP) access to the netCDF data. They simply stopped providing this data in the same way they provide all the other weather data they distribute. In short, anyone, including LuckGrib, who might want to offer this data to mariners is once again faced with a long, complex programming job... that would be hard to justify, especially since over the past year, the data availability had not been as consistent as it was earlier. Sometimes the data were not updated for 3 to 5 days.</p><p>In short, we are back in a mode of no ASCAT data in grib format, and this one is likely stick for a while.</p><p>It is true that this grib data have not been used as much as it should be by the overall marine community. We cannot take blame for that. For years we have taught that this is the most valuable way to check a model forecast—we have a long chapter in <i><a href="https://www.starpath.com/catalog/books/1886.htm" target="_blank">Modern Marine Weather</a></i> on ASCAT. But it is definitely not plug and play. It takes effort to coordinate (in both space and time) when useful data are available, especially when we want to overlay the grib versions of both model forecast and an actual ASCAT measurement.</p><p>Below is a sample of how nicely and precisely we <i>used to be able</i> to compare forecast and measurement using grib format.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh3mAZQgLL-m-lqeHe4Shnc8jUAZ19EzPL524yqhx96G82hxMKem8I3MlI2ydanEj8CZ9fpmqO16WJw7kYiqekFP27_TIAhyb56lvW2oE-g_WO_Ygn9qInYgBt5LxbFpvmtIqz1I2U-EnHuexsSxDgwGyu7o2VQ79T9mS2gG5dx9_NumTOZ3wMXKbGl/s1037/LG%20ASCAT%20at%2015h%20annotated.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="766" data-original-width="1037" height="472" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh3mAZQgLL-m-lqeHe4Shnc8jUAZ19EzPL524yqhx96G82hxMKem8I3MlI2ydanEj8CZ9fpmqO16WJw7kYiqekFP27_TIAhyb56lvW2oE-g_WO_Ygn9qInYgBt5LxbFpvmtIqz1I2U-EnHuexsSxDgwGyu7o2VQ79T9mS2gG5dx9_NumTOZ3wMXKbGl/w640-h472/LG%20ASCAT%20at%2015h%20annotated.png" width="640" /></a></div><i><div style="text-align: center;"><i>An ASCAT pass of measured surface wind speeds on July 2, 2022, downloaded from and viewed in LuckGrib. The colors are not wind speed, but rather a time scale of relative valid times. The red sector is enlarged below.</i></div></i><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkW8QHjCbwO6UBpVfCrvTQCYbYC2NAejclZfaxTvGKvwfySAFulVjt5Q50rI64Tf3CxfMls2Rw_Ts26lc1ewI5eCsKQo42U0mliSbOY21rPLVGniTo-7tUUZqq3GUeOgPcJwXqmuJmJMKZL-i1K8Fz01wUw3Ad3CUmgGjYr2T2NM948QFRKp8TZ7SY/s1229/ASCAT%20202207021151.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="879" data-original-width="1229" height="458" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgkW8QHjCbwO6UBpVfCrvTQCYbYC2NAejclZfaxTvGKvwfySAFulVjt5Q50rI64Tf3CxfMls2Rw_Ts26lc1ewI5eCsKQo42U0mliSbOY21rPLVGniTo-7tUUZqq3GUeOgPcJwXqmuJmJMKZL-i1K8Fz01wUw3Ad3CUmgGjYr2T2NM948QFRKp8TZ7SY/w640-h458/ASCAT%20202207021151.png" width="640" /></a></div><div style="text-align: center;"><i>In this image from LuckGrib, the colors represent wind speeds. These actual measured winds can be (could have been!) compared with the GFS forecasted winds. This pass took place at 1151z on July 2, 2022.</i></div><div style="text-align: center;"><i><br /></i></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOu1MncSzpItlB6Szj2e1h8CS0l8T-ELJgpwOVSCDaTfRDSg6Zk_SVM9n10eo3EnLd9y2BHRkJs2wo8wPAr0136CfC6PUuO3xRIprE_pdY5GqlkoO75UKMdSJgp5_PbtXJxZ5uYJ5KydVz2ZEdUMiRxmcxcWUbXUTtE9zRC8nJWJgvNYWnbps4ucMd/s1226/GFS%20202207021200%20annotated.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="882" data-original-width="1226" height="460" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhOu1MncSzpItlB6Szj2e1h8CS0l8T-ELJgpwOVSCDaTfRDSg6Zk_SVM9n10eo3EnLd9y2BHRkJs2wo8wPAr0136CfC6PUuO3xRIprE_pdY5GqlkoO75UKMdSJgp5_PbtXJxZ5uYJ5KydVz2ZEdUMiRxmcxcWUbXUTtE9zRC8nJWJgvNYWnbps4ucMd/w640-h460/GFS%20202207021200%20annotated.png" width="640" /></a></div><div style="text-align: center;"><i>Here is the 12z GFS wind forecast for the region shown above. This comparison is within minutes, so should be good. We added the red lines so we can see where we have actual measurements.</i></div><div style="text-align: center;"><i><br /></i></div><div style="text-align: center;"><i><br /></i></div><div>The value of the above comparisons in grib format is we can zoom in and look at specific digital values for a precise comparison.... and indeed we see here that the GFS analysis was very good, covering a large span of wind speeds and subtle wind angle changes. There will be times and places, where the ASCAT reveals weakness in the GFS forecasts.</div><div><br /></div><div>So that is the type of comparison we can no longer count on.</div><div><br /></div><div>But all is not lost... we just return to another section of <i>Modern Marine Weather</i> that explains how we can access the graphic presentation of the ASCAT winds with an email request at sea. Then these images can be georeferenced and displayed with the model wind forecasts overlaid on them for the same time of model evaluation. The challenges of setting up the best comparison is still the same, but these graphic ASCAT data remain available.</div><div><p>Programs like Expedition and qtVlm have very convenient ways to georference an image once, and then do a single button click for further updates. We have many of these georeferenced files already created, which takes that first step out of the process. </p><p>We should also not forget <a href="http://www.opencpn.org" target="_blank">OpenCPN</a>, which like qtVlm is free for both Mac and PC computers. It has a very good button-click georeferencing display option in its Weather_fax plugin for the Windows version. The general grib parameter display options are not as strong as in other programs, but the key wind comparison needed for this application works well, and its image display function in the Windows version of OpenCPN is one of the best.</p><p>Below are samples of overlaying a model forecast onto ASCAT wind measurements in graphic format as a way to evaluate the model forecast. These graphic data have been dependably available for many years.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggUb01_1fHAWfkI5CjDd9SOPhYvbm1TU6VkknBJwCSUkexaCjBaDhFpS46o-TL_hg7QZVF-_vFcQKW2cqDLgDPhWJAM6vyIXIJeVRkk9RbhMtk0WT3K5LXWDJ3oyVQsGemdhNGS4UoP8iVV_J3omyJTXXOiptPmL7gNnn_TyXfdGMje2QIwTHAbmAa/s3181/ASCAT122%20in%20qtVlm.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2161" data-original-width="3181" height="434" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggUb01_1fHAWfkI5CjDd9SOPhYvbm1TU6VkknBJwCSUkexaCjBaDhFpS46o-TL_hg7QZVF-_vFcQKW2cqDLgDPhWJAM6vyIXIJeVRkk9RbhMtk0WT3K5LXWDJ3oyVQsGemdhNGS4UoP8iVV_J3omyJTXXOiptPmL7gNnn_TyXfdGMje2QIwTHAbmAa/w640-h434/ASCAT122%20in%20qtVlm.png" width="640" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj79zcpY7kUoOjokAumn3mnkhOyT_mSSVpvfK0nQw3qiTBy9aMVd9OZIWhYtF2PMAvWywGJ3U0Q1j3DEe1PjgLb_XhTCb6ys2rJ5p7VvMlhJ-wpuYOnDZC5bqHx01zh8pa46e-8ZyL7_tTM5xoimG49v4Qvn0Aohh7Ro9nL9zet9sYGXAPEuy_cq6hb/s3157/ascat%20110%20in%20qtVlm.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2164" data-original-width="3157" height="438" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj79zcpY7kUoOjokAumn3mnkhOyT_mSSVpvfK0nQw3qiTBy9aMVd9OZIWhYtF2PMAvWywGJ3U0Q1j3DEe1PjgLb_XhTCb6ys2rJ5p7VvMlhJ-wpuYOnDZC5bqHx01zh8pa46e-8ZyL7_tTM5xoimG49v4Qvn0Aohh7Ro9nL9zet9sYGXAPEuy_cq6hb/w640-h438/ascat%20110%20in%20qtVlm.png" width="640" /></a></div><p style="text-align: center;"><i>Overlaying a GFS forecast on the graphic presentation of ASCAT wind measurements using qtVlm. These are the same data shown above, but not at the same locations. The graphic files are about 37 kb so they can be requested by email. The files names and how to get them are discussed in Modern Marine Weather. We also have a <a href="https://www.youtube.com/watch?v=6eK4rf-f4-c" target="_blank">video on the process</a>.</i></p><p><br /></p><p>We no longer have the digital read out of the ASCAT winds, but with little practice the color bar provided is adequate for the wind speed comparisons, where we have to accept ±2.5 kts as in agreement. Also it is important to note that the convenient grib format had a very long latency time, because it took a long time to process this data once accumulated. The graphic data, on the other hand, is updated much more quickly so we can always compare the latest forecast with an appropriate satellite pass.</p><p>Here are the various ways we can view such data in Expedition now and in the past when we had the ASCAT grib data. </p><p><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLrmh-1LS-7xqTTIgW55ZQqk4J8qWlltcHPoiyoL_dm_LazmmP-u3V8Bn8RlGFWcOJBtKRnWdFkOek6Gf5rdTLNeP8vhlnxOJBvgfkcaRsg-Ui5GOAJ6eZmVXACtHlSd7IfNYiubIefRkxPS-BFeWlfmla6Ls8MGsfoJPOF_SDyeHFlqeJjNw3Kps1/s3166/1%20Expeidtion%20asct%20grib%20and%20graphic%20overview.png" style="margin-left: 1em; margin-right: 1em; text-align: center;"><img border="0" data-original-height="1620" data-original-width="3166" height="328" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLrmh-1LS-7xqTTIgW55ZQqk4J8qWlltcHPoiyoL_dm_LazmmP-u3V8Bn8RlGFWcOJBtKRnWdFkOek6Gf5rdTLNeP8vhlnxOJBvgfkcaRsg-Ui5GOAJ6eZmVXACtHlSd7IfNYiubIefRkxPS-BFeWlfmla6Ls8MGsfoJPOF_SDyeHFlqeJjNw3Kps1/w640-h328/1%20Expeidtion%20asct%20grib%20and%20graphic%20overview.png" width="640" /></a></p><div style="text-align: center;"><i>Here we see the ASCAT as grib file overlaid on one of the ASCAT graphic image files from above.</i></div><div><br /></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVySJderiOTw5vK5CJKaRHp62g4Ic28zpctbB1qLbsnnBGavuBO95kggJHxnatdpLBd_t_h0xxjW4tWYKFfZL50khbcqfhWz-kHo1Wb4Y24f0n8oY03Ps6ZlHYP1iyuht5N5nm1OLmzPOIPeD6-piJGK9ONMm1crllZS-70SXi0GDV7cGBh6BcGeaI/s3168/2%20Expedition%20ascat%20grib%20and%20graphic%20close%20up.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1613" data-original-width="3168" height="326" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVySJderiOTw5vK5CJKaRHp62g4Ic28zpctbB1qLbsnnBGavuBO95kggJHxnatdpLBd_t_h0xxjW4tWYKFfZL50khbcqfhWz-kHo1Wb4Y24f0n8oY03Ps6ZlHYP1iyuht5N5nm1OLmzPOIPeD6-piJGK9ONMm1crllZS-70SXi0GDV7cGBh6BcGeaI/w640-h326/2%20Expedition%20ascat%20grib%20and%20graphic%20close%20up.png" width="640" /></a></div><div style="text-align: center;"><i>Here we zoom in to the pic above to see that the graphic data and the grib data are the same.</i></div><div><br /></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgzrfIqrRZSA8JmweZ4sbyRpbiHbdMMtoytJJxrigFTyCatv731BQ4qWQINeTtK9vAlTRvcJR9ASBYu7CoN6Np2NaljgZQxpaAYL4usLQiUw0X_2_VBa4XJxSMsCnIrnDecUxBdPAk9MlWdoXjrdmPFVekSEakJlK273_WyKtF-pA-HOjit6XWczAVd/s3165/3%20Expedition%20ascat%20and%20GFS.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1635" data-original-width="3165" height="330" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgzrfIqrRZSA8JmweZ4sbyRpbiHbdMMtoytJJxrigFTyCatv731BQ4qWQINeTtK9vAlTRvcJR9ASBYu7CoN6Np2NaljgZQxpaAYL4usLQiUw0X_2_VBa4XJxSMsCnIrnDecUxBdPAk9MlWdoXjrdmPFVekSEakJlK273_WyKtF-pA-HOjit6XWczAVd/w640-h330/3%20Expedition%20ascat%20and%20GFS.png" width="640" /></a></div><div style="text-align: center;"><i>Here we overlay the GFS grib forecast with the grib version of the ASCAT data which was, when we had it, an effective way to check the GFS. See below</i></div><div><br /></div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjc5fsjyz76gqcUeixjXyAePxymfSWgDyPzXSrrKvzVOZozIDvts76OTQ-54ewb4ubo35rqhL8PHPEeoCiMIkcWzx5RaKZfs62c9fWXf-QE_cMuhnX-y4Ht2F_9BhT75QuopltsBHq1zrE2UsmaEPVpCNCPDQcxY5c5tiOhizRmBvd6SK5pC1SEJcBQ/s3152/4%20Expeidtion%20GFS%20and%20ascat%20grib%20close%20up.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1619" data-original-width="3152" height="328" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjc5fsjyz76gqcUeixjXyAePxymfSWgDyPzXSrrKvzVOZozIDvts76OTQ-54ewb4ubo35rqhL8PHPEeoCiMIkcWzx5RaKZfs62c9fWXf-QE_cMuhnX-y4Ht2F_9BhT75QuopltsBHq1zrE2UsmaEPVpCNCPDQcxY5c5tiOhizRmBvd6SK5pC1SEJcBQ/w640-h328/4%20Expeidtion%20GFS%20and%20ascat%20grib%20close%20up.png" width="640" /></a></div><div style="text-align: center;"><i>Here we zoom in to see that for the most part the model is spot on but there are regions in this complex flow of light air where they do not agree, so if we are routing across this region we can take that into account.</i></div><div style="text-align: center;"><br /></div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDzwFSt_efdEVfsgCj2ZiIVV7BvsMOFkODrX29jOO7baax_GxECvDTR9bn2CN8BAWccOy7JuLGruV7dPM3V8s33XRUngYPBUiFIezzcr8Ap3vCDmISKK45CPyCQ7KI_7hQOOsOVEWiL0EE_m3_x5rOQTKwdnXQhr7HM5DRNBE_Ze90J06pehprR7KN/s3175/5%20Expedition%20GFS%20and%20ascat%20graphic.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1618" data-original-width="3175" height="326" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjDzwFSt_efdEVfsgCj2ZiIVV7BvsMOFkODrX29jOO7baax_GxECvDTR9bn2CN8BAWccOy7JuLGruV7dPM3V8s33XRUngYPBUiFIezzcr8Ap3vCDmISKK45CPyCQ7KI_7hQOOsOVEWiL0EE_m3_x5rOQTKwdnXQhr7HM5DRNBE_Ze90J06pehprR7KN/w640-h326/5%20Expedition%20GFS%20and%20ascat%20graphic.png" width="640" /></a></div><i><div style="text-align: center;"><i>Finally we come to the way we will make this comparison going forward by overlaying the model wind forecast onto the graphic image of ASCAT data. Expedition has a super easy way to georeference these images since they have a prominent graticule in view.</i></div></i><p><br /></p><p>I will try to follow through shortly and show how this can be done in the PC version of OpenCPN. </p><p>We have an index of ASCAT related info at <a href="http://starpath.com/ascat.">starpath.com/ascat.</a></p><p>In the meantime, I would like to thank Craig McPheeters, author of LuckGrib, for all the fine work he put into this project over the years. It was all in the right direction.</p><p>Nick White, author of Expedition, also played a key role in this unique chapter of marine navigation history that many ocean racing sailors benefited from. Maybe one day down the line we will get this data back from the government... in some form.</p><p>The time scale is not clear, but the next generation of electronic charts called S-100 has a component for universal weather distribution called <a href="https://ocean.weather.gov/S-41X/index.php" target="_blank">S-41X</a>. This system and indeed all weather and environmental data under the S-100 format will not use the grib format. They plan to use a more generalized format called <a href="https://en.wikipedia.org/wiki/Hierarchical_Data_Format" target="_blank">HDF5</a>. Thus if all data does indeed end up in HDF, we might get the ASCAT back in that format, which, presumably, by then we would all have nav apps that could read it.</p><p>Grib is a unique format developed by the WMO specifically to transmit weather data. But I have been assured by the WMO that they are adhering to S-100 standards when ready, and they are all HDF, not grib. I was informed that if anyone wanted grib in S-100, they can propose a change to S-100, and the next opportunity for that will be in five years. In short, they are killing their baby.</p><p>But this will be a while down the line. We might be fairly well into transmigration by then.</p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-67618755705307455982023-02-25T15:11:00.004-08:002023-02-25T17:04:35.227-08:00Rock Symbols in ENC<p>While working on another project, it seemed that it would be nice to have one more example of how rocks show up on an ENC, which is what we provide here. We have several earlier notes on rock symbols</p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p><a href="http://davidburchnavigation.blogspot.com/2018/08/chart-symbols-rock-or-coral-on-rncs-and.html" target="_blank">Chart Symbols: Rock or Coral on ENC</a></p><p><a href="http://davidburchnavigation.blogspot.com/2016/11/rock-talk-is-it-all-awash-or-not.html" target="_blank">Rock Talk 1, Is it Awash or Not?</a></p><p><a href="http://davidburchnavigation.blogspot.com/2016/11/rock-talk-2-rnc-to-enc.html" target="_blank">Rock Talk 2, RNC to ENC</a></p></blockquote><p>and this image from the last one, summarizes the situation.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggghnW9nTd9hodNar-DJZs61ScMUFvmVjeHhK4usrfCT8wpyJOWVuOiuL9gL39zruHFiGdQGxey1GWZSJ71uOVH7pBqC0pJTfkbMDopsMzw7BuMJOPUa4CYgzZ4lRTjQfY1gMNEBHzwv3VpAJzPp2e3jQt_JveYh468qrSPPCPnBqgff20nz9HG5b6/s1260/rock%20summary.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="375" data-original-width="1260" height="190" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggghnW9nTd9hodNar-DJZs61ScMUFvmVjeHhK4usrfCT8wpyJOWVuOiuL9gL39zruHFiGdQGxey1GWZSJ71uOVH7pBqC0pJTfkbMDopsMzw7BuMJOPUa4CYgzZ4lRTjQfY1gMNEBHzwv3VpAJzPp2e3jQt_JveYh468qrSPPCPnBqgff20nz9HG5b6/w640-h190/rock%20summary.jpg" width="640" /></a></div>The project that brought back the rock symbol question is the very subtle way that a properly presented ENC (i.e., according to IHO Standard S-52) decides when to show the isolated danger symbol to replace a standard rock symbol. <div><br /></div><div>So the goal here was to show the above samples in an actual charting program, where we can change our choice of safety contour and decide if we want to see these symbols inside of the safety contour, or just outside of it. That article is forthcoming. For now we just look at samples of the rock symbols in play and have this addition resource for a quick reference.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0LEZsmyRhtQvt3Ho3iHYlqeW3snWRMpQrocrsVOPcOXEf16N4hhFji7K_zwC5l2v9mNpxbdJIKBY4hTPLO-O-VtP84r8Xiv1D_pD5-UfqliXt6dcOeF4cBziKX8Yv02CX16LVSdjh8fpwLUV8GS95wgkhHM6kEI4aCRfZTHkJq1xvLLkDPTktBuS-/s2657/Rocks%20on%20ENC%20fixed.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1469" data-original-width="2657" height="354" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0LEZsmyRhtQvt3Ho3iHYlqeW3snWRMpQrocrsVOPcOXEf16N4hhFji7K_zwC5l2v9mNpxbdJIKBY4hTPLO-O-VtP84r8Xiv1D_pD5-UfqliXt6dcOeF4cBziKX8Yv02CX16LVSdjh8fpwLUV8GS95wgkhHM6kEI4aCRfZTHkJq1xvLLkDPTktBuS-/w640-h354/Rocks%20on%20ENC%20fixed.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;"><div><br /></div></div><div class="separator" style="clear: both; text-align: left;">Here we see the pick reports for each of the rocks that shows most, not all, of the information needed to decide when the symbol might be replaced with the danger symbol, example 5 above.</div><div class="separator" style="clear: both; text-align: left;"><br /></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-87808132729256679152023-01-20T13:11:00.000-08:002023-01-20T13:11:04.837-08:00Sail toward the wind shift<p><span style="background-color: white;"><span face="Arial, Helvetica, sans-serif">The hallmark of all good navigation is looking ahead. Under sail, the main thing we want to anticipate is wind shifts—currents and traffic are other important factors to plan around, but for now we look at the wind. We look at two practical examples of the general guideline when sailing to weather to sail toward the direction the wind is expected to shift toward. If the wind is anticipated to shift left, then we want to be on the left side of the course when that happens. </span></span></p><p><span style="background-color: white;"><span face="Arial, Helvetica, sans-serif">We can often anticipate wind shifts based on the terrain, with no weather pattern changes—such as channeled wind bending into an open bay on one side of the channel—but we also get insights from forecasts. An approaching Low generally brings winds backing to the south, which in a narrow channel running N-S will force a southerly wind to a south-southeasterly in the channel. When a front crosses such a narrow channel, the S-SE wind makes a 100% predictable sudden shift to the S-SW because wind always veers at a front.</span></span></p><p><span face="Arial, Helvetica, sans-serif" style="background-color: white;">In the first example we have a south wind across the area during the start and most of the way to the finish, but there is a strong adverse current in the channel, which means we must choose right or left side to stay out of the strong current in the middle—current is always weaker in the shallower water along the edges. But we have a forecast that the wind will be shifting to the SE or S-SE as a low approaches. We do not know when, but definitely before we get to the south end.</span></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAkOxOQ8wzEqB0DIY4Sd6UOU8KrctmgMGiNoR3Lk7GL480rspterOJkB8IO7qdcLhOwP2XDOlyxtA_6U8Azx90IMcTu9tmzbyZXHL5XZASoyt7WHRhbFI20Boc9hEsxrYCmwh_h1VBU2md2BCG8HLJm_7Qtle0aIgj7wy0EnM1g-iF0mReJwk8b-x_/s1266/sail%20to%20shift%201.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1266" data-original-width="628" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAkOxOQ8wzEqB0DIY4Sd6UOU8KrctmgMGiNoR3Lk7GL480rspterOJkB8IO7qdcLhOwP2XDOlyxtA_6U8Azx90IMcTu9tmzbyZXHL5XZASoyt7WHRhbFI20Boc9hEsxrYCmwh_h1VBU2md2BCG8HLJm_7Qtle0aIgj7wy0EnM1g-iF0mReJwk8b-x_/w318-h640/sail%20to%20shift%201.png" width="318" /></a></div><p></p><div class="separator" style="clear: both; text-align: center;"><i style="background-color: white;">S wind changing to SE. Here we show it happening just at the south end, but this is meant to illustrate where the boats are when the wind all along the channel went from S to SE.</i></div><div><span style="background-color: white;"><br /></span></div><div><span face="Arial, Helvetica, sans-serif" style="background-color: white;">We can choose the east or west side and then tack down the beach. But since we know the wind is shifting to the east (S to SE) we want to be sure to take the east side. See what happens when the shift occurs. The boat on the east side then has a fast reach right to the mark, whereas the boat on the west side has to keep on tacking to get there. This is a shift to the left, so we want to be on the left side.</span></div><span style="background-color: white;"><br style="font-family: Arial, Helvetica, sans-serif;" /><span face="Arial, Helvetica, sans-serif">Below we have another realistic scenario. The wind is now from the SE along the water way and it does not take long as you tack down the middle of the Sound to notice with binoculars that boats along the shorelines are not tacking, they are reaching right down the beach. They are taking advantage of the fact that wind on land is notably backed (shifted to the left) relative to wind on the water. This is a friction effect. And the transition does not take place right at the shoreline, but it extends out into the water to some extent and these reaching boats are riding along in that wind.</span><br style="font-family: Arial, Helvetica, sans-serif;" /><br /><span face="Arial, Helvetica, sans-serif">Thus in this situation, the logical route is along the shore to take advantage of that, and we could go down either shoreline. But in this scenario, we know from the forecasts that a front is going to pass over us before we get all the way down, so we have a logical side to choose. When a front crosses the sound the wind veers (shifts to the right) in a matter of minutes, so this wind going from SE to SW is shifting to the west, so we want to be on the west side—or put another way, shifting right, we want to be on the right.</span><br style="font-family: Arial, Helvetica, sans-serif;" /><br style="font-family: Arial, Helvetica, sans-serif;" /><span face="Arial, Helvetica, sans-serif">And you see again what happens when the two boats meet that shift, one on the east side has to start tacking to the mark, and the one on the west side has a fast reach right to it.</span></span><div><span style="background-color: white;"><br /></span></div><div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKTvtbvrd6k69q80JEK-W1s4MUiPYyZxAWRsdqXE1-4G5eDehRMqaoXtnYRVoEsYP3vKS42NQO_32x2Y4tEHMoKHfjpdZZrgaD_YYe9u3-gy0HE_1SF_2LheOqX0mQyOZvRKHPTgG0-Pts0KEx7vXgBOwsHCKnQFP00wlL-6FwiC75PrJxuCZCgZ-b/s1258/sail%20to%20shift%202.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1258" data-original-width="618" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgKTvtbvrd6k69q80JEK-W1s4MUiPYyZxAWRsdqXE1-4G5eDehRMqaoXtnYRVoEsYP3vKS42NQO_32x2Y4tEHMoKHfjpdZZrgaD_YYe9u3-gy0HE_1SF_2LheOqX0mQyOZvRKHPTgG0-Pts0KEx7vXgBOwsHCKnQFP00wlL-6FwiC75PrJxuCZCgZ-b/w314-h640/sail%20to%20shift%202.png" width="314" /></a></div><div><div class="separator" style="clear: both; text-align: center;"><i style="background-color: white;"><br />SE wind veering to SW. Here we show it happening just at the south end, but this is meant to illustrate where the boats are when the wind all along the channel makes this change from SE to SW.</i></div><div class="separator" style="clear: both; text-align: center;"><span style="background-color: white; text-align: left;"><div><i><br /></i></div></span></div><div class="separator" style="clear: both; text-align: left;"><br /></div></div><div><span face="Arial, Helvetica, sans-serif" style="background-color: white;">The summary is: if the wind is shifting to the right, we want to be on the right side; shifting left, we want to be on the left side. To think through the shift direction, draw the wind arrows point to point as shown below.</span></div><div><span face="Arial, Helvetica, sans-serif" style="background-color: white;"><br /></span></div><div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJGXU2hMhVyfs4VehIsoj-kw5iUxf-NCTmjWtM-JTN2Oeduou-CqUhUI875aSj2xVXPOR0hqjhTzZLdI-GlzzOGg45oHl2OodCSr_RcbUulncPnNw82YR6Lt3tHsnyrr9X4iN68A7NCnEUzoMqK0Fh8RsM3hmYpMNdAd6q_BBnTvTaycfBolUXStUi/s1214/sail%20to%20shift%203.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="984" data-original-width="1214" height="259" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhJGXU2hMhVyfs4VehIsoj-kw5iUxf-NCTmjWtM-JTN2Oeduou-CqUhUI875aSj2xVXPOR0hqjhTzZLdI-GlzzOGg45oHl2OodCSr_RcbUulncPnNw82YR6Lt3tHsnyrr9X4iN68A7NCnEUzoMqK0Fh8RsM3hmYpMNdAd6q_BBnTvTaycfBolUXStUi/s320/sail%20to%20shift%203.png" width="320" /></a></div><br /></div><div><span face="Arial, Helvetica, sans-serif" style="background-color: #dedfdf;"><br /></span></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-61499732141324969822022-11-12T06:55:00.004-08:002022-11-12T11:03:04.533-08:00Adding an image to a PDF<p><span style="font-family: Helvetica;"><span style="font-size: 18px;">The procedures presented here apply to any application of adding an image to a PDF, but we are focused on a specific marine navigation application. </span></span></p><p><span style="font-family: Helvetica;"><span style="font-size: 18px;">The <a href="https://www.starpath.com/NCC" target="_blank">NOAA Custom Charts (NCC) app</a> sometimes has difficulty placing compass roses in best locations.</span></span><span class="Apple-converted-space" style="font-family: Helvetica; font-size: 18px;"> </span><span style="font-family: Helvetica; font-size: 18px;">in which case you can just shut off the compass rose option when creating the NCC and then make one with the free app below. That will generate a NOAA-style compass rose as a PNG file with a transparent background that you can then paste onto the PDF NCC where you choose, and resize as desired.</span><span class="Apple-converted-space" style="font-family: Helvetica; font-size: 18px;"> </span><span style="font-family: Helvetica; font-size: 18px;">See Instructions on adding an image to a PDF.</span></p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b>Preliminary Notes</b></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">The NCC PDF includes the chart itself and several pages of chart notes. You can print all pages of the NCC on a regular printer paper and that gives you a compressed view of the chart, along with the chart notes. At this stage we can then concentrate just on page 1 of the NCC which is the chart itself.</p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><span style="font-family: Helvetica; font-size: 18px;">To remove all pages of the NCC except the chart (on page 1):</span><br /><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1) Open the chart in Google Chrome</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2) Choose Print, top right icon</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">3) Under Destination choose Save as PDF<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">4) Under which pages choose Custom and enter 1 in the field below it</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">5) Print, with the option to rename.</p></blockquote><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b>Owners of Adobe Acrobat Pro, with Mac or PC. This is the easiest solution.</b><span class="Apple-converted-space"> </span> </p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1) Open the PDF in Acrobat</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2) Choose Tools tab and then Edit PDF</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">3) Choose Add Image, navigate to the rose PNG saved, press Open.</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">4) Resize with a corner control; drag to desired location</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">5) Click Close top right of the display.<span class="Apple-converted-space"> </span>Done</p></blockquote><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b>Mac computers use Preview App. (Easy… but involves an unusual trick!)</b></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b><br /></b></p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1) Open the PDF in one Preview window and open the rose PNG in another Preview window.Do this by right clicking the files in the Finder and choosing Open with Preview.</p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2) In the rose window, click the screen to activate it, then do<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>cmd+A to select all, then<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>cmd+C to copy all, then<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>cmd+V to paste what you copied right back on top of the image,<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>then click the center of the image to activate it then</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>cmd+C to copy</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">3) go to the NCC window and click to activate it, then do cmd+V to paste the image onto the PDF</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">4) size as desired and more to desired position, then cmd+S to save and you are done.</p></blockquote><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b>PC computers install free graphics program called <a href="https://inkscape.org" target="_blank">Inkscape</a> and use it.</b></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><b><br /></b></p><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1) Open Inkscape, then menu File/Open and open the NCC PDF</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2) accept all defaults they offer and choose OK</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">3) Position the window so you can see the full PDF</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">4) Do ctrl+A to select All</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">5) Right click image and choose Lock selected objects</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">6) Menu Layer / Layers to open the layers window on the right.<span class="Apple-converted-space"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">7) Lock the top layer shown which is your NCC chart by clicking lock icon</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">8) Click + to add a layer; call it “compass rose”; Position “above current”, be sure it stays highlighted</p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">9) Menu File/Open and open the rose PNG</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">10) Click the rose to activate it, do ctrl+A to select all, then ctrl+C to copy all.</p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">11) Right click the screen and paste the rose</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">12) Position and size it as desired, then menu Save as, choose PDF format with name of choice, and done.</p></blockquote><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com2tag:blogger.com,1999:blog-714718709601620866.post-12967881063041995252022-11-02T12:58:00.008-07:002022-11-02T13:24:19.345-07:00Slow Water Rule<p>Our 50-90 Rule for estimating tidal current speeds between tabulated peak currents and slacks has been adopted by the US Power Squadrons, who have developed a form for its use. What is less well known is our Slow Water Rule for estimating the time the tidal current will remain below 0.5 kts at each slack time. This rule is actually based on the 50-90 Rule as is shown below.</p><p>The Slow Water Rule states that the current stays less than 0.5 kts on either side of slack for a time (in minutes) equal to 60 minutes divided by the peak current speed in knots. This period is usually different on each side of the slack for mixed semidiurnal tides. That very simple form assumes the period from peak to slack is equal to the global semidiurnal average of 3 hr, but there are many cases where this varies from just 2 to over 5. Thus to make the Rule more accurate, replace the 60 min with one third of the actual time interval between peak and slack.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwUV1GiNHxm0IuLXFC8GcGMJ6AUGSsriN2ShCYDfcsWfGs6SIQZU3sHkwJK2LFi4IAHto5Kxx_t-Z_jAvqqniSdaEu4GCZERmxWF3JT-RQPePJ-Pm_ammdBYSGB_9rWU0y84DTPnErxXcoCmnM9H9yaOVxjii7IFy0hjtbuuhdI04wLQ5NnLZ_0BlY/s1234/Slow%20Water.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1200" data-original-width="1234" height="389" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwUV1GiNHxm0IuLXFC8GcGMJ6AUGSsriN2ShCYDfcsWfGs6SIQZU3sHkwJK2LFi4IAHto5Kxx_t-Z_jAvqqniSdaEu4GCZERmxWF3JT-RQPePJ-Pm_ammdBYSGB_9rWU0y84DTPnErxXcoCmnM9H9yaOVxjii7IFy0hjtbuuhdI04wLQ5NnLZ_0BlY/w400-h389/Slow%20Water.png" width="400" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;">A main purpose of this post is to document how this Slow Water Rule comes from the 50-90 Rule, and that is shown in the hand drawn image below.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrQOd_HHdTE1cneCWYWQR5HlrihZhfIaI6CD-qmCntV7hMxaXiDSVR8ex0aU3oyX2RYy0-ERtdwsYIc2YpswkxzLFXWvl68IafV0pnHunsNLwY-Uoab_4LwQc1XsaJWV6SXnjt5yae8HuXqXHn5GttNy6pSNj_3aTesUu4QmFCkLaA0J2_sNjiack0/s3810/the%20big%20proof%20of%20slow%20water.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="3810" data-original-width="2960" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhrQOd_HHdTE1cneCWYWQR5HlrihZhfIaI6CD-qmCntV7hMxaXiDSVR8ex0aU3oyX2RYy0-ERtdwsYIc2YpswkxzLFXWvl68IafV0pnHunsNLwY-Uoab_4LwQc1XsaJWV6SXnjt5yae8HuXqXHn5GttNy6pSNj_3aTesUu4QmFCkLaA0J2_sNjiack0/w498-h640/the%20big%20proof%20of%20slow%20water.png" width="498" /></a></div><br /><div class="separator" style="clear: both; text-align: left;">In this sketch, t3 is one third of the slack to peak times. Alpha is the slow water estimate we propose.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Granted, we are looking backwards with these efforts to do navigation in the head, on the wing. Traditional paper charts are being completely replaced with electronic charts (ENC) and with that more and more mariners will be using some form of electronic chart system (ECS) for their navigation...more commonly called just a "navigation app" or "echart app." Popular free ones are OpenCPN and qtVlm, both of which include excellent tide and currents functions that display and plot the tides or currents—which pretty much answer any question we have about this crucial data. Commercial nav programs also, of course, include excellent tide and current presentations that answer tide and current questions with a couple mouse clicks.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">In short, traditional methods in all aspects of navigation are likely to be set aside as we move more into more electronic charting. We complained about this when GPS came on the scene, and there will likely be more complaining as the charts get transformed from traditional preprinted fixed editions to the new user-designed and user-printed NOAA Custom Charts (NCC). It will take a while for the NCC to settle in, but I am confident that in the end they will be in fact superior to the traditional charts they are replacing. </div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;"><br /></div><br /><p><br /></p>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-31786757002672306172022-10-18T08:19:00.008-07:002023-05-18T13:02:30.545-07:00Viewing HRRR Wind Forecasts in OpenCPN<p><i><b>Errata added May 18, 2023. </b></i></p><p><i>This article was intended to point out a limitation in the HRRR data provided by Saildocs and a <a href="https://davidburchnavigation.blogspot.com/2022/09/wind-direction-in-model-forecasts.html" target="_blank">couple other sources</a> in that the wind directions on the two US coasts could be off by some degrees, which we could easily account for. We do not know exactly when this was corrected but looking today we see that this correction has been made, so the HRRR data from Saildocs is now identical to what we would get from a direct NOAA download. This is good news for many sailors as Saildocs remains the most convenient free source of weather data and we all remain very grateful to them.</i></p><p>__________________</p><p>HRRR (high resolution rapid refresh) is one of the premier regional wind models from NOAA. The native data are distributed through <a href="https://nomads.ncep.noaa.gov/" target="_blank">NOMADS</a> in a grib2 format that uses a Lambert projection, which is very similar to a great circle projection. <a href="http://www.opencpn.org" target="_blank">OpenCPN</a> at present cannot read and display that format, as is the case with many echart programs and grib viewers. With that limitation in mind, <a href="http://www.saildocs.com" target="_blank">Saildocs</a> has done a service for mariners by converting the Lambert projection into a rectangular Lat-Lon projection that can be read in OpenCPN and other grib viewers.</p><p>The attached video shows the process of obtaining the HRRR grib from Saildocs, which is straight-forward, with a couple precautions to guard against large files. We propose a hybrid solution that lets OpenCPN create the email request that we then tweak a bit before sending to Saildocs.</p><p>There are two formats for the HRRR data. The standard model forecasts are computed every hour and there is a forecast made for each hour going forward, out to 18 hr. There is just over a 1.5-hr latency, meaning that a forecast computed at say 15z (z=UTC) will have the first of the 18 hourly forecasts valid at 15z, but we will not be able to download this forecast until about 1635z.</p><p>The second format is same as above, but at the synoptic times of 00, 06, 12, and 18z the forecast extends out to 48 hr. Saildocs calls the standard version HRRR and the extended version is HRRRX. The latency for this one is just over 2h (0205). Thus the 48h forecast ran at 12z will be available at about 1405.</p><p>The main point I want to stress here, however, is the native HRRR CONUS (continental US) forecasts obtained from NOMADS present the wind direction relative to the orientation of the grid, which in the Lambert projection does not line up with the North-South meridians, and this correction is not accounted for in the HRRR data from Saildocs. An overview of the corrections is shown below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggGYh46YBfX_VOWvtAdJx_NNZotjCLjjTRLJHBvderJpZ4L0njYCi0KBhU7GJupSjUwWH-sRxmWPThZ0oS-p53zPnYPCr8V5RZ54B_vUw_pezWHCecxQIdFtXcL88c1dTtG9DxELXi5Bk3LHaDtvlbbS3qkOuCHUQRq9lFLaemwixmigoWVohvO6U2/s7234/HRRR%20map%20coverage.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="4055" data-original-width="7234" height="358" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEggGYh46YBfX_VOWvtAdJx_NNZotjCLjjTRLJHBvderJpZ4L0njYCi0KBhU7GJupSjUwWH-sRxmWPThZ0oS-p53zPnYPCr8V5RZ54B_vUw_pezWHCecxQIdFtXcL88c1dTtG9DxELXi5Bk3LHaDtvlbbS3qkOuCHUQRq9lFLaemwixmigoWVohvO6U2/w640-h358/HRRR%20map%20coverage.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><i style="font-size: small; text-align: left;">The HRRR CONUS coverage map is from <a href="http://luckgrib.com/models">luckgrib.com/models</a>. I have overlaid the approximate orientations of the grids.</i></div><p>Saildocs HRRR wind directions in the East CONUS are too small and we must add these corrections depending on location. In Cape Cod, for example, the correction is about +15º, so when you read a wind direction of 100, the actual HRRR forecasted wind is 115. In the coastal waters of the West CONUS, we subtract these values to get the actual HRRR wind directions. In San Francisco Bay, for example, the correction is about -15º so an HRRR wind from Saildocs of 100, would mean the intended HRRR forecast is for 085.</p><p>These shifts of roughy ± 15º along the coasts are not really so significant for a static forecast, keeping in mind that the the model data themselves and all the buoys we read the wind from are only accurate to ± 10º or so. But this offset is something to be aware of when doing optimum weather routing computations as this shift can have a significant effect on those results.</p><p>The nominal average of about 14º to 16º right along the coast is correct for the actual grid orientation but the projection conversion process introduces some spread to this. The values pictured above should be considered ±3º or so.</p><p>We focus on OpenCPN in the video demo below, but this same correction would be called for when viewing the Saildocs HRRR gribs in any viewer. </p><p>I should stress that the programs <a href="http://www.expeditionmarine.com" target="_blank">Expedition</a> and <a href="http://www.luckgrib.com" target="_blank">LuckGrib</a> are two that provide their own access to the native HRRR data and their presentations are correct as displayed. </p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/kn3RS63v8ug" width="320" youtube-src-id="kn3RS63v8ug"></iframe></div><p style="text-align: center;"><i>Video demo of the topics above.</i></p><p><br /></p>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-19890018296797997542022-10-14T16:12:00.003-07:002022-10-14T17:57:44.148-07:00Compare Wind Forecasts in qtVlm<p>For routing analysis and computations we have several global models we can choose from, as well as several regional models. An important step is choosing what might be the best forecast. Elsewhere we have an in-depth article and video on <a href="http://davidburchnavigation.blogspot.com/2020/02/evaluating-weather-forecast-slides-from.html" target="_blank">Evaluating a Weather Forecast</a>, but our first step here is just comparing what the different models forecast.</p><p>We will use qtVlm for this, which is the free program we use in our <a href="https://www.starpath.com/catalog/courses/1804bc.htm" target="_blank">Marine Weather</a> and <a href="https://www.starpath.com/catalog/courses/1926.htm" target="_blank">Electronic Chart Navigation</a> courses. Similar analysis can be done with <a href="http://www.expeditionmarine.com" target="_blank">Expedition</a> or with <a href="http://www.luckgrib.com" target="_blank">LuckGrib</a>, and other programs as well. </p><p>Step 1. Download the data. We use three global models available from <a href="http://www.saildocs.com" target="_blank">Saildocs</a>, accessible from within qtVlm, but in practice you may choose other products or sources.</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEhE750Ju_axUze1LJLywxOEOu87tDuakm_kzuUq0p5pkM2x6ZbMPNQyUspfhjuiIixGTHLSUo2oD_bKhF5GLknptEe90TYVWqEOeHACDcFe1IsIID5k3K4fgQUk17jFrL98Y4iEQWZmWPW5AXDr-H5N_xdxgWweuopS2cYHoCMRsE1t8lITgjTUM01l" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1685" data-original-width="1563" height="640" src="https://blogger.googleusercontent.com/img/a/AVvXsEhE750Ju_axUze1LJLywxOEOu87tDuakm_kzuUq0p5pkM2x6ZbMPNQyUspfhjuiIixGTHLSUo2oD_bKhF5GLknptEe90TYVWqEOeHACDcFe1IsIID5k3K4fgQUk17jFrL98Y4iEQWZmWPW5AXDr-H5N_xdxgWweuopS2cYHoCMRsE1t8lITgjTUM01l=w595-h640" width="595" /></a></div><br />Then select the models (blue means on, yellow means off, for all of qtVlm):<p></p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEgPlbVMsuGnRS7Jo2lDVG8z_owf49qCwFWDoM1vFQOgmIZ9_FP3IS92GtRnbQELXGVA_gL3ZXUk9Qa5vxQFiG9SyPP9AON1PhgnJ3iR4B6GbUZDEojKSnrp8Bk9KmwuNmUHQg-sn8f5p1hgkKg6YIuYEMvQYgXDt2Paz_tFwd0_Hd2ryxmGacERrICU" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1426" data-original-width="2788" height="328" src="https://blogger.googleusercontent.com/img/a/AVvXsEgPlbVMsuGnRS7Jo2lDVG8z_owf49qCwFWDoM1vFQOgmIZ9_FP3IS92GtRnbQELXGVA_gL3ZXUk9Qa5vxQFiG9SyPP9AON1PhgnJ3iR4B6GbUZDEojKSnrp8Bk9KmwuNmUHQg-sn8f5p1hgkKg6YIuYEMvQYgXDt2Paz_tFwd0_Hd2ryxmGacERrICU=w640-h328" width="640" /></a></div><br />Here we choose wind and pressure from GFS, COAMPS, and ECMWF. We are after wind comparisons, but the pressure can help us understand what is taking place. Pressing send email creates the request we send to saildocs and they send back by return mail the 3 model forecasts in grib format. We asked for the best resolution that each model offers and restrict the comparison to the time interval of the shortest one, namely COAMPS at 4 days. In each case we ask for a forecast every 3 hr.<p></p><p>The generated email then looks like this shown below. We could create it manually as we explain in <a href="https://www.starpath.com/catalog/books/1886.htm" target="_blank"><i>Modern Marine Weather</i></a>, but the auto generate function is a convenient tool.</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEitLCnOeoYOXilTS4nERRMWJfb5k2vCe4hgQVyp_-0VM2hfV6w-bEK75w-plIeXSUl_obMmenSjLTQGzLh59FaPgaYwc6SCn8gUFdiObEIHBraOW6iY2vriaclaoLxWAtsRw1_-KN4-5-TZN6ghg712S66PtM7E8G7vbzSKGowGf68wJRgUu9CnBJ0k" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="685" data-original-width="1142" height="384" src="https://blogger.googleusercontent.com/img/a/AVvXsEitLCnOeoYOXilTS4nERRMWJfb5k2vCe4hgQVyp_-0VM2hfV6w-bEK75w-plIeXSUl_obMmenSjLTQGzLh59FaPgaYwc6SCn8gUFdiObEIHBraOW6iY2vriaclaoLxWAtsRw1_-KN4-5-TZN6ghg712S66PtM7E8G7vbzSKGowGf68wJRgUu9CnBJ0k=w640-h384" width="640" /></a></div>We get back three emails with the grib files as attachments, which we then drag into a folder to be used by qtVlm, which looks like this:<p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEiHGkPANjCzdvpX7_XaVbLwwpZBDFVeeeY9046NmIDOwoDtG8WU-k1ytr5gn01WUXE56_O57D76jjDYDVuRcsmvFJ6bio-_M2DjyQLUHnaMRyYf_8TwzzDGM3UGqwt0mRkY94QQp_OWgBU-XANODXXqKAaHw-mcxt8LdnlT2sHqatIG9tUEOkABxbd0" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="408" data-original-width="1364" height="192" src="https://blogger.googleusercontent.com/img/a/AVvXsEiHGkPANjCzdvpX7_XaVbLwwpZBDFVeeeY9046NmIDOwoDtG8WU-k1ytr5gn01WUXE56_O57D76jjDYDVuRcsmvFJ6bio-_M2DjyQLUHnaMRyYf_8TwzzDGM3UGqwt0mRkY94QQp_OWgBU-XANODXXqKAaHw-mcxt8LdnlT2sHqatIG9tUEOkABxbd0=w640-h192" width="640" /></a></div><div><br /></div>Now we load each of these into qtVlm, starting with slot 1. The order is up to you.<div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEj-okCvvLE1OPjSoTrqfJOFuUT-O-4m96mBQ5fBBTHW-oGPiBxNgoDsDVXMA24twprTYc-Updo9AkAHHbflcLFY17v6DJbu4YuVMrpdRDCD_CLZNhubjele0Oy-usnoijePU_jWawveYOXSJ7CepmwKcYtnQEEaKFQwPCwZw3dy4HKvZEPO2y6VdwOO" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1082" data-original-width="1334" height="519" src="https://blogger.googleusercontent.com/img/a/AVvXsEj-okCvvLE1OPjSoTrqfJOFuUT-O-4m96mBQ5fBBTHW-oGPiBxNgoDsDVXMA24twprTYc-Updo9AkAHHbflcLFY17v6DJbu4YuVMrpdRDCD_CLZNhubjele0Oy-usnoijePU_jWawveYOXSJ7CepmwKcYtnQEEaKFQwPCwZw3dy4HKvZEPO2y6VdwOO=w640-h519" width="640" /></a></div><div><p>At this stage you navigate to the file in the folder above and say OK. Then do the same with slot 2 and slot 3 for the other two forecasts.</p><p>Once all are loaded, they will be listed in the title bar and you can use menu Grib/Grib Information to inspect the contents of each if desired. The wind barbs themselves might not being showing if we do not yet have them turned on.</p><p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEhtANU8PEi1atzcl3bartX_X2InBANP_WqP9y6HvnOXjIQ0c_0Vq0LNZQFN58-K6mGA-yLRPUzRq7IuFuC7bPfN0GP0N0NCMV48AK2qMb57XQMp5MfonD-9ptaIor0vS2Y5B963WLYQxYfBdAsILMHC_IOV3fSzz_INI5-vEioNWDPKzB6a8vF-IoHE" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1674" data-original-width="1994" height="537" src="https://blogger.googleusercontent.com/img/a/AVvXsEhtANU8PEi1atzcl3bartX_X2InBANP_WqP9y6HvnOXjIQ0c_0Vq0LNZQFN58-K6mGA-yLRPUzRq7IuFuC7bPfN0GP0N0NCMV48AK2qMb57XQMp5MfonD-9ptaIor0vS2Y5B963WLYQxYfBdAsILMHC_IOV3fSzz_INI5-vEioNWDPKzB6a8vF-IoHE=w640-h537" width="640" /></a></div><br />Next we want to use menu Grib/Grib configuration (windsock toolbar) to set up the wind display to enhance the comparison. Use these settings:<p></p><p></p><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEggT3f4KeYimOIZ9e90xdqkphVCUnKP3AaTpzo2f9fYv9lz0p7lZKSvBBiDeQZNY4yvNvmMy-NDkh9NOVIULfHYpJl-9pakkUL8t0_XVaHKbBAQvaK51frP1gzvRlIcVYhsMl451S4rMXH9phbc_8RXQvO5cgWlwUCb6U95P5eV4Yd1mWRWFRjc8fzb" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1780" data-original-width="1475" height="640" src="https://blogger.googleusercontent.com/img/a/AVvXsEggT3f4KeYimOIZ9e90xdqkphVCUnKP3AaTpzo2f9fYv9lz0p7lZKSvBBiDeQZNY4yvNvmMy-NDkh9NOVIULfHYpJl-9pakkUL8t0_XVaHKbBAQvaK51frP1gzvRlIcVYhsMl451S4rMXH9phbc_8RXQvO5cgWlwUCb6U95P5eV4Yd1mWRWFRjc8fzb=w531-h640" width="531" /></a></div><br /><div style="text-align: left;">The colors have to be done one at a time. The three arrow colors are slots 1, 2, and 3. Set one; close it; set the next. The logic of these color choices will be clear shortly.... or maybe not!</div><div style="text-align: left;"><br /></div><div style="text-align: left;">Next set the isobar display using the isobar/isotherm tab.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEgoVxVvKtsafWHF4X3KoOdxYoq8RxPjTbOHVnhiYZNo4RZWsj9r0W2CWBTsDvSaXZSwVILSYcVb_BMdsHxPa-jGaO-mDqwx4VqHe2ud1F7n9wL5m0Aqb32Wj6r2P38c7yTKDSQP9Wb6cqvPJTCzebiSP-CU8U2oAbHv7WZED5prQaEmKqAzAPEDT1OW" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1546" data-original-width="894" height="640" src="https://blogger.googleusercontent.com/img/a/AVvXsEgoVxVvKtsafWHF4X3KoOdxYoq8RxPjTbOHVnhiYZNo4RZWsj9r0W2CWBTsDvSaXZSwVILSYcVb_BMdsHxPa-jGaO-mDqwx4VqHe2ud1F7n9wL5m0Aqb32Wj6r2P38c7yTKDSQP9Wb6cqvPJTCzebiSP-CU8U2oAbHv7WZED5prQaEmKqAzAPEDT1OW=w371-h640" width="371" /></a></div><br />The bottom slider controls the thickness. Midway is good.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">Now we can turn on the wind and see the first comparison:</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEiwofnbYpqiUcOrUP9qS-1Sih5mmfCK2zQjgBAJjBwSc8aqQa8PQtJdkzg6O-xd-XQulE9izYfA9_w0Qha57lkH1A8dojcI4dQPFw1r4rQcLSHE5_W4nOGharvVzm5coUUsLKtAB0oEK57YnyRSBumK1BAxS6XJKTGORQaxuVX73EBJsINQw7MzslXB" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="2034" data-original-width="2240" height="582" src="https://blogger.googleusercontent.com/img/a/AVvXsEiwofnbYpqiUcOrUP9qS-1Sih5mmfCK2zQjgBAJjBwSc8aqQa8PQtJdkzg6O-xd-XQulE9izYfA9_w0Qha57lkH1A8dojcI4dQPFw1r4rQcLSHE5_W4nOGharvVzm5coUUsLKtAB0oEK57YnyRSBumK1BAxS6XJKTGORQaxuVX73EBJsINQw7MzslXB=w640-h582" width="640" /></a></div><br /><br /></div><div style="text-align: left;">Black, red, and green are 1, 2, and 3, which can be checked in the title bar. We see good agreement in some places and poor agreement in others. This is the first forecast (effectively a surface analysis) which we see from the slider being far left. Change times with the slider or use the clock icon to digitally set to a specific time. This way you can scan through to see how the agreement stands, looking at a large area, or zoom in to look at a more local area.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">For time comparisons and a more digital comparison, right click where you want to see a meteogram, and then choose "Compare models," as shown below. </div><div style="text-align: left;"><br /></div><div style="text-align: left;">The meteograms always start at the location of the on screen display, indicated by the grib time or slider location.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">Also we see now the intended motivation of the color selections. These meteograms have fixed colors so we have just forced wind barb colors, which we can change, to match the graph colors that we cannot change. Else we have 6 colors to sort out, or worse, the same colors meaning different things.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEgAS20VXex27dcyDUE9oN4RoNwwnsdYxS6JVfLUATP1Y4Z1RZMUchNSWMErol3UQ-CqVz9Khs_Ib2aiZwLukPaxa1PloapGi6zhCHJr-huwUHli58gFQ9tXY_uTJjiIm4zlTtZ-A3Cw9qpUO0XpaNLA4WUkhuXTatHtXsWNhYc3xXzZtgLrAG9frNly" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1398" data-original-width="2131" height="420" src="https://blogger.googleusercontent.com/img/a/AVvXsEgAS20VXex27dcyDUE9oN4RoNwwnsdYxS6JVfLUATP1Y4Z1RZMUchNSWMErol3UQ-CqVz9Khs_Ib2aiZwLukPaxa1PloapGi6zhCHJr-huwUHli58gFQ9tXY_uTJjiIm4zlTtZ-A3Cw9qpUO0XpaNLA4WUkhuXTatHtXsWNhYc3xXzZtgLrAG9frNly=w640-h420" width="640" /></a></div><br />This is the forecasts for <b>Point AA</b>. A cursor over the point brings up the values, faked here in that only one at a time can show. We see mixed agreement at this point over time.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEhkOh5WAWSzYNtZ8mbwAOnjGMAJGXyr7CLJf972wc4V-SM_nsZIILadNk_opg0K09qdQQaLk9GP3ZrSwuIfYhtg3hGKuMkl5XS4Kto1JM41KPtiKrOySp4SV49HjGWuI6DFMYzzKBYkTA5ROMij3Y5BytrBm-WTj_fDr1PWdFOnP-GkUIH4XaLErUaY" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1476" data-original-width="2248" height="420" src="https://blogger.googleusercontent.com/img/a/AVvXsEhkOh5WAWSzYNtZ8mbwAOnjGMAJGXyr7CLJf972wc4V-SM_nsZIILadNk_opg0K09qdQQaLk9GP3ZrSwuIfYhtg3hGKuMkl5XS4Kto1JM41KPtiKrOySp4SV49HjGWuI6DFMYzzKBYkTA5ROMij3Y5BytrBm-WTj_fDr1PWdFOnP-GkUIH4XaLErUaY=w640-h420" width="640" /></a></div><br />This is the forecasts for <b>Point BB</b>, which is pretty good agreement for the first two days.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEiY-BtFFQBF4p4Vosu07WG_LsMEFDZhhHNj41YHgJsIGYJUewH1kmlgy8MLym8Gy5iVXZSiUhHcCaQ-_ZRYFhFEZhJbfaCRbedxChxm5ktfIX-vwhZUOkrG2jYQmAwA8fowno4i7LzlBN7Ji71cGKeiO60aG19QxIMh5Zmgy9E2Gol0pKgdqwTaNQM0" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="1438" data-original-width="2262" height="406" src="https://blogger.googleusercontent.com/img/a/AVvXsEiY-BtFFQBF4p4Vosu07WG_LsMEFDZhhHNj41YHgJsIGYJUewH1kmlgy8MLym8Gy5iVXZSiUhHcCaQ-_ZRYFhFEZhJbfaCRbedxChxm5ktfIX-vwhZUOkrG2jYQmAwA8fowno4i7LzlBN7Ji71cGKeiO60aG19QxIMh5Zmgy9E2Gol0pKgdqwTaNQM0=w640-h406" width="640" /></a></div><br />This is the forecasts for <b>Point CC. </b>We see the models agree and disagree over different areas and times, but this is just a demo of how this works. Generally we would be focused on a more localized area.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">Referring back to the wind barbs view above we see some areas agreeing and others not. This would generally lead us to look at the isobars. But there are some nuances to getting the most out of this, in that for now we cannot directly compare isobars from different models. We have to look at each individually, and we will likely find one model more useful than others. Here are the three models with their isobars for the surface analyses at 12z (first forecast).</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEiT7lnpAvazpNhTACOvMNDW-o-gPe7DDXseTBtUFniDvI5iRGsTmYl6utMsBNcGtJwWa-KO6ZdKT0_vG2SEgvvxTAF-sXWsFzpjQhLJHikuuyxfFHpvYaZ3uFjOVVy50zrvlIRxL8JAyxXJlavSnkk_Av1VIBsg5Vz39Lr41Ck4XaXTF_9A8yfn5eRu" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="2041" data-original-width="2244" height="582" src="https://blogger.googleusercontent.com/img/a/AVvXsEiT7lnpAvazpNhTACOvMNDW-o-gPe7DDXseTBtUFniDvI5iRGsTmYl6utMsBNcGtJwWa-KO6ZdKT0_vG2SEgvvxTAF-sXWsFzpjQhLJHikuuyxfFHpvYaZ3uFjOVVy50zrvlIRxL8JAyxXJlavSnkk_Av1VIBsg5Vz39Lr41Ck4XaXTF_9A8yfn5eRu=w640-h582" width="640" /></a></div><br />Slot 1, black, is the GFS.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEjaYpZOayRfNqAvzM88ia7BkwTLvs75j_XlSEH8UnvdxNDXkuDBBW_3L9F5xBN2LeogO4y6P20bR_0wvkU7T9l_uH8DYhf0T6AuAnAVtS8IF4ZFroy9PweJrm1YPPAIkYeLu6us4MbbZMzyMo4ihu79pS6P3pQHu6TjH4FaL8hCgLwQgyQGDAqrGPPm" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="2027" data-original-width="2249" height="577" src="https://blogger.googleusercontent.com/img/a/AVvXsEjaYpZOayRfNqAvzM88ia7BkwTLvs75j_XlSEH8UnvdxNDXkuDBBW_3L9F5xBN2LeogO4y6P20bR_0wvkU7T9l_uH8DYhf0T6AuAnAVtS8IF4ZFroy9PweJrm1YPPAIkYeLu6us4MbbZMzyMo4ihu79pS6P3pQHu6TjH4FaL8hCgLwQgyQGDAqrGPPm=w640-h577" width="640" /></a></div><br />Slot 2, red, is the ECMWF.</div><div style="text-align: left;"><br /></div><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/a/AVvXsEgEjqfMsGVZ22L1pAeRdwDCGgvTmS0yvwrT-wzM-H3HYI1QOl_Lxp7l3wrfu8NNZZ4QXApembj-yIQzanFcRMt-LY8SxjKRGg7f7gTl3FxPv-UaJfIvASLsorjx_jokJIEU5l4r3JgxVcSw59d-FGQciJ_0d8jihxN6ttCkHRItzYFLBadFCy8dgaDU" style="margin-left: 1em; margin-right: 1em;"><img data-original-height="2029" data-original-width="2234" height="582" src="https://blogger.googleusercontent.com/img/a/AVvXsEgEjqfMsGVZ22L1pAeRdwDCGgvTmS0yvwrT-wzM-H3HYI1QOl_Lxp7l3wrfu8NNZZ4QXApembj-yIQzanFcRMt-LY8SxjKRGg7f7gTl3FxPv-UaJfIvASLsorjx_jokJIEU5l4r3JgxVcSw59d-FGQciJ_0d8jihxN6ttCkHRItzYFLBadFCy8dgaDU=w640-h582" width="640" /></a></div><br />Slot 3, green, is the COAMPS. This is a US Navy regional model, which accounts for why it does not cover the full extent of the other two, which are global models. The transient weak Low at point BB does not last long.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">In you own boat at sea or along the coast, you will likely have an have an <a href="https://www.starpath.com/marinebarometer/about.htm" target="_blank">accurate barometer in your pocket</a>, so you have compelling information about which of these models might be closer to the truth.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">qtVlm also offers a very convenient way to <a href="https://www.starpath.com/qtVlm/#buoys" target="_blank">download and plot all the buoy and ship reports</a> around you which is more data to add to your own wind and pressure measurements to help pin down the best forecast.</div><div style="text-align: left;"><br /></div><div style="text-align: left;">Below is a video illustration (11:19) of the above:</div><div style="text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/Fy7vy7wF0qo" width="320" youtube-src-id="Fy7vy7wF0qo"></iframe></div><br /><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div><div style="text-align: left;"><br /></div></div><br /><br /><p></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-35380859776822891342022-09-17T15:13:00.021-07:002022-09-27T14:25:46.489-07:00S-63 Encrypted International Charts—Sources and Installation<p><span style="font-family: Helvetica;"><span style="font-size: 18px;"><i>These notes are intended as background for several videos illustrating the purchase and installation of S-63 charts. </i></span></span></p><p><span style="font-size: medium;"><span style="font-family: Helvetica;">• </span><span style="font-family: Helvetica;">We are </span></span><span style="font-family: Helvetica; font-size: 18px;">spoiled in the US. Our ENC charts are S-57 format, unencrypted and updated weekly, but we are not forced to update them if we do not want to. They run on any compatible device, in any compatible electronic charting system (ECS), and they are free to all users.</span></p><p><span style="font-family: Helvetica; font-size: 18px;">• To my knowledge, New Zealand is the only other nation to offer their official ENC free to the mariners (discussed below). </span></p><p><span style="font-family: Helvetica; font-size: 18px;">• Inland ENC (IENC), on the other hand, are free from every nation. These cover rivers and some limited coastal waters around the estuaries. US IENC cover only the "Western Rivers," as defined in the Navigation Rules. These charts have their own standards, similar to ENC, but not the same.</span></p><p><span style="font-family: Helvetica; font-size: medium;"><span>•</span><span> </span>In contrast</span><span style="font-family: Helvetica; font-size: 18px;">, almost all international ENC are encrypted in the </span><a href="https://iho.int/uploads/user/pubs/standards/s-63/S-63_2020_Ed1.2.1_EN_Draft_Clean.pdf" style="font-family: Helvetica; font-size: 18px;" target="_blank">S-63 format</a><span style="font-family: Helvetica; font-size: 18px;">, and must be purchased. They are restricted to a specific navigation program (ECS or ECDIS), running on a specific computer or dedicated hardware, and they must be updated (repurchased) on a fixed scale of 3 mo, 6 mo, or 12 mo. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Most </span><span style="font-size: 18px;">places where we buy S-63 charts have some form of graphic index to select from, but we might want more detail about individual charts than they offer, thus we recommend finding charts from the IC-ENC version that you can get to from </span><a href="https://www.starpath.com/getcharts/" style="font-size: 18px;" target="_blank">starpath.com/getcharts</a><span style="font-size: 18px;"> (item 22a). See video demo below.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/6SxucTAt4cw" width="320" youtube-src-id="6SxucTAt4cw"></iframe></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><i>How to learn what international charts are available (5:20)</i></div><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: x-large;"><br /></span></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Expir</span><span style="font-size: 18px;">ed charts can still be viewed, but updates are no longer available. A warning must show in the nav program when accessing expired charts; this is a requirement of the S-63 compatibility certification of the ECS, reflected in the existence of a file called IHO.pub located somewhere in your ECS folder structure.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>To acc</span><span style="font-size: 18px;">ommodate the encryption and restrictions on S-63 charts, the purchase and installation of the charts are more complex than it is for use of S-57 charts.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Fort</span><span style="font-size: 18px;">unately, New Zealand offers all of their official ENC free to users in the S-63 format, so we can all practice both the "purchase" and installation of S-623 charts. We outline that process in our new book </span><a href="https://www.starpath.com/catalog/books/1996.htm" style="font-size: 18px;" target="_blank"><i>Introduction to Electronic Chart Navigation, 2nd Edition</i></a><span style="font-size: 18px;">. The free sample of that book includes an outline of the process.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>D</span><span style="font-size: 18px;">espite these extra steps required, prudent mariners sailing in foreign waters will likely want official copies of the most important charts. Others might be tempted to use only the convenient and relatively low cost third party products, but there are many documented examples of errors or inadequacies of these charts. Indeed, though rarely read, every time you open one of these apps you must accept the End User Agreements, samples of which are in our textbook.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiX-yKUEDc9D_d1UnRU_nm5WVyn4_uIgnzxUreCTLCfEusHZFkbynOV6cxFXrmfr0hvZ9cUfTA8u0t_7ec9s_YtHshWi61O7NzvFHuW1Yt209oXxAu5WFQfrsrucXC8Ac-W6LTCRFPvn8Rqya8AROpo6lHm_AIfdzfLlJ30QfODK1yLh2LYe3bGP-97/s1289/chart%20EULAs%20.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1289" data-original-width="1067" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiX-yKUEDc9D_d1UnRU_nm5WVyn4_uIgnzxUreCTLCfEusHZFkbynOV6cxFXrmfr0hvZ9cUfTA8u0t_7ec9s_YtHshWi61O7NzvFHuW1Yt209oXxAu5WFQfrsrucXC8Ac-W6LTCRFPvn8Rqya8AROpo6lHm_AIfdzfLlJ30QfODK1yLh2LYe3bGP-97/w530-h640/chart%20EULAs%20.png" width="530" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><br /><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>In sh</span><span style="font-size: 18px;">ort, prudent seamanship calls for use of official, updated charts, and indeed for just a few of them it is not that expensive: roughly 10 euros each for a 3 month subscription. The price of the charts are pretty much the same from any source.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Obv</span><span style="font-size: 18px;">iously not sure how this evolves, but now, 9/17/22, is a good time to buy them. The euro is on parr with the dollar (roughly 23% less than last year) and most of these are sold in euros.</span></p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Can li</span><span style="font-size: 18px;">kely get by with the 3 month versions—keeping in mind we are used to paper charts that were updated sometimes only on a year or two basis, since we can check the Local Notices to Mariners or Light List. The Light Lists now are typically updated online weekly.</span></p><p class="p2" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 22px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>W</span><span style="font-size: 18px;">hat is needed? The two main documents needed that are unique to your installation have names:</span><span class="Apple-converted-space" style="font-size: 18px;"> The </span><b style="font-size: 18px;">User permit</b><span style="font-size: 18px;">, which is a 28 character number and a </span><b style="font-size: 18px;">permit.txt</b><span style="font-size: 18px;"> file, which lists key numbers that relate your installation to the chart set supplied to the source. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Th</span><span style="font-size: 18px;">e user permit comes from your ECS (navigation software) supplier. For commercial programs that you bought, this permit is usually free. It is either already in the program somewhere, or you write to where you bought the program and ask for it. The user permit will apply to just your copy of the program running in a specific computer. If you change computers, you may need to ask for a new user permit. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Whe</span><span style="font-size: 18px;">n using free ECS such as OpenCPN or qtVlm, there is an extra step required because these ECS can be loaded into any computer. This is resolved by requiring a onetime modest purchase for a plugin (10 to 20 euros) that configures the program to handle S-63 charts. In that process you run a function that creates a fingerprint file or number that identifies your computer and you submit that with the plugin purchase to get a key that opens up your nav program to handling S-63 charts. Both of these programs listed allow for the plugin to be used on 3 computers. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>More </span><span style="font-size: 18px;">on shared usage: the S-63 charts are tied to the User Permit used when the charts were purchased. So the limitation on their display depends on the rules established by the navigation software. As noted, qtVlm and OpenCPN each allow a User Permit to be used on three separate installations. Commercial nav software generally associate the User Permit with the software instance. Some have rules that allow only one instance of the program, others allow for two simultaneous instances—a main computer and a backup. Most allow for unregistering the program on one device and moving it to another, in which case the User Permit would go with it.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>Mo</span><span style="font-size: 18px;">st ECS offer a set of instructions for the installation of S-63 charts, usually with a sequence of links that should put all the components in the right place. The final folder structure of the charts is crucial. If you find that the packaged instructions do not work as expected, then create the file structure manually as we explain in our textbook, and then copy that folder to the charts folder or S63 folder in your ECS folder structure and link to that one to access the charts. This should also facilitate the updating of the charts as they become available.</span></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: 18px;"><br /></span></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: 18px;">• Updating S63 ENC can be tricky, because update files (ending .003, .004, etc) refer to a specific base set (.000). With good internet connections, the easiest route is just download the latest base set and latest update set and install them, as if starting from scratch.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>A</span><span style="font-size: 18px;">gain, the </span><a href="https://www.encservice.linz.govt.nz/" style="font-size: 18px;" target="_blank">free S-63 charts from NZ</a><span style="font-size: 18px;"> is an excellent way to practice with obtaining S-63 charts and the associated files needed. The permit.txt file is the crucial one, but there are a couple others that are useful as well. To use the system, you need to first set up a free account using a valid email address. It is safe and non-invasive. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/AFMUKFJ6Y_g" width="320" youtube-src-id="AFMUKFJ6Y_g"></iframe></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div style="text-align: center;"><i>Download free NZ ENC in S-63 format (8:58)</i></div><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>We</span><span style="font-size: 18px;"> illustrate an actual purchase using </span><a href="https://www.chartworld.com/shop" style="font-size: 18px;" target="_blank">ChartWorld</a><span style="font-size: 18px;"> online. There are numerous places to purchase these, including storefront and online companies, but ChartWorld (in Germany) has an easy interface and offers all formats of charts globally, not just the S-63. They are also part of SevenCs, which is a world leader in ENC production and display. To use the system, you need to first set up a free account using a valid email address. It is safe and non-invasive. </span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/MY3hLFL2h4o" width="320" youtube-src-id="MY3hLFL2h4o"></iframe></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><i>Purchase and download S-63 charts from ChartWorld (12:15)</i></div><br /><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>To d</span><span style="font-size: 18px;">ownload the charts from ChartWorld: you will receive a link from them like ftp://www.chartworld.com/DC12345, where the last bit is your installation ID, unique to a User Permit. To access this in Win10 or Win11, open File Explorer and paste this in the address bar. You can then drag them to a folder of choice. All charts you buy now or later with that User Permit will be located there.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-size: medium;"><span>•</span><span> </span>B</span><span style="font-size: 18px;">elow are a few video illustrations of installing S-63 charts.</span></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">Expedition ---------------------------</p><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/lhR1YNbkzKU" width="320" youtube-src-id="lhR1YNbkzKU"></iframe></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><i>Loading free S-63 Charts from NZ into <b>Expedition</b> (9:56)</i></div><div><br /></div><div><br /></div><p class="p1" style="font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><br /></p><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/jhiNjSQKVRM" width="320" youtube-src-id="jhiNjSQKVRM"></iframe></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><div class="separator" style="clear: both;"><i>Loading S-63 charts from ChartWorld into <b>Expedition</b> (10:16)</i></div><div class="separator" style="clear: both;"><i><br /></i></div><div><br style="text-align: left;" /></div></div><span style="font-family: Helvetica; font-size: 18px;">qtVlm ---------------------------</span><div><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/nS5Lqqax9pA" width="320" youtube-src-id="nS5Lqqax9pA"></iframe></div><br /><div class="separator" style="clear: both; text-align: center;"><i>Loading S-63 charts into <b>qtVlm</b> (19:52), both free NZ charts and two AU charts from ChartWorld</i></div></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: Helvetica; font-size: 14px; text-align: start;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><span style="font-family: Helvetica; font-size: 14px; text-align: start;"><br /></span></div><div class="separator" style="clear: both; text-align: left;"><span style="font-family: Helvetica; font-size: 14px;">Summary of the qtVlm process.</span></div><div class="separator" style="clear: both; text-align: center;"><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— read this article!</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— prepare the program</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— buy a user permit (15 euros)</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— send user permit and hardware ID to qtvlm to get activation key<span class="Apple-tab-span" style="white-space: pre;"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span> </span><span> </span>— check that all is ok</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— get the charts and move then to the right folder</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— practice with free NZ charts</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— then use purchased charts as from ChartWorld</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— assign the folder to the charts</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— this is done from the S63 tab, not the main vector chart tab</p></div><div class="separator" style="clear: both; text-align: center;"><i><br /></i></div><div><i><br /></i></div><div><span style="font-family: Helvetica; font-size: 18px;">OpenCPN ---------------------------</span></div><div><span style="font-family: Helvetica; font-size: 18px;"><br /></span></div><div class="separator" style="clear: both; text-align: center;"><iframe allowfullscreen="" class="BLOG_video_class" height="266" src="https://www.youtube.com/embed/9V-5wTVGoec" width="320" youtube-src-id="9V-5wTVGoec"></iframe></div><br /><div style="text-align: center;"><div style="text-align: left;"><div class="separator" style="clear: both; text-align: center;"><i>Loading S-63 charts into <b>OpenCPN</b> (12:22).</i></div><div><i><br /></i></div></div></div><div style="text-align: left;"><span style="font-family: Helvetica; font-size: 18px;"><div class="separator" style="clear: both; font-family: Times; font-size: medium; text-align: center;"><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">Summary of the OpenCPN process.</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— read this article!</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— prepare the program</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— update plugins list and install the S-63 plugin</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span> </span><span> — purchase user permit from o-charts.org (12.5 euros)</span><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span class="Apple-tab-span" style="white-space: pre;"> </span>— create hardware fingerprint, use it at o-charts.org to get an install permit<span class="Apple-tab-span" style="white-space: pre;"> </span></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span> </span><span> </span></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;">— buy the charts / unzip the folders / move them to your charts folder</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span> </span><span> </span>— setup / charts / s63 charts / keys-permits / import cell permit assign user permit and install permit</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><span> </span><span> — </span> setup / charts / s63 charts / chart cells / import charts assign chart folder</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; text-align: start;"><br /></p><div><br /></div></div></span></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-63399027584243277102022-07-22T19:24:00.001-07:002022-07-22T19:24:14.366-07:00Start Saying Goodbye to Your Favorite Paper Chart<p>This should not be a surprise. NOAA told us they were <a href="https://cdmcs.org/wp-content/uploads/2019/11/NOAA-Sunset-Plan.pdf" target="_blank">Sunsetting Traditional Paper Charts</a> back in 2019 and that they would all be gone by the end of 2024. After these charts are gone, we will rely on electronic navigational charts (ENC) and the new NOAA Custom Charts (NCC) that we design on our own using an online NOAA app and <a href="https://davidburchnavigation.blogspot.com/2022/03/printing-noaa-custom-charts-ncc.html" target="_blank">print on our own</a> in a size and quality we choose.</p><p>This note is an alert that this is happening at an accelerated rate... plus we have a way now to know the status of our favorite charts. In item (1) of our <a href="https://www.starpath.com/getcharts" target="_blank">starpath.com/getcharts</a> resource, go to the paper chart index on the left and you will see something like shown below for the San Juan Islands area.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAPJPNSOyaOxBOu1qvC_JgNmcOp06GbEbHJe65GK_d1YLMng5XIYWthphudIGZbT6ewix85EOOU5WDvIxJyJqxAcs9a84JxJmjYutAz0B18_A1-as78sHML1rqTcu_pvBRbCmjnB_p5v8t7OWFB9IZ_G_WJohe-kwNbSrLBkWHEJ5REWZ4l8ll9Zax/s2219/San%20Juans.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1628" data-original-width="2219" height="470" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgAPJPNSOyaOxBOu1qvC_JgNmcOp06GbEbHJe65GK_d1YLMng5XIYWthphudIGZbT6ewix85EOOU5WDvIxJyJqxAcs9a84JxJmjYutAz0B18_A1-as78sHML1rqTcu_pvBRbCmjnB_p5v8t7OWFB9IZ_G_WJohe-kwNbSrLBkWHEJ5REWZ4l8ll9Zax/w640-h470/San%20Juans.png" width="640" /></a></div><p>Notice a few things new. Several of the charts are now gray and their traditional names have been changed to be preceded by the letters "LE." This means "last edition." I have clicked one of them (it turns orange) to show its details on the right. Take notice of the text in red. This chart is no longer updated and will be removed on Oct 5, 2022—2 month and 13 days from today. The same with all of the gray ones.</p><p>These are not random, obscure charts. These are the main working charts for the San Juan Islands, and it is not just this area. The extent of cancelation is even higher on the Gulf and East Coasts.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu0DqnPrwI75iWZmjfJxH3y-xOMm_bohLHYij6VPPEf6BpCkXlNK3bySri3JXuYGH_FLowMPrBRgtJAPpMu1Nu_hZitC2hcxrAun86AqV48f589f9sAWe60tHNzHmrfUevwDJCR5ZhMdw0EzOZ19MoR4jYYP9ZbfgoIQ5lzEGPHxImidhYGOpPZmlO/s1028/gulf%20coast.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="552" data-original-width="1028" height="344" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhu0DqnPrwI75iWZmjfJxH3y-xOMm_bohLHYij6VPPEf6BpCkXlNK3bySri3JXuYGH_FLowMPrBRgtJAPpMu1Nu_hZitC2hcxrAun86AqV48f589f9sAWe60tHNzHmrfUevwDJCR5ZhMdw0EzOZ19MoR4jYYP9ZbfgoIQ5lzEGPHxImidhYGOpPZmlO/w640-h344/gulf%20coast.png" width="640" /></a></div><div><br /></div>All gray charts west of Alabama are schedule for cancellation same time as the West Coast (Oct 5, 2022). Those on the East Gulf Coast and on the SE Atlantic are scheduled for Jan 4, 2023—that is 5 months 14 days from today.<div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVuQ2m8XzczSMDWR-ktzUUCM8ywp-TstJJZh8FHkN8--Waj5cPJTLU3sr1B0GXEDN5vySiT0AWh4OvyroQght4zlZsmy8NNmY2RmmeF6KgmcTI7ZnqQQ8wJEooyqPv6NQhHd3TwO3ycbcysexUSyxV7eS1T05GOtsIUWEWJeUaJXoI4xcOIoPzubVV/s1063/Central%20atlantic.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="699" data-original-width="1063" height="420" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgVuQ2m8XzczSMDWR-ktzUUCM8ywp-TstJJZh8FHkN8--Waj5cPJTLU3sr1B0GXEDN5vySiT0AWh4OvyroQght4zlZsmy8NNmY2RmmeF6KgmcTI7ZnqQQ8wJEooyqPv6NQhHd3TwO3ycbcysexUSyxV7eS1T05GOtsIUWEWJeUaJXoI4xcOIoPzubVV/w640-h420/Central%20atlantic.png" width="640" /></a></div><br /><div>The Central Atlantic has a lot charts leaving on Nov 16, 2022, which is just 3 months and 25 days.</div><div><br /></div><div>When these traditional charts are gone, there are two NOAA Print on Demand outlets that have announced on their websites that they are set up to print the new ENC versions of these same charts. In other words, they are considering making their own NCC that are the exact aspects, scales and sizes so you could just ask for the chart number. </div><div><br /></div><div>But it is not clear if that is the best solution. The NCC app gives the chart designer a lot of freedom on what is included and what exact area is covered and at what scale. I think it will be best for mariners to learn how to use the NCC app to make their own decisions on what paper charts they want to replace the old ones. </div><div><br /></div><div>Luckily, there is an easy place to go to learn to use the NCC app, namely the Starpath online <a href="https://www.starpath.com/catalog/courses/1926.htm" target="_blank">Course on Electronic Chart Navigation</a>, which includes our unique textbook on the subject. We focus on the actual ENC usage, but do have an extended lesson on how to make the NCC along with practice exercises and individual support.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5fQU4R5ylrz_erVY7W0YpJxe-xu3sbZXNQ3dgC4-IE1Ht-gDhdSvFvllnGBFE52UTnDjnlLYiixnH67C5tGPI-UfyQFZyABkF12xwkL5HncMLlzeUz9eOIohlXHgiVN_bwxXduIRVMwDCrV3DLBFH3SXiGgC-yKoFQfG1IMwPFlXIAJeQwxXPAcfL/s425/big_screen_splash_sm.jpeg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="367" data-original-width="425" height="276" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5fQU4R5ylrz_erVY7W0YpJxe-xu3sbZXNQ3dgC4-IE1Ht-gDhdSvFvllnGBFE52UTnDjnlLYiixnH67C5tGPI-UfyQFZyABkF12xwkL5HncMLlzeUz9eOIohlXHgiVN_bwxXduIRVMwDCrV3DLBFH3SXiGgC-yKoFQfG1IMwPFlXIAJeQwxXPAcfL/s320/big_screen_splash_sm.jpeg" width="320" /></a></div><br /><div><br /></div><div>Needless to say, the goal of the <a href="https://nauticalcharts.noaa.gov/publications/docs/enc-transformation.pdf" target="_blank">US National Charting Plan </a>is not to navigate on these NCC, but rather to navigate on the ENC, which is what we teach. But NOAA knows that many mariners, if not most, do indeed want to have access to a paper chart at all times, just in case—and that is the main purpose of the NCC.</div><div><br /></div><div>Beside NOAA doing away with the traditional paper charts in lieu of ENC and NCC made from them, the USCG is helping this transition along. They have just competed a call for comments on their <a href="https://www.federalregister.gov/documents/2022/03/28/2022-06416/electronic-chart-and-navigational-equipment-carriage-requirements" target="_blank">proposed new ruling</a> that commercial vessels <i>must</i> have on board a functional way (ECS or ECDIS) to use the official ENC. Third party charts that dominate the recreational echart world, do not count.</div><div><br /></div><div>In short, we have arrived at the moment where we have gone from a time when the ENC were a legal alternative to paper charts to a time when ENC are the required means of navigation. We stand by to learn how this specific ruling evolves, and what all vessels will be covered, but it will indeed be enacted.</div><div><br /></div><div>The side message to this note is this: knowing how to find these LE dates, if you want a copy of the last valid traditional paper chart, now is the time to order it. Right now you can still even get a PDF of it.<p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com4tag:blogger.com,1999:blog-714718709601620866.post-63124116343341677712022-07-05T14:45:00.054-07:002022-07-06T16:53:40.126-07:00Magnetic Variation on Electronic Navigational Charts (ENC)<p>Magnet variation (magvar) is crucial to marine navigation. It is the difference between true north and the direction of north read from a compass card, which is shown on the compass rose on a chart as the difference between true north and magnetic north. We might like to navigate by all magnetic bearings since we drive the boat by the compass, but we cannot avoid dealing with true bearings at times. Tidal current directions are always given in true, as are wind directions from any official forecast or observation. Charted navigation ranges are given in true, as are the visible boundaries of sector lights. Any cel nav solution must be worked out in true bearings, and so on. </p><p>In short, we might avoid them whenever possible, but we have to deal with true bearings. But that is navigator talk. Normally you would communicate related results or desired courses to the helmsman in magnetic and you would expect all logbook entries to be in magnetic.</p><p>Working with magvar is just one more aspect of navigation that is improved with the use of ENC. Magvar is an ENC charted object, and we can cursor pick any place on the chart to see what the local value is at that point. ENC get the latest values (magvar drifts slowly with time) from the <a href="https://www.ngdc.noaa.gov/geomag/" target="_blank">World Magnetic Model</a> (WMM), which is updated every 5 years on the even 0s and 5s. The most recent is 2020; next update is 2025.</p><p>The image below shows how magvar varies over the country, along with its annual rate of change.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjrEouSBJBmOP3Fz8MEyKUockBYGisdahCNRN_qVNdFDxZKDbJxnnkCuhbmCugfPKJPrugUgMqc3LOTZ7W_YnhQOE1ZKIKwCf_6DkN70EKCHIewnhXUl2UUY4v--M7xO_DOeDxfFAue4I32hBh0Xo81f6xToVKXNXwpr9KxHMwIFpmVJUYaUthwfxoM/s1397/WMM2020.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1076" data-original-width="1397" height="492" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjrEouSBJBmOP3Fz8MEyKUockBYGisdahCNRN_qVNdFDxZKDbJxnnkCuhbmCugfPKJPrugUgMqc3LOTZ7W_YnhQOE1ZKIKwCf_6DkN70EKCHIewnhXUl2UUY4v--M7xO_DOeDxfFAue4I32hBh0Xo81f6xToVKXNXwpr9KxHMwIFpmVJUYaUthwfxoM/w640-h492/WMM2020.png" width="640" /></a></div><br /><div class="separator" style="clear: both; text-align: center;"><i>You can get a high res PDF copy including one for AK at the WMM link above. </i></div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">The line separating E and W variation goes through New Orleans. The 2020 value at the NW corner of the US at Cape Flattery is about +16º (16º E) with an annual rate of change in minutes of about -6'/yr (6' W)—at present rate, every 10 years it will change by 1º; but the rate of change also changes with time. The spatial distribution of this change is on the edge of a col (saddle point) at this location, so it is not easy to read from this plot, but the actual values are well known at all locations (see WMM link). The reference year for these values is 2020.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: left;">Why ENC are so much better than paper charts on this topic is tied to the fact that traditional paper charts are being discontinued and all will be gone by the end of 2024. Consequently they are not being updated except in crucial matters—getting the magvar wrong by a degree or so once in a while is not really crucial in most circumstances.</div><p>Most current (July 2022) ENC have the 2021 values encoded in the charts (corrected from 2020 WMM), whereas the latest paper charts or the raster navigational charts (RNC) made from them could be very old. Two examples are below.</p><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1mjEmc3jH47Offhucnqj0qJ37lOikvS-BKLKMT0pawtxE1mqlfVKuH02b9qhc7KQWdqwqvNxcdez7cAWp6fyL1-CVNOe8Pf9ZFsZ4DE1l9ZGuRK83Vc3Q_dkDPPH4YKSEz6nlsdOx-D2eaX6zHKL7Z8YHwwpHOBEmJSxbjMoNUIq_c95zD17AoG3B/s784/Latest%20Neach%20Bay%20chart%2018484.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="754" data-original-width="784" height="616" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj1mjEmc3jH47Offhucnqj0qJ37lOikvS-BKLKMT0pawtxE1mqlfVKuH02b9qhc7KQWdqwqvNxcdez7cAWp6fyL1-CVNOe8Pf9ZFsZ4DE1l9ZGuRK83Vc3Q_dkDPPH4YKSEz6nlsdOx-D2eaX6zHKL7Z8YHwwpHOBEmJSxbjMoNUIq_c95zD17AoG3B/w640-h616/Latest%20Neach%20Bay%20chart%2018484.png" width="640" /></a></div><div><br /></div>This latest edition of 18484, Neah Bay at Cape Flattery, WA, refers to magvar data from 2006. If we use that to predict the present value, which you would have to do if this is the only chart you have, even with it being the most current version, we would figure 2022 -2006 = 16 yr x 11'/yr = 176' = 2.93º. In 2006 it was 18º 15' = 18.25º, so the 2022 value would be 18.25 - 2.93 = 15.32º E, which would round to 15º E. Paper charts refer to the change as annual "increase" or "decrease," whereas with ENC we have to think though the algebraic use of + and – signs. E is +; W is –.<div><br /></div><div>Checking the latest ENC for this area we get </div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhpHk2_SbYlsvVg6EvFBAeX69zMjN-FSsJYjgOpmmfJMk07W6GlE3tAhrJHM2xrflN7bG2e23Okr8xNTq20tBsO-HETjnTsq33o3Ob_H_JOnJHqaFXV0RYqdCdmN42K5issT0obWN22Wnp9xc0C3QHtyacu8aLuNnp7j0tNhet4d8M5hX6y6Xtiw77a/s922/Neah%20bay%20on%20ENC.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="491" data-original-width="922" height="340" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhpHk2_SbYlsvVg6EvFBAeX69zMjN-FSsJYjgOpmmfJMk07W6GlE3tAhrJHM2xrflN7bG2e23Okr8xNTq20tBsO-HETjnTsq33o3Ob_H_JOnJHqaFXV0RYqdCdmN42K5issT0obWN22Wnp9xc0C3QHtyacu8aLuNnp7j0tNhet4d8M5hX6y6Xtiw77a/w640-h340/Neah%20bay%20on%20ENC.png" width="640" /></a></div><br /><div>Thus we see that the actual variation is 16º E - 1yr x 6'/yr, which is no notable change, leaving 16º E. In short, this latest paper chart has magvar wrong by just 1º even using this older data.</div><div><br /></div><div>Neah Bay is remote, but Elliott Bay is not, being the access to Seattle. Below is the latest Elliot Bay Chart, 18449.</div><div><br /><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5DAXmbhz6AKn4G4vXk3wrasGMVSoNoehWI0LKb05lsNwWM4Nyr-h6bUo_b2BXXdYBDhkQHg9LtegsjyCCfunIU9_AS35MiTDj7SNto3Eoac6Ql5kMiicLGdlDv5LxAQS17uHvq0BoboY6F0BEzNwLy4d_riZ2LhpNhNIQ93N_WE86ZmLukwo5Sd1y/s791/latest%20Eliott%20Bay%2018449.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="753" data-original-width="791" height="610" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj5DAXmbhz6AKn4G4vXk3wrasGMVSoNoehWI0LKb05lsNwWM4Nyr-h6bUo_b2BXXdYBDhkQHg9LtegsjyCCfunIU9_AS35MiTDj7SNto3Eoac6Ql5kMiicLGdlDv5LxAQS17uHvq0BoboY6F0BEzNwLy4d_riZ2LhpNhNIQ93N_WE86ZmLukwo5Sd1y/w640-h610/latest%20Eliott%20Bay%2018449.png" width="640" /></a></div><br /></div><div><p>This latest printed chart value is from 2017, which means corrected from 2015 WMM. At 2022 we have 5 yr x 9'/yr = 45'=0.75º, which implies a current magvar of 15.25º E. We find the actual value from the latest ENC below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhNm4MSTURvxWGkuyfvlRLz3oP3TXaZzzodJhaFZH7nnuBrt-YfEavkypJAFfAG93BLTQ68fTNzGJcHxKBWVDb4RFOKCBeq0MTGh1PxyoRG1FxptEb5hdpuB1PnSaMdy7UQZT_9ablhzQojEiWC2mfQOBvPJHUgHKC7-BI16ysyDHXnRMdFVCJlZcnu/s1505/elliott%20Bay%20magvar%20ENC.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1077" data-original-width="1505" height="458" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhNm4MSTURvxWGkuyfvlRLz3oP3TXaZzzodJhaFZH7nnuBrt-YfEavkypJAFfAG93BLTQ68fTNzGJcHxKBWVDb4RFOKCBeq0MTGh1PxyoRG1FxptEb5hdpuB1PnSaMdy7UQZT_9ablhzQojEiWC2mfQOBvPJHUgHKC7-BI16ysyDHXnRMdFVCJlZcnu/w640-h458/elliott%20Bay%20magvar%20ENC.png" width="640" /></a></div><p>An ENC pick report using the NOAA online ENC viewer. Here we are reminded that some ECS use only plain language names of objects and attributes, whereas others use only the so called acronyms (actually just abbreviations) for them... and some use both or offer the option, which is my preference.</p><p>The object is magnetic variation (MAGVAR); the attributes to this object are:</p></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p style="text-align: left;">LNAM = "Long Name" a combination of several object ID parameters (not related to navigation; it is an <i>object identifier</i>, not an object attribute; rarely if ever shown in ECS)</p></div><div><p style="text-align: left;">RYRMGV = Reference year for magnetic variation</p></div><div><p style="text-align: left;">SORDAT = Source date, which, in this case, is date the data was posted</p></div><div><p style="text-align: left;">SORIND = Source indication, which is where the data comes from, an internal NOAA doc number.</p></div><div><p style="text-align: left;">VALACM = Value of annual change in magnetic (in arc minutes per year)</p></div><div><p style="text-align: left;">VALMAG = Value of magnetic variation (in whole degrees for an <i>area</i> magvar report.)</p></div></blockquote><div><p>We always use RYRMGV for figuring the annual change, not the Source Date. In 2022, we have only a 6' change, so the actual variation at the moment is still 15º E, which is essentially what we got from the old data on the printed chart. <span style="font-family: inherit;">This m</span>eans that the rate of change changed at Neah Bay, but not so much here in Elliott Bay. It also shows why updating this magvar data is not a crucial paper chart update in most cases, and hence is not being done.</p><p>______</p><p>With those basics behind us, there are a couple more subtleties regarding magvar in ENC. Below is an expanded section from our book <a href="https://www.starpath.com/catalog/books/1996.htm" target="_blank"><i>Introduction to Electronic Chart Navigation</i></a>.</p><p class="p1" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px;"><b><span style="font-family: inherit;">2.13 Magnetic variation</span></b></p><p class="p2" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify;"><span style="font-family: inherit;">We can get spoiled using ECS navigation, because we just push buttons to switch back and forth between magnetic and true directions. In a dark sense, we don’t even need to know what the variation is. It is rather like not needing to know how to divide, since we have a calculator in our phone, or how to spell, when there is a spell checker in everything we write with. This is just a small part of the slippery slope of electronic navigation, but still one to be avoided.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><i>Magnetic variation</i> (MAGVAR) is an S-57 object that can be encoded into an ENC as either an area or a point object. Area examples are shown in Figure 2.13-1. </span><span style="font-family: inherit;">Some hydrographic offices include this data, but others (i.e., Canada) do not—inland ENC (Appendix 8) do not include MAGVAR in either the US or international versions. When MAGVAR is present as an area object we can find the value of the variation with a cursor pick at almost any place on the chart. The object has a </span><i style="font-family: inherit;">scale minimum</i><span style="font-family: inherit;"> attribute (SCAMIN), so it might not be reported on all display scales. On NOAA charts, the SCAMIN value for MAGVAR seems to be the same as used for the soundings, so if you can see soundings you can see the variation symbol, and if not, you can’t—assuming the soundings display has not been turned off.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><br /></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh-efmK2kc4KCJqtaGwrk6oeSAkhCDkhjgUw65xu34X3Y1DeBojLGgAeQVy0diuK3AxRKgEJwF1f4OO6ouu_vV8Lm2ud6VEixIEgH8tqlLCGFOsQ2mBP1BgMzTt_zkp3Xla4980vMfpJZFcoiCIyKZfU5x71_TbEkEf2MV45k8pi8oL-lagRWlNFviJ/s1608/2.13-1%20area%20plus%20both%20symbols.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1458" data-original-width="1608" height="580" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh-efmK2kc4KCJqtaGwrk6oeSAkhCDkhjgUw65xu34X3Y1DeBojLGgAeQVy0diuK3AxRKgEJwF1f4OO6ouu_vV8Lm2ud6VEixIEgH8tqlLCGFOsQ2mBP1BgMzTt_zkp3Xla4980vMfpJZFcoiCIyKZfU5x71_TbEkEf2MV45k8pi8oL-lagRWlNFviJ/w640-h580/2.13-1%20area%20plus%20both%20symbols.png" width="640" /></a></div><b><br /></b><span><b>Figure 2.13-1</b>. <i><span style="font-family: inherit;">Another example of the value of the NOAA Online ENC viewer (Section 1.8), which has the instructive feature of outlining the boundaries of line and area objects when selected. Thus we can see the extents of MAGVAR area objects. We have made a composite of the reports to illustrate this pattern. In actual use, only one report at a time can be viewed. The MAGVAR areas can have other shapes, and the symbol changes locations as you view the area in different perspectives (called a “centered” symbol) The left-side inset shows the area object MAGVAR symbol; the right-side inset is a point MAGVAR object, showing just the value at that specific point. A hollow version of either symbol marks magnetic anomalies. The acronyms used in the pick report are explained in Figure 2.13-2. The scale minimum attribute of MAGVAR is typically the same as the soundings, so don’t expect the MAGVAR object to show up in a report if soundings are not showing. This image is from 2016.</span></i><br /></span><p></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><br /></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">There are symbols for magnetic variation seen periodically on the chart, although, as noted, we do not need to click it specifically to get variation. The symbols mark the identifying locations of the various MAGVAR area objects. These are the areas over which the variation is the same within one degree. These symbols are sparse in regions where the variation is not changing by one degree over the geographic span of the ENC cell. There is no correlation between the location of these MAGVAR symbols on an ENC and the placement of compass roses on the corresponding paper charts. On any ENC where we see a lot of these symbols it means the variation is changing by about 1º between the symbols.<span class="Apple-converted-space"> </span></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">A sample cursor pick report for a MAGVAR area object is shown in Figure 2.13-2. This is in principle the same data we get from a compass rose on a paper chart—if the paper charts were being kept up to date in this regard, but they are not. Often we do not need the value any more precisely, and since we are unlikely to be using old ENCs (as opposed to sometimes using old paper charts) it would be rare we needed to correct for the annual change in ENC values.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><br /></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQb99-RFfLxkLUEgkUFv1cSalTsqOpadjc1r8BaJLToNFf0mPOu_7UGkJwLI-9Q0yno2xahylQtEWBQzoYFrseqQtQzHqkC5ZdQv6iI1vkDmHTQ2u9NfMlS38nBOKg42lvoHESO-y-f9Qx4SNOz7fSlZuun8j0P_c-6MpTFJ5W0aJzwVVc70CJ0iN-/s1016/2.13-2%20NEW%20mag%20var%20symbols%20w%20area%20symbol.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="792" data-original-width="1016" height="498" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQb99-RFfLxkLUEgkUFv1cSalTsqOpadjc1r8BaJLToNFf0mPOu_7UGkJwLI-9Q0yno2xahylQtEWBQzoYFrseqQtQzHqkC5ZdQv6iI1vkDmHTQ2u9NfMlS38nBOKg42lvoHESO-y-f9Qx4SNOz7fSlZuun8j0P_c-6MpTFJ5W0aJzwVVc70CJ0iN-/w640-h498/2.13-2%20NEW%20mag%20var%20symbols%20w%20area%20symbol.png" width="640" /></a></div><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><br /></p><p></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><b>Figure 2.13-2.</b> <i>Cursor pick report for object Magnetic variation (MAGVAR) at a point. Point symbols include a text label showing the Value of the variation (VALMAG) to the hundredth of a degree at that specific point (15.83º E) and the Reference year (RYRMGV). The actual point report (top) includes values to the tenth of both variation (positive values are east) and its attribute Value of annual change in magnetic variation (VALACM), which is always in arc minutes, with an annual change toward the east being positive, and toward the west negative. </i></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><i> The bottom part of the report is for the area value of the variation at that location, which will always be rounded to the nearest whole degree that is the average value for the local area. You would get this same area report by cursor picking any place near this point on the chart.</i></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><i> Also shown is a MAGVAR area symbol, which is larger, in a fainter magenta, with no label. These symbols mark the area where the variation is constant to within 1º. These are elusive symbols (called “centered”), because they move on the chart as you change the display, staying as near the center of your screen as possible. They are identifying an area on the chart, not a point. Some ECS choose not to include these MAGVAR area symbols, as we can always get the variation with a cursor pick on the chart that reports the variation in that area. The only value of the symbol is to mark where the average variation is changing by 1º. More symbols (as seen farther north) indicate more change in the variation. Note that it can happen that an area average (15º in this case) is not the same as a point value in that area rounded to the nearest degree (15.8º in this case).</i></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><i> Also shown for comparison is the symbol for a point report of a magnetic variation anomaly. An area of anomaly symbol is the same as the area symbol shown, but in outline only (see Chapter 4, Section B).</i></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><br /></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">The exception comes when doing a compass calibration, in which case we want this as accurate as possible.<span style="font-family: inherit;"> </span></span><span style="text-align: left;"><span style="font-family: inherit;">On paper charts, variation is marked East or West and the change is marked "Increasing" or "Decreasing," but on electronic navigational charts (ENC) only algebraic signs are used. East is + and West is –. Thus when the variation and the change have the same sign, the value is increasing with time; when they are opposite, the variation is decreasing. A value with no sign is +.</span></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">The correction is done in the normal manner. Using the value of annual change from Figure 2.13-2, in 2024, which is 3 years after the reference year, the correction would be 3 </span><span class="s1" style="font-family: inherit; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal;">x</span><span style="font-family: inherit;"> -6’ = -18’ = -0.3º. The variation is 15º E, correcting to the west, so the corrected value is 15º - 0.3º = 14.7º E in 2024. We do not use the high precision point value for this because that is the value at just that one point, which is unlikely to be where we are at the time. If the correction and the variation are in the same direction, then it is getting bigger with time.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">A main takeaway here is that even though the charts are updated weekly and the computer knows the time and date, we must still treat magnetic variation obtained from an ENC as if we were reading it from a paper chart. With that said, we note that this data is typically no more than a year old in ENC, so it is rare that we would need this correction.</span></p><p class="p4" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: center;"><b><span style="font-family: inherit;">Direct computation of magnetic variation</span></b></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">One reason some hydrographic offices might decide they do not need to encode the magnetic variation is because many ECS programs (and presumably some ECDIS as well) have incorporated special software that can compute the magnetic variation accurately for any location and date. One example of such a program is <i>geomag.exe</i> from the National Centers for Environmental Information (NCEI). This program can be downloaded for personal use, even if not used as part of an ECS. See References.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">With this, or a similar program, running in the background, a user can interrogate any location on the chart to obtain the magnetic variation—even without an ENC loaded for that location. What information you get from that and how you execute the request depends on the specific ECS that has this feature. It could simply report the variation in plain language or present something similar to an ENC pick report. When using this supplemental ECS feature to find variation for a specific date and place, it would be good practice to check that it agrees with the value given in the ENC for the location.<span class="Apple-converted-space"> </span></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">These details of magnetic variation are important because ship captains are trained to make compass corrections accurate to a few tenths of a degree, which requires correspondingly accurate data. Accurate (point values) of the variation can be found at <a href="http://ngdc.noaa.gov/geomag/WMM">ngdc.noaa.gov/geomag/WMM</a>. The World Magnetic Model (WMM) used for this is updated every five years. This program provides the variation values shown on ENC, so they should always agree.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">Compass navigation is another advantage of ENC over traditional paper charts still in place that are in the process of being discontinued. Most paper charts do not have the latest MAGVAR data, some based on 2010 WMM data or even earlier models.</span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;"><br /></span></p><p class="p3" style="font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px 0px 5.4px; text-align: justify; text-indent: 13.5px;"><span style="font-family: inherit;">___</span></p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com5tag:blogger.com,1999:blog-714718709601620866.post-80212768450806206012022-07-04T11:03:00.002-07:002023-05-18T13:11:47.680-07:00Wind Direction in Grib2 Model Forecasts<p><i><b>Errata added May 18, 2023. </b></i></p><p><i>This article was intended to point out a limitation in the HRRR data provided by several sources in that the wind directions on the two US coasts could be off by 10 to 15º. Several sources corrected the output shortly after this observation was reported, but others took longer to correct. Now we can report that all sources we originally referred to have updated the data output. It appears now that the HRRR data from the various public sources are now identical to what we would get from a direct NOAA download. We have not confirmed this for the other models in grib 2 Lambert format shown below.</i></p><p>____________</p><p>Our lead weather instructor, Dave Wilkinson, serves on the race committee boat for setting up the race courses based on local forecasts in Port Townsend, WA. He uses several regional models for this, but his workhorse has been NOAA's High Resolution Rapid Refresh model (HRRR). He obtains the forecasts and views the data by two independent methods: the LuckGrib app running in an iPad and the qtVlm app running on a PC laptop. LuckGrib provides the HRRR data directly from their server; qtVlm offers an internal method of getting the data from a third party server.</p><p>As races came and went, he noticed a consistent discrepancy in these two presentations of what we all thought was the identical forecast—double-checking that the area covered, and, more importantly, the computational time of the forecasts were the same. HRRR is updated every hour and extends out 18 hr.</p><p>qtVlm has a convenient way to make this comparison as we can load both grib files and show them overlaid with different wind barb colors, as shown below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh1Z_wmAmMt-GaIWRVMo5w6LQ6vVsOgYp2GY4yA0kpkDyaNOOwO3qiBHapL-eq1Gvdfp8F0q8v6aiSNK8EjARLom8nctsxuSSZg94Go2W7vSkmgfspiTX5mkcxMUxeyEmJ2PtUDpsLZOFqtR4QnZl83guwVR_LFGVh3jdN-p3llc8iISj5OT89QQlik/s1052/LG%20v%20SD%20HRRR.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="817" data-original-width="1052" height="498" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh1Z_wmAmMt-GaIWRVMo5w6LQ6vVsOgYp2GY4yA0kpkDyaNOOwO3qiBHapL-eq1Gvdfp8F0q8v6aiSNK8EjARLom8nctsxuSSZg94Go2W7vSkmgfspiTX5mkcxMUxeyEmJ2PtUDpsLZOFqtR4QnZl83guwVR_LFGVh3jdN-p3llc8iISj5OT89QQlik/w640-h498/LG%20v%20SD%20HRRR.png" width="640" /></a></div><div><br /></div>The white barbs are from the LuckGrib server; the red wind is from a third party server we call S1 below. We see there is a consistent shift in direction. We have an even better way to compare this in qtVlm by opening a meteogram on the page and then selecting Compare gribs.<div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjztqr1zwCLvh-BDUP9zDbA3orFe1NxInTLeGKgqW_JV7g_BI8lAR5SKo_EJZJSyeybjt_KwPInctN-aUICberlbCiWyH3iAsgC3CAZ_rXUheFLlLCndyr-WHIu9l7eP_0RNDBos170HzL-iJ_2InFij7geKtJDTPKwwY1WiEbhPGt4_G-uy3QmDZgo/s1047/meteogram%20view.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="818" data-original-width="1047" height="500" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjztqr1zwCLvh-BDUP9zDbA3orFe1NxInTLeGKgqW_JV7g_BI8lAR5SKo_EJZJSyeybjt_KwPInctN-aUICberlbCiWyH3iAsgC3CAZ_rXUheFLlLCndyr-WHIu9l7eP_0RNDBos170HzL-iJ_2InFij7geKtJDTPKwwY1WiEbhPGt4_G-uy3QmDZgo/w640-h500/meteogram%20view.png" width="640" /></a></div><br /><div>Now see more specifically that the wind speeds are about the same, but the directions are shifted about 18º in this example. The slight differences in speed can be understood as noted later, but the direction issue must be sorted out—as we learn below, this difference depends on both the source of the HRRR forecast and the grib viewer software in use. Also looking ahead, I am using HRRR model data for this analysis, but this issue also applies to other popular model data such as NAM, and others.</div><div><br /></div><div>To my knowledge, there are 5 or 6 popular sources (apps and online services) for the HRRR model data, one of which is the primary source at <a href="https://nomads.ncep.noaa.gov/" target="_blank">NOMADS</a>, which individual navigators can access though the NOMADS Grib Filter option—although that is more work than the push-button sources available elsewhere.</div><div><br /></div><div>For now we look at four sources, LuckGrib (LG), NOMADS (NM), and two others, S1 and S2, and for now we use three different apps to view the data, LuckGrib and two other apps we found that can read and display the files in question called here app A1 and app A2—although it is important to stress that neither one of these apps advertise that they support the type of file we are looking at. Thus anomalies in display are not unexpected. I make these comparisons in any event because it is common for navigators to use files from various sources in their weather analysis, especially when the data may be unique high-resolution products.</div><div><br /></div><div>Shown below are only part of the data studied: just 1 location and 1 time—conclusions were double-checked using a second time and a second location in each of the four data files. The data being used are from the 12h forecast (of the 19 available) computed on 9/9/22 at 21z, making it valid at 09z on 9/10/22. My reasons for singling out the LuckGrib products will be clear shortly.</div><div><br /></div><div>First a look at what we see, then a look into what might be taking place.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOpgEJfFVeVKy2XUuXCZ0Til05SE4HZufqx5Sb8uqyn7Lg4bIk3Cve7Jm9AiZpNJN_WG07k3wvXN9PooSPt9_O7s43Twey0eSPV67q0JWLmrkjDm0Eka05hJCtJN0wUcx5o5Ri6C7nnP4t9x_C-O8HQeIiWzJZ_8xv9tr4ji0_83noaJ7kp57XjyUC/s510/Data%20summary%20table.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="385" data-original-width="510" height="387" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgOpgEJfFVeVKy2XUuXCZ0Til05SE4HZufqx5Sb8uqyn7Lg4bIk3Cve7Jm9AiZpNJN_WG07k3wvXN9PooSPt9_O7s43Twey0eSPV67q0JWLmrkjDm0Eka05hJCtJN0wUcx5o5Ri6C7nnP4t9x_C-O8HQeIiWzJZ_8xv9tr4ji0_83noaJ7kp57XjyUC/w510-h387/Data%20summary%20table.png" width="510" /></a></div><div><br /></div>We are looking at a unique model forecast that originated at NOMADS, but in its travels and display something changed. Every app and every source agrees on the HRRR wind speed forecast for this location and moment in time, but they do not agree on its direction. The directions in this table should all be the same. The fact that they differ means something is wrong. We see three directions (290, 306, and 322) and I think we can understand where the differences come from. <div><br /></div><div>The HRRR model data is in the grib2 format, but more to the the point here, the model is computed on a Lambert conformal map projection (grid), which is similar to a great circle projection, whereas all grib1 data (and some grib2 data) are on a rectangular coordinate system, similar to Mercator or an equidistant Lat-Lon grid. </div><div><br /></div><div>Looking into how the grib files are presented and wind directions defined, I believe that this coordinate system distinction is the source of the issue at hand, along with its broader implications. Most of the US high-res models use this Lambert grid, which is notably tilted relative to a rectangular grid over many areas. Below shows the coverage distribution of several grib2 models that are not on a rectangular grid. Images from the LuckGrib Model Explorer function.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0xE05OUOLz-6OtHP3nHJZkJf8q6LRtBsawr-ePzwUmDiihkkSRhaaep9Is0LeRt3VajZczlU6IZd5tFsdoAnKskNVJ2VrZPthn02n3S7fqrfYsX89M_9PL1BxT4mJRMcSYjt5-UXlQC9C0ZFYpCdaKY5aeXWc5VdxMj5FI-ITYGxVzmJHUWXFncTi/s1207/conical%20coverage.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="681" data-original-width="1207" height="362" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0xE05OUOLz-6OtHP3nHJZkJf8q6LRtBsawr-ePzwUmDiihkkSRhaaep9Is0LeRt3VajZczlU6IZd5tFsdoAnKskNVJ2VrZPthn02n3S7fqrfYsX89M_9PL1BxT4mJRMcSYjt5-UXlQC9C0ZFYpCdaKY5aeXWc5VdxMj5FI-ITYGxVzmJHUWXFncTi/w640-h362/conical%20coverage.png" width="640" /></a></div><br /><div>The issue comes about because the model computation in grib2 has the option to specify wind directions relative to either the grid orientation or what they call N/S. It is possible to look into a grib file to see what choice has been made using a tool called <a href="https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/" target="_blank">wgrib2</a>. We then ask for the parameter vector_dir. Here are those results for the four sources in hand:</div><div><br /></div><div><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">C:\Users\macdavid\Desktop\wgrib2>wgrib2 -var -vector_dir LG_HRRR.20220909.21.grb2</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1:0:UGRD:winds(N/S)</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2:335:VGRD:winds(N/S)</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">C:\Users\macdavid\Desktop\wgrib2>wgrib2 -var -vector_dir NM_NOMADS_direct_HRRR.grb2</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1:0:UGRD:winds(grid)</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">2:545:VGRD:winds(grid)</p><p class="p2" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px; min-height: 17px;"><br /></p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">C:\Users\macdavid\Desktop\wgrib2>wgrib2 -var -vector_dir S2_HRRR_09_09Sep22_224022.grb</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1.1:0:UGRD:winds(grid)</p><p class="p1" style="font-family: Helvetica; font-size: 14px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;">1.2:0:VGRD:winds(grid)</p></div><div><div><br /></div><div>We cannot run wgrib2 on the S1 file, because that source is actually a custom one that has been converted from grib2 to grib1, which is in general a tremendous service for mariners who do not have an app that will display grib2 data.</div><div><br /></div><div>Wind in most grib files (not all) is presented as a vector, giving the E-W speed (UGRD) and N-S speed (VGRD) in m/s, then the display app computes the direction and magnitude from these components.</div><div><br /></div><div>What we learn from this is that NOMADS themselves publish the HRRR data with the reference direction aligned with the grid and not with N/S. This is also true for the source we call S2. Source S1 is automatically N/S, because that is all that grib1 has.</div><div><br /></div><div>And we see the notable thing about the LuckGrib data that is what led me to focus this note around that product. LuckGrib has taken the raw data from NOMADS and converted it to N/S, so if the data are exported to another app that is limited to N/S they will still be able to read it properly. </div><div><br /></div><div>So to read the wind direction properly from a NOMADS model with grid reference, we either need to get it from a source that has converted grid to N/S, or we need an app that recognizes this parameter (vector_dir) and makes the correction at the display stage. LuckGrib actually does both. Note in the table above that LuckGrib gets the right wind direction from NOMADS directly (NM) as well as from S2, both of which are grid referenced files, and it gets it right from its own data (LG) which are N/S referenced.</div><div><br /></div><div>Looking at the table above for app A1, we see that it gets the direction right for LG data, but it gets it wrong for the other 3 sources, and I think there are two independent reasons for that. First, it seems that this version of app A1 is not making the grid to N/S correction, so it treats all wind as referenced to N/S, which for gridded data at this location is an error of 16º (discussed below). But these S2 and NM results also agree with the S1 results, which is grib1 and thus automatically N/S. This seems to tell us that S1, when they converted grib2 to grib1, did not make the conversion of the reference. So that the S1 source of HRRR is off in wind direction by the local angle between the grid orientation and N/S.</div><div><br /></div><div>When we turn to the app A2 results, we see something still different. It has the direction right with the LG data, which is N/S, and it has the same error in the S1 data as app A1 reported, so it seems that it is reading N/S info correctly, but on the two sources with grid reference it does not get the right value. Instead of converting the 306 to 290 by subtracting 16º it appears that it added the 16 to the 306 to get 322. In other words, it appears that this version of app A2 is making the grid to N/S correction, but applying the correction in the wrong direction.</div><div><br /></div><div>At least that is how I see it, based on the pictures below.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6hthD0riDRjWmCACSB-x-NsBPc5invBnwzDWaCeKNOrzdGIByoPzIb9TqsZRULkpWg4yxUIm_E-xLxP2NAcKZqdkV1w3lJjz6CQUO2T19bWBHfE6OlB5VyLgUC4o82tul8UZbp4edy5qZ7hbECHNuELW42L4DHBxB3m3s5MWYPHPK0ytwKkxNeO0H/s2451/grid%20orientation.PNG" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1966" data-original-width="2451" height="514" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6hthD0riDRjWmCACSB-x-NsBPc5invBnwzDWaCeKNOrzdGIByoPzIb9TqsZRULkpWg4yxUIm_E-xLxP2NAcKZqdkV1w3lJjz6CQUO2T19bWBHfE6OlB5VyLgUC4o82tul8UZbp4edy5qZ7hbECHNuELW42L4DHBxB3m3s5MWYPHPK0ytwKkxNeO0H/w640-h514/grid%20orientation.PNG" width="640" /></a></div><br /><div>This display in LuckGrib puts a dot on the actual grid points. Wind barbs in between are interpolated. Most grib viewers let users show such a display or show just wind on the actual grid points, which is an important option to prevent us from over estimating the resolution of the actual data.</div><div><br /></div><div>By drawing a line (or leg of a route) between grid points you can read the orientation of the grid at your location, in this case it is 344T, which is 16º to the left of north.</div><div><br /></div><div>Below we look at wind directions relative to the grid and to the N/S line. </div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcx4rfsDQB2osSbJQrzyoePASstF_Whp3t1lDjrqP0w7f_s6qsoF9rvXSUulEXt3LtkcCmaT9vEmbnXk2Rnx1RCaEI-6MEs6-Zqu7lTToP1tOGDS929w2BN86V7jmVrlE5Isr9LjEAA6h0dIsFxN2xnSQL8jNoOHIOPFl_zQLIZvpK_PEO1mgXk_FV/s2118/figure%20wind%20angles.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1934" data-original-width="2118" height="584" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjcx4rfsDQB2osSbJQrzyoePASstF_Whp3t1lDjrqP0w7f_s6qsoF9rvXSUulEXt3LtkcCmaT9vEmbnXk2Rnx1RCaEI-6MEs6-Zqu7lTToP1tOGDS929w2BN86V7jmVrlE5Isr9LjEAA6h0dIsFxN2xnSQL8jNoOHIOPFl_zQLIZvpK_PEO1mgXk_FV/w640-h584/figure%20wind%20angles.png" width="640" /></a></div><div class="separator" style="clear: both; text-align: center;"><br /></div><div class="separator" style="clear: both; text-align: left;">Since we do see app interactions with the grib files, I double-checked the actual content of the grib files using another function of wgrib2. This one lets us ask for the specific values it contains for a specific location. Here we ask for the UGRD and VGRD components at the point and time in question.</div><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqyNu0IzWDvmWgxXBKBWaoIUSYaNB1eC7V6IREnC7MMBJRvsGXCiAbqMa4Xxl-q2JlBSh_rFszbqqYbll9pNKqeOrwJsb_nIevMgoaZjzDW3aBm3T4s_LLQoWS7SUQhsNiVu-0FW3JvtW7wtGcePAnC-c2a5IEU-fyvKK7xu1VvESLpH7OqbGOl9Eu/s752/wind%20vectors%20from%20ugrib2.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="199" data-original-width="752" height="170" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqyNu0IzWDvmWgxXBKBWaoIUSYaNB1eC7V6IREnC7MMBJRvsGXCiAbqMa4Xxl-q2JlBSh_rFszbqqYbll9pNKqeOrwJsb_nIevMgoaZjzDW3aBm3T4s_LLQoWS7SUQhsNiVu-0FW3JvtW7wtGcePAnC-c2a5IEU-fyvKK7xu1VvESLpH7OqbGOl9Eu/w640-h170/wind%20vectors%20from%20ugrib2.png" width="640" /></a></div><br /><div class="separator" style="clear: both; text-align: left;"><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkFkb5YIJAlsQiv6i3wH_MLqmQ106eS7l-G9bvNzE0ifOCSdkQ2ALEKsBz4jJuWkHLH-yQnv8eN05C0MI45kK_sBeyNnn86VQMXMk4L2jRbIySOL5jD5-jAnXGbb39yz_eANb_8G853Q-C1VpmVdgzrEDAs2AjegnyOs1yvqTCzvi6dKbQb-X1X425/s674/graphic%20of%20wgrib2%20wind%20data.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="264" data-original-width="674" height="250" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjkFkb5YIJAlsQiv6i3wH_MLqmQ106eS7l-G9bvNzE0ifOCSdkQ2ALEKsBz4jJuWkHLH-yQnv8eN05C0MI45kK_sBeyNnn86VQMXMk4L2jRbIySOL5jD5-jAnXGbb39yz_eANb_8G853Q-C1VpmVdgzrEDAs2AjegnyOs1yvqTCzvi6dKbQb-X1X425/w640-h250/graphic%20of%20wgrib2%20wind%20data.png" width="640" /></a></div>This is a section of a homemade spreadsheet that computes the magnitude and direction of the wind from U and V. We see that the data in the files are indeed what we expected them to be based on grid vs N/S reference.<br /><div class="separator" style="clear: both; text-align: left;"><br /></div><div>I have checked with the developers of app1 and app2 who both agreed with observations above, and both have made updates that correct this display issue. So with good data, all apps we know of now show the right display. If any questions remain about your data source, look at the coverage maps above to get a rough estimate of the grid angle at your location, which can then be measured as noted, and kept in mind if discrepancies are noted.</div><div><br /></div><div>A wind direction error of up to 18º or so is, on its own, not that much of an error when we realize that the model forecasts and the buoy and ASCAT reports we use to check them are only ±10º or so. The bigger factor comes into play with optimum routing computations where a consistent shift of 10º can make a big difference, and having it off prevents us from getting the best evaluation of a model forecast. Thus if you have more than one source of HRRR data you might run the route using both of them to see if there is any difference.</div><div><br /></div><div>Thanks again to Dave Wilkinson for his sharp eye and prudent navigator's practice to not let something slip by that does not make sense.</div><div><br /></div><div>And thanks to Craig McPheeters, developer of LuckGrib, for ongoing invaluable discussions of grib files and weather models. His overview of models available at <a href="https://luckgrib.com/models/" target="_blank">luckgrib.com/models</a> and related blog posts on details remain the standards we refer to.</div><div>––––––––––––––––</div></div><div><br /></div><div>To check these ideas with another model we look at 3-km NAM CONUS model on the East Coast, just off of Cape Cod, where the grid is tilted the other direction, to the right. We get the model data from LG and from NM. This model is not so readily available as the HRRR—noting we are not using the more popular 12-km NAM. Using the same abbreviations as above, we have the table below summarizing the results.</div><div><br /></div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjWlYkt_KYK__6v807uhxoopiR-zwpSFBIue-v42Y0ptAs0yBLL9NFRjUv01-7WFdSCQnw5ojSm9wxcip6tFHyvr5kLleDcdsdcfbjnWLFORANzA_FcBQcdrqFe8_gaR90GUCZ_2sM307mvdc72HlM1V-sYkMdb3O6lQHDvG12d3SRGFpbwS-PZro2b/s520/NAM%20summary%20table.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="245" data-original-width="520" height="302" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjWlYkt_KYK__6v807uhxoopiR-zwpSFBIue-v42Y0ptAs0yBLL9NFRjUv01-7WFdSCQnw5ojSm9wxcip6tFHyvr5kLleDcdsdcfbjnWLFORANzA_FcBQcdrqFe8_gaR90GUCZ_2sM307mvdc72HlM1V-sYkMdb3O6lQHDvG12d3SRGFpbwS-PZro2b/w640-h302/NAM%20summary%20table.png" width="640" /></a></div>The apps A1 and A2 are the same as above. We see the actual forecast was for 211, and we see in app A1, which does not correct for the effect, a display of 193. We are on the East Coast with the grid to the right (see coverage maps above), so the grid based wind direction is too low. The correction must be added on the East Coast and subtracted on the West Coast. Then app A2 version we have, which does make a correction, sees a grid wind of 193 and corrects in the wrong direction to display 176.<div><br /></div><div>This seems consistent with what we see in HRRR on the West Coast to confirm this research, but again I stress that both app A1 and app A2 have fixed this display issue in their latest builds.</div><div><br /></div><div>If you want to test this, here is a link that will get you HRRR model forecast that you can customize as you like, keeping in mind these are hi res files that get big fast. The date used has to be present day or yesterday, and the latest run will be about 2hr old. Time and date UTC.</div><div><br /></div><div><p class="p1" style="background-color: white; font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="s1" style="font-kerning: none;">https://nomads.ncep.noaa.gov/cgi-bin/filter_hrrr_2d.pl?file=hrrr.t<span style="color: red;">18z</span>.wrfsfc<span style="color: red;">f03</span>.grib2&lev_10_m_above_ground=on&var_UGRD=on&var_VGRD=on&subregion=&<span style="color: #2b00fe;">leftlon=-70.6</span>&<span style="color: #2b00fe;">rightlon=-70.0</span>&<span style="color: #2b00fe;">toplat=42.0</span>&<span style="color: #2b00fe;">bottomlat=41.7</span>&dir=%2Fhrrr.<span style="color: red;">20220917</span>%2Fconus. </span></p><p class="p1" style="background-color: white; font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span class="s1" style="font-kerning: none;"><br /></span></p><p class="p1" style="background-color: white; font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-family: Times; font-size: medium;">This is the 3rd hr forecast computed on 9/17/2022 at 18z (valid at 21z 9/17) for the lat lon box shown. Change these inputs as needed and paste into a browser to get the grid referenced HRRR forecast. </span></p><p class="p1" style="background-color: white; font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"><span style="font-family: Times; font-size: medium;"><br /></span></p><p class="p1" style="background-color: white; font-family: Helvetica; font-size: 18px; font-stretch: normal; font-variant-east-asian: normal; font-variant-numeric: normal; line-height: normal; margin: 0px;"></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAztv9gC0F-NSsX-_WoOUtVVFAnIl92ik4LNDkzMFw9F0Ynz5eXrXDUnKR_5BnCwfeVUkcQkbrcperaXqAaX8kRB7TBFict1Oq639kOcwZMi6ZRQAbyY2HKTKOT6S1vn0KcRjHkNwAseOHAqjxYaeSPH5XZ5yrl9fW_OZT4GPaG8EjA9CFx0ev8led/s518/capecodbay.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="498" data-original-width="518" height="308" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAztv9gC0F-NSsX-_WoOUtVVFAnIl92ik4LNDkzMFw9F0Ynz5eXrXDUnKR_5BnCwfeVUkcQkbrcperaXqAaX8kRB7TBFict1Oq639kOcwZMi6ZRQAbyY2HKTKOT6S1vn0KcRjHkNwAseOHAqjxYaeSPH5XZ5yrl9fW_OZT4GPaG8EjA9CFx0ev8led/s320/capecodbay.png" width="320" /></a></div><br /><span style="font-family: Times; font-size: medium;"><br /></span><p></p><div><br /></div><div class="separator" style="clear: both; text-align: center;"><br /></div><br /><div><br /><div><br /></div><div><br /></div><div><br /></div><div><br /></div><div><br /></div></div></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0tag:blogger.com,1999:blog-714718709601620866.post-24327333954614592742022-05-27T22:16:00.002-07:002022-05-28T07:57:52.894-07:00Ocean Position Plotting Sheets<p><i>First we go over a bit of background and then show how you can create neat plotting sheets using the new NOAA Custom Chart (NCC) app online.</i></p><p>During an ocean passage it is very convenient if not crucial to keep a paper plot of your position as you cross. This is true for a small yacht as well as it is for a large commercial ship or navy ship. The traditional solution for yachts have been to use Universal Plotting Sheets. They are about 13" x 13", spanning 4º of latitude. The user then sets up the meridians to match the latitude selected using a nomogram printed on the sheets. They were originally a DMA product No. VPOSX001, now long discontinued, although still available from <a href="https://www.starpath.com/catalog/accessories/1825.htm" target="_blank">third party sources</a>. There are also numerous <a href="https://www.starpath.com/celnavbook/" target="_blank">free PDF versions</a> online.</p><p>These sheets have been and continue to be used to plot celestial navigation sights to find a position, and then the DR track of the vessel can be made right on these same sheets. In modern times, they are still valuable for plotting GPS positions and planning routes based on forecasted winds, even when cel nav is not in use.</p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj80jcP4zu1OqkDFpjY0oIoTPgOosicaVXJ35FkGHTpYipaWgC-wKvJuSKz4dJNhmVSe4Qbtxw7tc0rAU-u0AbRlsmoYAsfuc3w-WPyKoyX1G6WbWpNrRSbn37skS4MkjqkQ3X5RpKuGi-C-kB8FSLUfB-leaWQGg7In_V-ZBE1oQHK_P-W_1q4Aple/s1502/UPS.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1502" data-original-width="1212" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj80jcP4zu1OqkDFpjY0oIoTPgOosicaVXJ35FkGHTpYipaWgC-wKvJuSKz4dJNhmVSe4Qbtxw7tc0rAU-u0AbRlsmoYAsfuc3w-WPyKoyX1G6WbWpNrRSbn37skS4MkjqkQ3X5RpKuGi-C-kB8FSLUfB-leaWQGg7In_V-ZBE1oQHK_P-W_1q4Aple/w516-h640/UPS.png" width="516" /></a></div><b>Figure 1</b>. <i>Instructions for setting up a Universal Plotting Sheet from our text </i><a href="https://www.starpath.com/catalog/books/1887.htm" target="_blank">Celestial Navigation</a><i>. The meridians and parallels are not labeled, but the user sets them up this way for any location. The book explains how these charts can be scaled up to 30' per 3 inches or even 6' for more accurate cel nav plotting. Scaling down to cover larger areas, you might double it just fine, (3" = 2º), but much more will begin to introduce distortion into the plotting.</i><div><i><br /></i><p>There are other options for yachts to keep track of ocean position when cel nav plotting is not needed. See optional downloads in Item (30) at <a href="http://starpath.com/getcharts">starpath.com/getcharts</a>. We also have there high-res great circle maps of the world we made for mariners at Item (31) on that page. If you want to print any of these on larger paper, see our note on <a href="http://davidburchnavigation.blogspot.com/2022/03/printing-noaa-custom-charts-ncc.html" target="_blank">Printing NCC</a>, which reviews costs and paper sizes.</p><p>The key issue with ocean plotting sheets is: do you need to analyze cel nav sights, or not. If you need cel nav plotting then we need sheets that can be expanded to a large scale, just a a couple hundred miles across. This can be done on any size vessel using the Universal Plotting Sheets shown above, but for ships with large chart tables and large budgets there are other options... and with that, we tiptoe into the topic at hand.</p><p>You can buy blank charts of the ocean on real chart paper at real chart sizes. There are two NGA series of Position Plotting Sheets that cover these; they are available, purchased individually, at official print on demand (POD) printers such as <a href="https://mdnautical.com/211-nautical-charts" target="_blank">Maryland Nautical</a>, <a href="https://www.paracay.com/nautical-charts-and-books" target="_blank">Paradise Cay</a>, or <a href="https://nautical.eastview.com/Default.asp" target="_blank">EastView Geospatial.</a> </p><p>The first style (part number WOBZP9xx) are just blank charts. You choose the Lat range and the charts are labeled with those values, which sets the miles scale, and then you fill in the Lon lines you want. The sheets are 36" x 48". This series has 16 charts, numbers 920 to 936.</p><p>A sample is shown below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiXkRFd5tOK9AbdYmY03_wj4Y_KVjNgnzkGo9WYeBYv3knHd0IKh-Oz07PzArYb-LF4WeBAqgJB_hdAGNywBAvsl81ZWVVdsg2HsR4If4OtPDZrqgfl-the1g-PV0dWMejhWTPh4hW8pa9OIHnZt9xJ9do4S874IC7Ekx8obDKxXzKMUtJJztsn_L8S/s2559/926_35%20to%2041%20to%20print.jpeg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="2559" data-original-width="2000" height="640" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiXkRFd5tOK9AbdYmY03_wj4Y_KVjNgnzkGo9WYeBYv3knHd0IKh-Oz07PzArYb-LF4WeBAqgJB_hdAGNywBAvsl81ZWVVdsg2HsR4If4OtPDZrqgfl-the1g-PV0dWMejhWTPh4hW8pa9OIHnZt9xJ9do4S874IC7Ekx8obDKxXzKMUtJJztsn_L8S/w500-h640/926_35%20to%2041%20to%20print.jpeg" width="500" /></a></div><b>Figure 2.</b> <i>An NGA WOBZ Position Plotting Sheet WOBZP926. Recall you can zoom for details, by clicking the pic, then right click, open in new tab, the zoom.</i></div><div><br /></div><div><br /><p>An index of WOBZ options is shown below:</p></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>No.<span style="white-space: pre;"> </span>( Latitude)<span style="white-space: pre;"> </span>Mid Lat<span style="white-space: pre;"> </span>Scale = 1:x,xxx,xxx</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>920<span style="white-space: pre;"> </span>(4ºS to 4ºN)<span style="white-space: pre;"> </span>0º<span style="white-space: pre;"> </span>1,095,679</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>921<span style="white-space: pre;"> </span>(3º to 11º)<span style="white-space: pre;"> </span>7º<span style="white-space: pre;"> </span>1,087,566</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>922<span style="white-space: pre;"> </span>(10º to 18º)<span style="white-space: pre;"> </span>14º<span style="white-space: pre;"> </span>1,063,344</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>923<span style="white-space: pre;"> </span>(17º to 24º)<span style="white-space: pre;"> </span>20º30'<span style="white-space: pre;"> </span>1,026,715</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>924<span style="white-space: pre;"> </span>(23º to 30º)<span style="white-space: pre;"> </span>26º30'<span style="white-space: pre;"> </span>981,220</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>925<span style="white-space: pre;"> </span>(29º to 36º)<span style="white-space: pre;"> </span>32º30'<span style="white-space: pre;"> </span>924,1984</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>926<span style="white-space: pre;"> </span>(35º to 41º)<span style="white-space: pre;"> </span>38º<span style="white-space: pre;"> </span>864,517</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>927<span style="white-space: pre;"> </span>(40º to 46º)<span style="white-space: pre;"> </span>43º<span style="white-space: pre;"> </span>802,596</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>928<span style="white-space: pre;"> </span>(45º to 50º)<span style="white-space: pre;"> </span>47º30'<span style="white-space: pre;"> </span>741,597</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>929<span style="white-space: pre;"> </span>(49º to 54º)<span style="white-space: pre;"> </span>51º30'<span style="white-space: pre;"> </span>683,493</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>930<span style="white-space: pre;"> </span>(53º to 57º)<span style="white-space: pre;"> </span>55º<span style="white-space: pre;"> </span>629,889</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>931<span style="white-space: pre;"> </span>(56º to 60º)<span style="white-space: pre;"> </span>58º<span style="white-space: pre;"> </span>582,040</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>932<span style="white-space: pre;"> </span>(59º to 63º)<span style="white-space: pre;"> </span>61º<span style="white-space: pre;"> </span>532,576</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>933<span style="white-space: pre;"> </span>(62º to 65º)<span style="white-space: pre;"> </span>63º30'<span style="white-space: pre;"> </span>490,222</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>934<span style="white-space: pre;"> </span>(64º to 71º)<span style="white-space: pre;"> </span>67º30'<span style="white-space: pre;"> </span>841,029</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>935<span style="white-space: pre;"> </span>(70º to 75º)<span style="white-space: pre;"> </span>72º30<span style="white-space: pre;"> </span>660,992</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>936<span style="white-space: pre;"> </span>(74º to 78º)<span style="white-space: pre;"> </span>76º<span style="white-space: pre;"> </span>531,840</p></div></blockquote></blockquote><div><p><br /></p><p>Recall that a chart scale of say 1:80,000 means that 1 inch on the chart corresponds to 80,000 inches on the land it covers. In a standard universal plotting sheet we have 3" on the sheet = 1º of Lat, and 1º of Lat = 60 nmi x 1852 (m/nmi) x 100 (cm/m) x (1/2.54) (inches/cm) = 4,374,803 inches.</p><p>So 3" on the sheet = 4,374,803 inches, so the UPS scale = 1:1,458,267. We see that these WOBZP9xx plotting sheets have a larger scale by about a factor of 2 to 3, which makes sense since they both cover 4º and one is about 3 times larger.</p><p>The other series of NGA Position Plotting Sheets (Series (WOXZP9xx) are a bit like an oversized compromise between the NGA plotting sheets shown above and the Universal Plotting Sheets. A sample is shown below.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgjD4cw80DYaco2Z4kk5TDuryNdLcb_Exup5W-f9WojLnZGInkbB6jCD_Eo17j5Ne5PHeLOmJek-IWi9ygRd6e68_rjfx9V4bJnFkTyrwTkyPI6jLZbEw83Ilz685A-78cbPDwNbeceLcVb0V5flTDuA86bVElhSKpPqb62TbE2jxmDUZs3EuflWHjh/s2000/972_35-38NS%20to%20print.jpeg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1732" data-original-width="2000" height="554" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgjD4cw80DYaco2Z4kk5TDuryNdLcb_Exup5W-f9WojLnZGInkbB6jCD_Eo17j5Ne5PHeLOmJek-IWi9ygRd6e68_rjfx9V4bJnFkTyrwTkyPI6jLZbEw83Ilz685A-78cbPDwNbeceLcVb0V5flTDuA86bVElhSKpPqb62TbE2jxmDUZs3EuflWHjh/w640-h554/972_35-38NS%20to%20print.jpeg" width="640" /></a></div><br /><p><b>Figure 3.</b> <i>An NGA WOXZ plotting sheet WOXZP972.</i></p><p>Below is an index of sheets available in this series (also 16 options, numbered 960 to 975). Except for the first one, each is set up for N or S latitudes. These charts are printed on 20" x 36" paper but active part of the chart is about 20x20. We are told the the US Power Squadrons use these sheets in their cel nav courses.</p></div><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>960 LATITUDE 2N to 2S</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>961 LATITUDE 0 to 4 </p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>962 LATITUDE 4 to 8</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>963 LATITUDE 8 to 12</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>964 LATITUDE 12 to 15</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>965 LATITUDE 15 to 18</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>966 LATITUDE 18 to 21</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>967 LATITUDE 21 to 24</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>968 LATITUDE 24 to 27</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>969 LATITUDE 27 to 30</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>970 LATITUDE 30 to 33</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>971 LATITUDE 33 to 37</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>972 LATITUDE 35 to 38</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>973 LATITUDE 38 to 41</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>974 LATITUDE 41 to 45</p></div></blockquote><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px; text-align: left;"><div><p>975 LATITUDE 45 to 49</p></div></blockquote></blockquote><div><p>Again, both of these NGA plotting sheets styles can be ordered from any POD printer. They are purchased individually, on good chart paper, and sell for $12 up to $26 depending on item and source.</p><p>_________</p><p>Now we get to the fun part!</p><p>When NOAA announced the <a href="https://cdmcs.org/wp-content/uploads/2019/11/NOAA-Sunset-Plan.pdf" target="_blank">discontinuation (sunsetting) of traditional paper charts</a>, they also announced an intriguing new way that mariners can make their own paper charts, covering the regions they care about. The program is called <a href="https://devgis.charttools.noaa.gov/pod/" target="_blank">NOAA Custom Charts (NCC)</a>. It is still under development with a few notable issues to be resolved, but I am confident that by the time these are our only option for any paper chart (December 2024) they will in fact be superior to the traditional paper charts we are accustomed to.</p><p>The key point for our present interest is the fact that we can define the scale we want and the location we want to cover, plus we can control what details of the chart are shown. For plotting, we will shut off as much of the chart info as we can, and then choose a scale convenient for either cel nav plotting or ocean tracking.</p><p>Below is an example made—at a student's request—for the waters west of Puerto Vallarta.</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhyW6nZbJxCI5bs4NZmcvUQeRoIpCJU4FJNXq75ZdWHU_R_-kiHw5jyMyxcn5yp1oWKklYabi4BBaCnxIv7F8Xl7Eq-cwCpIOLRTmifwZi4cgqHxr47FRR8bTEs9m8tR8XAUZXylxP6dIySdDCDHnpgoXHw9QbO6IlcjAPPxEuyvPMJT4p2X7GwJlBJ/s1399/NCC%20plot%20sheet%20w%20rose.png" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="905" data-original-width="1399" height="414" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhyW6nZbJxCI5bs4NZmcvUQeRoIpCJU4FJNXq75ZdWHU_R_-kiHw5jyMyxcn5yp1oWKklYabi4BBaCnxIv7F8Xl7Eq-cwCpIOLRTmifwZi4cgqHxr47FRR8bTEs9m8tR8XAUZXylxP6dIySdDCDHnpgoXHw9QbO6IlcjAPPxEuyvPMJT4p2X7GwJlBJ/w640-h414/NCC%20plot%20sheet%20w%20rose.png" width="640" /></a></div><br /><p><b>Figure 4.</b> <i>Plotting sheet made with the NOAA NCC app. The compass rose has been added.</i></p><p>When you make the sheet (chart) it will not have this compass rose on it, but you can add one if you want by downloading this file <a href="http://www.starpath.com/downloads/Compass_Rose_for_plot-sheet.png" target="_blank">Compass_Rose_for_plot-sheet.png</a>, which has a transparent background. Pasting images into a PDF is easy with Adobe Acrobat, but takes takes special steps using free apps from Mac or PC. Depending on what plotting tools you use, you might not need a compass rose (ie triangles or Weems plotter do not need one.)</p><p>We have a video on <a href="https://youtu.be/ktFbPGihAao" target="_blank">How to make the NCC</a> (27:19) and, again, we have an article on <a href="http://davidburchnavigation.blogspot.com/2022/03/printing-noaa-custom-charts-ncc.html" target="_blank">How to Print NCC</a> on big paper.</p><p>Another way to make a plotting sheet is to print the screen from a favorite nav program viewing base map only set to the region you want as shown below.</p><p><br /></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgxYYOTyDQw1nejiJ_C3f2WjsniA_4_ZrSAQq9zFvEYRFkqIotfYq6xPtDNcmFpq0ugzAtup0uZBMsngJhbftqTGp1fXn7HeoDJzAtkLbIpM2069bmCknsZeAtHtfUERFVC8MyIdt5Y-U5MeVS9m-Pn44hjQu4CSMwLY65tO-18RaZMe4Xos619JLCc/s3174/qtVlm_plot_sheet.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1719" data-original-width="3174" height="346" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgxYYOTyDQw1nejiJ_C3f2WjsniA_4_ZrSAQq9zFvEYRFkqIotfYq6xPtDNcmFpq0ugzAtup0uZBMsngJhbftqTGp1fXn7HeoDJzAtkLbIpM2069bmCknsZeAtHtfUERFVC8MyIdt5Y-U5MeVS9m-Pn44hjQu4CSMwLY65tO-18RaZMe4Xos619JLCc/w640-h346/qtVlm_plot_sheet.png" width="640" /></a></div><br /><b>Figure 5.</b> <i>Plotting sheet made from exporting the screen of a nav program. This is from the free program <a href="https://www.meltemus.com/index.php/en/" target="_blank">qtVlm</a>. You can also do the same from the free program <a href="https://opencpn.org/" target="_blank">OpenCPN</a>. For this method, you might want to mock up a lat-lon interpolator scale. Graphic tools for changing colors once the image is captured are helpful. In the above we turned the dark blue ocean to white, and the while lines to gray.</i><p></p><p><br /></p><p>Post comment/questions here as needed. As time or need arises, I can add a video on this process of making plotting sheets with NCC app.</p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p><p><br /></p></div>David Burchhttp://www.blogger.com/profile/13413234790483329339noreply@blogger.com0