Sunday, July 15, 2018

Finding Watch Rate

For celestial navigation we need to know UTC, formerly called GMT. We have many ways to get accurate UTC on land with internet connections. Offshore we need either an HF radio or satphone or we need to have a watch whose rate we know.  All watches gain or lose time at some rate. A chronometer is a watch whose rate (gaining or losing) is constant. The cheapest quartz watch has a rate of a few seconds every 10 days, and it is constant, which qualifies it as a "chronometer." A very expensive quartz watch might half that rate.

In another note and video we show various ways to get accurate time and demonstrate that these various sources do indeed give the same time, which took some coordination of sources. That article includes a 2015 rate measurement of a Timex quartz watch, done in a way similar to what is described here.

For now we illustrate the process of measuring the rate of a watch, in part to support our GPS Backup Kit that includes a rated watch. This shows the method we use to rate the watches we include in those kits. Chances are, some users of that kit will have their own quartz watch, which is probably more sophisticated (and expensive!) than the one we include. However, it is also just as likely that the watch in hand does not have a known rate. It is likely right to the nearest minute, but for cel nav we need to know UTC accurate to the second. Ask yourself now how much you would bet that the watch on your arm—or a crew member's arm—is accurate to the second, or that its rate is known the the correction can be computed.

On the other hand, these days it is actually not as likely that someone is wearing a watch as it was 10 years ago. Many folks have replaced watches with cellphone time, which is accurate when connected to a network. However, once we head off to sea, it is prudent to go back to wearing a traditional watch, preferably with known rate.

We start by setting all the watches to the same time, in this case, within ± 0.5s. The standard source for UTC used here is an iWatch connected to a cellphone network. We have confirmed that the watch is indeed ticking off the correct UTC seconds—although we did notice that every once in a while it appeared to hesitate a fraction of a second at the transition, but this did not affect its transition for the following second.

So with a wireless connection to your phone, it will likely be the most convenient source of accurate time. Or your computer time when linked to the internet.  Computers and watches off shore away from any wireless connection, however, are essentially just stand alone watches, although in principle they should have a clock circuit in them that would be as good as a random quartz watch.

Off shore—or on land, for that matter—the primary source of accurate UTC is a GPS signal, which includes UTC as well as your position.  GPS is the source the phone and internet companies rely on to provide us with accurate time on our devices.

We are effectively using the GPS here to rate a watch in preparation for losing the GPS.  That is, we are talking here about rating a watch so we can do accurate celestial navigation, which would typically mean that for some reason we have lost all GPS navigation.

Below shows the watches lined up on Day 1 when they were set. The iWatch is on ZD = +7, so it was on day July 6, while the others are set to UTC, which would be 7 hr later on July 7.

The three Casios are model F-91W, which is perhaps the most famous of all watches. It is at least very popular.  Dating from 1991, they are still popular and sell for $10 (Walmart or Amazon), with millions having been sold globally. They are water resistant, with a 7-year battery.  The only weakness is this type of resin band, which if worn daily will only last a year or so before they harden and break. It is easy to replace the band with a more durable style. Also the light in them is not very useful; but they are accurate watches, known to last for many years.  The other is a Timex Expedition with several ocean crossings to its credit. Essentially this same model is available today for about $32. Its virtues are a super good light (Indiglo, a timex innovation) and true waterproof. Two time zones are also standard, but not needed. They are, however, no more accurate than the F-91Ws, as is the case with most quartz watches.

Next we want to start a notebook to keep track of the data. (If you use spreadsheets, the notebook data can be later transferred to the computer as a good way to organize the data and maybe make a graph of the results.)  We do not need any special math for this exercise.  We find time differences, then then divide by some time period to get the rate in seconds per, say, 10 days. This can all be done perfectly well with a few written notes in your logbook.

Here is the second data point two days later.

So far we have not learned much, but this will take a couple weeks.  For best values it will take longer. Good timekeeping would call for us checking and recording its error to extend the rate measurement as long as we have accurate UTC available.

This check does not have to be done every day, but some regular check every 3 days or so will lead to a better evaluation—see, for example, the plot of watch error vs time given in the link above. It will also be clear, even on the second data point, that the seconds do not turn over on the watches (test case and reference time) simultaneously, so we are only ± 0.5s at this point.  A way to get around this is illustrated below.

After a while it won't matter much if you are off ± 0.5 sec on each reading, we can still get a good average value over an extended time. But if you care to home in on the rate more quickly, then one trick that is a good estimate of the fractions of a second is to take a series of rapid cell phone pics of the two time sources. In this case is would be a phone taking a series of pictures like the one above.  Then make a table as shown below, this one made on the 7th day. It shows the watch errors from 8 pics, each taken just as the iWatch changed seconds.

Remember we are after the correction we apply to the watch to get UTC, so a positive number means we add this to the watch time; negative, we subtract it.  In this example, in the second pic of watch B, it read 2 second below the iWatch standard, so it is a +2. Now we can summarize what we have to date.

The set time and the first data point did not use the average cel pic method, so they are not quite as accurate, but this will not matter in the end, as we shall see.

The time difference between first and latest times in decimal days (dT) is figured from the times converted to decimal days (d.dd), which can be adequately approximated as d.dd = d + h/24.  The "10d-rate" is latest watch error (WE) divided by time elapsed since setting (dT).

We don't have good rates yet, but we have learned a lot. First, these $10 Casio F-91W classic watches (A, B, C) are indeed very good. They are all gaining time (except maybe C, which is spot on after 7.8 days), but at a very low rate, and the Timex Expedition is losing time. So far it looks like -2.3 s/10 days, but it is too early to tell.

I will post this note now, and then update it every few days till we get convincing rates. We should know these pretty well in another week or so... but again, for best cel nav practice, this should be considered an ongoing process where you keep a record of the watch error and date, from which you can continually update the rate... or better put, home in on a more and more accurate rate.

If you are using your own phone to compare to a watch, then you would need a second phone to take the pics. Or hang the phone over a computer monitor and use pictures of that for the series, such as shown below.  That is in fact how we did it for the rating example in our Celestial Navigation textbook in Figure 11.5-2. Most computers have some option to show the time on the screen... again we have to assume you have internet connections and have the system clock set to update automatically.

I will return to this post every few days for a while to update the rates.  For those following it, you will then see how the accuracy of the rate improves with time—or better still, find a watch and rate it yourself as an exercise in practical cel nav.

But looking ahead, here is what we are after (using new timing numbers):

We want to know the rate of the navigation watch and the date we set it. Say it turns out to be +2.4 s every 10 days, and we set it to have no error on Aug 8.  Then on Dec 3 we want to know the watch error so we can find a correct UTC.

First we figure the number of days between Dec 3 and Aug 8. You can count this out, or use the day numbers from the back of the Nautical Almanac—yes, there is such a table there, probably originally for this very purpose, although there are quite a bit of data in the Almanac that was at one time used frequently that we don't use much these days.

Dec 3 (day 337) - Aug 8 (day 220) is 117 days. Then 117d x 2.4/10d = 28.1s.  We read the time from the watch and add 28 seconds to get the right watch time, then we add the zone description (ZD) of the watch to get UTC. The ZD might be, for example, ZD = +4h, corresponding to EDT.

Belt and suspenders.
We are wearing the watch (watch time, WT) we navigate by, and it is set to ZD = +4.  That, by definition, means UTC = WT + 4h, assuming no watch error (WE). But we know all watches gain or lose time at some rate. In this case we know the rate is +2.4s every 10 days, and we set the watch to the correct ZD+4 time on Aug 8.  It is now Dec 3.

We did a sun sight and the WT was 18:20:05.  So we add 28s (as noted above) to get 18:20:33 on Dec 3. Then we add 4h to get 22:20:33 as the correct UTC to use when we look up data in the Nautical Almanac.

The data tables below will be updated every few days, with comments on progress. The date of last entry is given in the table.

The rates are getting better known, but still not stable.  After we see the 10d rate the same for a while we know we are getting closer.  We need to know this to the tenth of a second so we can correct the watch farther into the future. 

Now that we know they are good, we added 3 more of the F-91s (D,E,F), and started tracking them 1.79 days ago, so we will have enough calibrated watches for the backup kits.

Next time you look here there will be a new table in this space, 
which we will update every few days, with comments on progress.

Wednesday, June 20, 2018

Dip Short, Distance Off, and Running Fix

We use dip short primarily for practicing with cel nav sights on land using a shoreline of a lake or bay that happens to be too close to use, so the shoreline is blocking out the true sea horizon.  We can determine if a shore is far enough away to ignore this issue using the square root of our height of eye (HE) above the water surface.  The minimum distance for a good sea horizon is D in nmi must be greater than the Sqrt (HE in feet).  Standing on a dock with HE = 16 ft, if the shoreline I use for a horizon is 4 nmi or more away, then I can use that shoreline as if an open ocean horizon.  Nothing special would be required for cel nav sight reductions.

Indeed, for D > Sqrt (HE) we are not actually seeing the shoreline; the curvature of the earth is blocking it and we are seeing a true horizon.

Beyond its use in sextant practice on land, dip short remains a practical tool for the navigator's bag of tricks. It was more important to large vessels that relied on cel nav than it is to smaller ones because with a low HE you have to be right up on the land to have the shoreline blocked, i.e., at 9 ft HE you can be within 3 nmi of the beach and still get good sights using the shoreline.  On a taller ship this would be more like 5 or 6 miles.

In any event, if we have not had any good navigation for quite a while and come up on land during the night that we cannot identify, changes are we would hang out till day break to carry on.  And if the sun came up over the land as in the pic below, we would want a sun sight to start figuring out where we are, but if the shore is too close, we have to solve for dip short.

But to do this, we need both HE and the distance to the shoreline (D) which we do not offhand know so far. We do not even know what land we are up against; no chart, no info.  And that is the subject at hand: how to find our distance off in this case so we can use dip short.

Below shows the sun over the land. We are some distance off, standing at HE = 9ft and we guess we are closer than 3 nmi.  

Step 1 is take a bearing to an identifiable land mark on the shoreline. Let's say it was 347 T. This is plotted below on the same picture, but just assume you are doing this on a black sheet of paper, on which you represent the landmark as just a dot on the paper.  We choose to have the top of the page be 000T and then use any protractor to lay off the 347 line of position (LOP).

 Now we head off on course 294 T and mark the time and our speed, or note the log, and watch for the bearing to change at least 30º or so. In this example when we found that the bearing the chosen landmark was 012 T after traveling 0.6 nmi. That second LOP is plotted below.

Now we  have enough info to figure our distance off as shown in the next picture.  We do a running fix, which is described first, then we go over a more automatic solution.  We now have to establish a miles scale for our blank piece of paper. It can be anything; just pick something convenient.

We know when we started to move we were on the 347 LOP. Then we sailed 0.6 nmi in direction 294  and ended up on the 012  LOP.  There is just one place where that can be true, and when we find that we have found our position, relative to the landmark, which gives us the distance off we want.

We cannot have been at point A, because 0.6 nmi toward 294 does not get us to the next bearing line. We also cannot have been at point B, because our run would have put us past our known LOP.  You can set the dividers to 0.6 nmi and use parallel rulers to find the right place, i.e., point C, but there is an easier approach.

The systematic solution that does not require hunting around is just pick any point A, and plot out the 0.6 nmi in direction of your course, 294, and make a dot.  Then use the parallel rulers to move the initial 347 LOP over to that point, as shown above with the dotted line.  Where that line crosses the 012 LOP is your running fix.  Now use your miles scale to figure the distance off the landmark, which is what we needed. We still have no idea where we are, but we do know we are 1.1 nmi off that landmark and that is what we need to find dip short.

 The regular dip correction shown in the Nautical Almanac is computed from the formula shown below for dip.  With our HE of 9 ft, this would be a correction of -2.9' to the sextant height. At any height above the water we are seeing over the true horizon, so our sextant heights are too big and we have to account for this with the dip correction.

When the land is too close, we do not see the proper horizon for our HE. The horizon is too close so our sextant height is even bigger than it would be for a true horizon, so we will have a bigger dip correction, the so called dip short. That is a given from the formula below, which is in our cel nav textbook and many other places.

When we put in our HE and measured D we see that the dip short we should use is 5.1', which is also a negative correction.  So doing this right in this particular example, we gained just over 3 nmi accuracy in our cel nav sun sights.  Below shows the horizon we used (solid line of the shore) compared to what we corrected to with dip short (dotted line).

Saturday, June 16, 2018

Davis Mark 3 Sextant Part 1 — How to read the Angle Scales

This is one of several notes with associated videos on the use of the Davis Mark 3 sextant. We have a more general book on How to Use Plastic Sextants, but now we are focusing in on the Mark 3.  The reason for this focus is a bigger challenge we set for ourselves in our new booklet that teaches mariners how to use a sextant and find position at sea with no previous training at all. In fact, to the extent we succeed, you do not need this article or video! Just get the book and open it when you need it. To that end, we put together a kit that includes one of these sextants, this new booklet, and a few other things to serve as a GPS Backup Kit.

But for now, however, we address those who, for whatever reason, wish to use a Mark 3 sextant. There is a manual that comes with the Mark 3, rather detailed even, but it is our experience from teaching cel nav for so many years to so many thousands of students that the stock manual is not enough. So our GPS Backup Kit includes the book below, which explains how to use the Mark 3, which must start with how to read the scales, the subject at hand.

Back and front covers of our new book, available in print or ebook format.

Later we address how to calibrate the sextant and take sun and star sights, but now we just look at how to read the scales.  Below is the angle the sextant measures, called sextant height (Hs).

Below is a picture of the Mark 3 with parts identified. There are two adjustment screws (#1 and #2) which we discuss later. The angle we measure Hs will be in the form 39º 20', which is about what the one below is set to. 

We read the degrees part of the angle from the arc scale, and the minutes part of the angle from the vernier scale at the bottom of the index arm that slides along the arc. A vernier scale is a way to estimate fractional positions between two lines. The linear version we use today was invented by Pierre Vernier in the 1630s, likely based on a circular version used by Portuguese navigators in the late 1500s. The vernier scale has interval separations slightly larger than those on the scale it is interpolating. In the picture below we see that 30 intervals on the vernier scale span 31 intervals on the arc scale.

Start by looking at sample A below (click the pic for a better view). This is a reference showing what 0º 0' looks like. The checks we make for sextant calibration at each sight (index error) will be just a slight variation of this alignment, as discussed below.

Click the image for a bigger view.
The degrees part of any sight is read from the arc scale relative to the 0' mark on the vernier scale. In sample B we see from the degrees scale that the angle is bigger than 32º, but somewhat less than halfway to 33º. In other words the minutes part of the angle will be less than 30' It is always valuable to estimate what the minutes are before actually checking to see what they are. In this case, for example, you might decide is this just a bit bigger than 32 or is it almost 33 or is it near halfway, just below or above halfway, and so on.

Use the vernier scale to to get a better measure by finding which of the tic marks on the vernier scale most closely lines up with any of the degree marks above it. Zooming in on the image (which we do with a small magnifying glass when underway) we see that 20' or 22' could be considered aligned, with 18' or 24' definitely not aligned. Note too that the out of alignment marks will be off in the opposite direction on either side of the best aligned one... or maybe two, as in this case.  We called this one 22, but you could argue in this case that 21' might be best, since 20 and 22 were pretty close.

In Sample C this is a little easier with the degrees being almost 32, but not quite, so degrees part is 31º and the minutes alignment is best at 46'. Again, notice that the 45' and 57' are off in opposite directions.

A possible blunder to make in these measurements is to count the degrees scale backwards. In sample C that would be reading the angle as just bigger than 28º. It pays to double check we are doing that right. In other configurations it could be more misleading.

Another challenge we face is when it looks like the degrees line up exactly as in sample D. It would be a mistake to call this 28º 00' and go on.  When the degrees line up very closely (as they will with all of the index error measurements) then we must turn to the vernier scale to see if it is large minutes or small minutes that line up. Small minutes alignment means you are just over 28º; large minutes alignment means you are just under 28º and the actual degrees part is 27º, not 28º. Zooming in on D we see that 4' is the best alignment, so the angle is 28º 4'.

The next three samples are what we see when measuring the index correction, discussed later. In sample E we can see from the degrees scale alone that we are just above 0º and checking the vernier we see the amount above is 6', with 4' and 8' off in opposite directions. This value of Hs would be 0º 6'. When doing an index correction measurement we would call this 6' "On the scale."

In sample F we have similar case, but in this one it is easier to see that both 4' and 6' are equally unaligned but they are better than all the others, so this would be called 5' On the scale.

In sample G we cannot tell from the 0º alignment if this is above or below 0, so we check the vernier to find that the 52' mark is best aligned, and again we check that the alignment on either side is off in the opposite directions.  With these large minutes aligned, we have effectively -1º + 52' = -8'.  In other words, this alignment is just 8' Off the scale, which is how we would record it.

For index corrections we have then either small minutes aligned which are called "On the scale" or we have large minutes aligned and we subtract that from 60' and call the result "Off the scale."

That is how the scales are read. This must be done carefully if we want to get out the full potential of the Mark 3 sextant. A small magnifier helps.  Also we stress multiple places that whenever possible we should not rely on just one measurement. For good work we should take 3 or 4 sights each time so we can average the results.

We will add more articles and videos on the use of this sextant, but they will now assume we know how to read it.

Friday, June 15, 2018

Race to Alaska (R2AK) Navigation

A question came up in our online nav course about R2AK navigation. As it turns out we have worked on this route in great detail a couple years ago as we assisted the Team MAD Dog in preparation for their record setting race. I started to answer this longish question in our class discussion forum, but decided it could be of broader interest, so I put these notes here.

All such planning starts with the waypoints. We made this set for the full race that the team then transferred into their two handheld GPS units and also into Navionics "Boat US and Canada" app on two iPhones and one Android phone. This app for about $50 includes all Canadian charts. They are not as good as the official Canadian echarts, but those cost $200 or $300 from EC... although there is a very good set from RosePoint Navigation for $99 the last I Checked, but these may only work on Coastal Explorer.

PS. There are some tricks to getting an external gpx file into a mobile app version of Navionics. On the other hand, it is relatively easy to get them into a Garmin handheld.

R2AK-1.gpx is the full route Port Townsend to Ketchikan, with logical waypoints numbered and named from start to finish.  R2AK-2.gpx and R2AK-3.gpx are two short alternative legs if the main route has bad weather in these regions. [ Might have to right click and choose save, else you might look directly at the xml files.]

These files can be loaded into OpenCPN or other enav program and then studied in on the screen and on the routes manager display. Without Canadian charts, however, the details are hard to discern. The WA and AK charts are free downloads.

You can also drag these gpx files onto Google Earth to see what the routes look like in detail. There is some thought that there could be very many routes to Ketchikan, and probably so, but there is a logical direct route if weather permits.

Next we used current predictions from the Canadian Current Atlas that we customized and made into ebooks that could be loaded into their phones. Then each day and hour is a book mark, easy to find. Recall that MADDOG was all navigated from the trampoline of an open catamaran. No nav station and no other electronics.

Once into AK waters, we used our own publication called Southeast Alaska Current Atlas, and did the same ebook layout with that data. Both of these convenient current presentations paid off.

This was an unassisted race, so they had to get their weather info underway themselves, but we could run a day or so out at least as starting points. We considered the key was getting to Seymour Narrows on time, so this was focused upon.  They hit the Narrows, exactly as the current turned in their favor, flying by at 20 kts as the fishing boats hanging out for the right time were pulling their anchors.

Now we have better wind data than we had at that time.  For US waters WA and AK you can use the HRRR (our 19 hr, updated hourly) or 3-km NAM for longer runs.  The best data once into Canada would be HRDPS - High Resolution Deterministic Prediction System, which is 1.3 km run every 6h out to 2 days.  You can get that from LuckGrib on an iPad and transfer it to Expedition for routing... or just look at it on the luckgrib app. OpenCPN and other popular nav programs may not be able to read it.

Our new book Modern Marine Weather 3rd edition includes detailed discussions of latest high resolution models.

Another key factor to the navigation of this route is floating logs and sometimes deadheads—meaning logs floating vertical with not much showing.  The latter is a risk throughout the Pacific Northwest, but the probability of colliding with one is small.   The when seen they should be reported to the Coast Guard who will then tag them with a flag.

On the other hand, after a very high tide, many of the logs that were beached find there way back into the water,  and these can be a serious impediment in some channels.  I have seen cases there where we had to navigate through a maze of these at dead slow.  This means navigation at night in these conditions takes special care.

As always, a careful reading the the US and Canadian Coast Pilots (the latter called Sailing Directions) is mandatory for this route.  The US versions are free downloads, the Canadian Sailing Directions for BC come in four volumes.  There are also numerous cruising guides to these waters with discussions of anchorages and small boat facilities.

If questions come up on this navigation, then post a question here or in the classroom.

We have other more general weather notes on the inside passage at  Be sure to get a copy of the BC Mariner's Weather Guide which is a top left link on that page. This shows where all the reporting stations are located.

See also this link that gives a gpx file of the current station locations.

Sunday, June 10, 2018

Weems & Plath Expanding Square SAR Course Identifier #113

A common search pattern used in search and rescue (SAR) operations is called the expanding square pattern, which is just what it says it is. You travel along calculated route legs in a pattern that expands every 3rd leg so you systematically cover the the search area in an expanding pattern, as shown below.

To plan out such a pattern you need to know your (1) initial heading, (2) the distance you want between the squares, (3) your boat speed, and (4) the time you plan to start. 

In one sense, the next logical step is turn to your electronic charting program, put in the way points, select an average speed and start time, and look at the resulting route plan, which will tell you the headings and times to turn onto each leg.  Then print that plan or take a picture of it with your cellphone.  In fact, the route itself will be laid out on the chart plotter and you can just follow it.

Or you could simply start off with the boat's track showing, and create the route from your displayed and saved vessel track... But that is not the point at hand.  Several agencies, including the USCG who certify assist vessels require that you solve this manually, without relying on any electronics, and we can't truly fault them for that. We want this done right, and we don't want it fouled up if the electronics hick up, or we accidentally push the wrong button on some display.

A device for doing this is offered by called the #113 Course and Leg Identifier, and this note explains how to use that.  The instructions that come with the device are sparse and strangely enough several other descriptions of the device I have seen do not add to it.  So we were happy to add a few notes on this when asked.

Below is a picture of the device. The middle disk rotates so you can point it in the initial heading of the search pattern, in this example 030.  This device implies this will be on a heading that is a multiple of 15º, but it does not matter if you use true or magnetic headings for this.

For higher res image, click above, then right click and choose open in new tab,
then click that for a big picture of the device. 
Or download pdf of this note.


Step 1. Point the track to your initial heading in multiples of 15.

Step 2. Choose your track spacing and boat speed, and enter the table (bottom left) of times for various leg separations and boat speeds. In the example used here, we choose legs 1 mile apart with a vessel speed of 10 kts.  This gives us a time increment of 6 min.  Had we chosen a separation of 1.5 miles with a boat speed of 12 kts, the tine interval would be 7m 30s.

[This is just the boat speed in minutes per mile (60/S) multiplied by the distance run.]

This  6 min the main time interval used to figure the times for each turn.  Each leg has a multiplier on it. For leg 5 this is x3. See green circles in the figure.

Step 3. Mark the start time at the base of leg 1, and then compute the turn times for each leg and write them with pencil in the spaces provided as shown.  Pencil marks on this disk can be erased for later use.

This step takes some care as mistakes are easy, so double check your work.  Then you head off on the headings indicated and turn at the times you have marked.

Obviously, if you do have a chart plotter, watch it to be sure you are making a square search pattern.  Current or leeway could through this off, so you would have to adjust the headings as needed.

Below is this example, roughly laid out in OpenCPN (ie legs and headings not precisely set; just click, click, click...) and the corresponding route plan, which can be used to check if we were right with the Course and Leg Identifier.

Thursday, June 7, 2018

Free Barometer App Designed for Mariners

Not many mariners realize that the most accurate barometer on the boat might be in their pocket... in their cellphones. There are not any native functions in iOS or Android phones that show the pressure reading, so you would not know it is there by just surfing around the functions of your phone.  It is not listed anywhere.  A third party app is needed to test that you have one.  There are many free versions of such apps, which brings up point one. Why do we make another one when there are so many already?

We want to let folks know about this tremendous resource, but then we are asked for an app recommendation, and the problem begins. Until now, the apps have all been too complicated. They want to do more, so they end up doing things like getting your elevation from the GPS, which even with a WAAS satellite connected is not accurate enough for barometer work adaptable to marine navigation.  In these days of remarkable weather services via model forecasts, we want the pressure accurate enough that we can tell if a surface analysis map is correct or not, so we can believe the forecast. In the tropics, we can also use accurate values of the pressure as a very powerful means of tropical storm forecasting.

Even worse than that, some apps get what is often called a "reference pressure" from the nearest airport, which are readily available online. Again, these maneuvers end up with pressures that are not accurate enough for our needs. There are even a few Andorid apps I have seen that do this without even saying what they are doing  In short, we could not find any app that we could recommend to our students to take advantage of this powerful, free resource.

Also only a few of the apps include a way to offset the sensor pressure, which, as good as the phone barometers are, they do, almost all, need some small adjustment (up to ± 2 mb or so) to be set right. So having an easy, transparent way to do that precisely is crucial.

Then the other common drawback is most apps do not have a way to store the data at all, let alone storing it with the time and location that we need.

So we have addressed these issues in the Starpath Marine Barometer App for iOS and Android.

Below are the four screens available to the user.

There is a rather detailed help file included with it to explain the features. You can set a sensor offset and we explain how to do that, and for sea level pressure you must manually enter the station elevation, ie how high is the phone above sea level when using it. In tidal waters we explain how to find the right elevation in the presence of tides.

You can store a pressure permanently, along with the time and your location.  Note that any app that uses the barometer has to be given access to Location Services in an iPhone, even if they are not using the location for anything.  Since there is no separate way in the phone to give access to the barometer sensor, this is the way Apple chooses to cover that permission.

We use the GPS sensor only for Lat, Lon, nothing else.  Also when you store the pressure it also records your sensor offset so you have a record of when that was done and under what conditions.

The only sort of bonus addition is we do add an option for averaging the pressure over the past 14 seconds. This covers the change in barometer elevation in a seaway. I will post a video soon that shows these functions in action—it's a mini workout; you do it on the stairs. This averaging can be shut off for use on land.

We do include the clear caution that the convenience of this accurate digital pressure from your phone—or any other electronic barometer—is not intended to replace a trusted aneroid barometer. All electronics are vulnerable to the rigors of the ocean environment. We compare a good mechanical instrument like the Fischer precision aneroid barometer to a sextant. It is a maritime investment that will remain dependable for generations.

We also note that the pressure data stored in the phone is intended to be transcribed to the ships printed logbook at your early convenience.

For more information and links to the app stores see Starpath Marine Barometer.


We have learned the Apple SE phones do not have a barometer. The barometer sensor seems to start with the ver 6 iPhones and newer. Also please note that not all tablets (Apple or Android) have a barometer, even newish ones. If you load the device and everything works except it shows no pressure then chances are your device has no barometer. This can usually be checked by a good inquiry about barometers in your model of phone.

Saturday, May 26, 2018

Christopher Columbus and the Spherical Earth

Columbus has long been a pet peeve of mine. I regret the unfounded adulation he is given in schools, which seems to persist into public thoughts on him. He would be better remembered as the founder of the African slave trade between Europe and the Americas—although I can say that in the city of Seattle, Columbus Day has been officially changed to Indigenous Peoples Day.

Reading his log, we find that Columbus was indeed a true master at ocean dead reckoning, which in the day was the key to good navigation, as it remains today. But that is the end of what good we can say about him. He was a charlatan, motivated by religious dogma and personal greed... but I digress.

He believed the flat earth theory promoted by the church at a time that every educated person in Europe knew the earth was a sphere, and many knew the circumference of it, which was measured remarkably accurately in 250 BC (see end notes here). In writing about Columbus in numerous places, I have made that statement, and just moved on. Today, cleaning up the office—a regular routine here carried out every two or three decades—I ran across a folder I had made in the early 90s, showing roman coins depicting a globe, which were at one point gathered to support that argument. So now with the convenience of the blogosphere, I present these below and throw away the old prints as a step in cleaning house. They are apparently from a coin collectors catalog.

In other words, the concept of the earth as a spherical globe was so well known in Roman times that it appeared on common coins.

To supplement this thought, here is a figure from our Celestial Navigation text explaining the first accurate measurement of the size of the earth... which I just realized now, has another of my digs at Columbus! 

The measurement relies on basic principles of celestial navigation, known in 250 BC, but much expanded upon by 1490—when Columbus was traipsing around with some obscure map based on a religious cult that he did not know how to interpret. Needless to say, many people of his time and for another 300 years, wise and unwise, were overly influenced by the church, but they did not go as far as Columbus did to risk this voyage so he could get rich enough to fund his own crusade. They did not tell me about that in school.

Figure 10.3-3.  How Eratosthenes measured the circumference of the earth in about 250 BC. On the solstice he knew the sun was overhead in Syene (Aswan, 24º 4’N, 53º 7’E), as the sun was known to shine directly down deep wells. It was also known that in Alexandria, 428 nmi to the north, on the same day at noon there were indeed shadows cast by the midday sun. He measured the sun’s zenith distance of 7.2º there at that time, probably from the length of a shadow as shown. The distance between these two cities was well known at the time. Since the measured angle turned out to be 1/50th of a full circle, the full circle must be 50 × the distance between the cities.

We are not sure exactly how well he knew it at the time, but we now know there is 428 nmi of latitude between these two cities. This gives a result that is almost spot on the right circumference. Celestial navigators know the circumference of the earth automatically, because of the definition of a nautical mile, namely: 360º × 60 nmi/1º, or 21,600 nmi.

The main uncertainty in figuring his exact accuracy stems from not knowing the exact definition of his unit of length. Using all variations of this and accounting for the offset to the west of Alexandria, his error was somewhere between 0.1% and 16%. In any case, very good. The factor of exactly 1/50th is just a numerical accident from the geometry of the measurement.

What is rather more curious than the greatness of this achievement—Eratosthenes had so many, in various sciences—is that Columbus was totally oblivious to this information 1240 years later. Other navigators before Columbus and after him were well aware of the dimensions of the earth. It was not long after Columbus’s voyages that this knowledge all resurfaced and world charts proliferated.

Syene had been a famous city in Egypt, long before the Aswan dam. Since ancient times it was the southern frontier, located at the first cataracts that blocked farther navigation of the Nile to the south. It was a 3 to 4 week sail from Syene to Alexandria. The stones of the pyramids were quarried near there and shipped north.

Tuesday, May 15, 2018

Paper Charts vs. Electronic Charts — Some Thoughts

This is an outline of a talk given at Captain's Nautical Supply on May 15, 2018 in support of the Coho HoHo program. 

Those reading online will need to do some reading between the lines... or better still, get a copy of our book on the subject! 

First a survey

• Who uses what?  PC, Mac, or Tablet...

 • What nav programs are used: OpenCPN, Coastal Explorer, Time Zero, Navionics, iNavx...

This is a short overview followed by details in questions, with a
reminder that Starpath offers courses in all aspects of marine navigation


• These days there is no reason not to have every chart we might ever want stored on our computers.

• Tablets are handy, but we can still do very much more with a computer.

• Generally PCs are still more functional than Macs for navigation and weather... the only exception is for Mac, which also has an iOS version for PC users with an iPad. This is the state of the art GRIB source and viewer without parallel, on any platform. Mac or iOS costs $20. (I have no financial ties to this product, but am a strong proponent... if you care about weather, it will change your life!)

• Main reference: Introduction to Electronic Chart Navigation. This is a unique resource with information not found in other sources. Crucial to successful use of ENC.

Main value of electronic chart systems (ECS) over paper charts 
demo in OpenCPN )

• Boat position tracks on the chart, as well as AIS targets

• CPA can be displayed on most ECS (electronic charting systems)

• Depth and other alarms can be set (easier on vector charts)

• Layout routes and compute route table (leg distances, headings, times, names)

• Shows tides and currents

• Quick range and bearing lines for piloting

• Easy to keep up-to-date

• In US waters and other places as well, they are free whereas paper charts always expensive

• Usually easier to see in various conditions (bright light on the deck is an exception)

• Vector charts have more detail than paper charts

Main value of paper charts over echarts

• They work when they are wet.

• They don't require power.

• The view is always the same and cannot be screwed up by wrong device settings, or a mis-click at a crucial time.  (Generally tricky situations are best navigated with echarts, with the best paper chart laid out on the chart table.)

• As such, a minimum number of paper charts are required for any voyage.

Overview of our resource

• For echarts and paper charts

• Check out the interactive viewer for chart selection

• Check out seamless ENC viewer to see how ENC charts work... i.e., click an object.

• Check out pdf charts... they can be stored in your phone or tablet.

Using ENC (electronic navigational charts) 

compared to 

RNC (raster navigational charts)

• See Fig 1.2-1  [ Figure references are to our electronic charts book above ]

• See also Fig 2.6-2

• ENC require an all new approach to chart reading.

• User controls what is shown on the chart (Base, Standard, Custom).

• User controls the scale of the chart, which is effectively done with RNC as well but this is more consequential with ENC.

• All objects have an attribute SCAMIN, which is the smallest scale the object will be displayed upon. This is a subtly to be reckoned with. A light with SCAMIN 21999, for example, will show when the display is zoomed to 1:20,000, but will not show if you zoom out to 1:23,000.

• OpenCPN has an excellent presentation of the S-57 format.

• Descriptions of symbols are not printed on the chart, but included in object and attribute descriptions accessed by a "cursor pick."

• Everything on the chart is an object, which has attributes, one of which is often a category, which in turn has a list of options.  See sample in Table 1.7-4. Check links below to see more.

• All objects and attributes are assigned a 6-letter "acronym,"  i.e., UWTROC

• These descriptions are in our Appendix, but also online at   Another source online with a different layout is  This one is based in Russia and we are told may not always be up to date, but sometimes its layout makes it easier to find something... also these values do not change very often.

• Where have all the towers gone?  There are no towers on ENC. They are now an object LNDMRK, with attribute CATLMK = tower. See other attributes of LNDMRK at caris.

• Chart names of ENC are different and convey more info that those of RNC.

• Regions covered by specific ENC are irregular and do not correspond directly with RNC.  See Fig. 1.5-1 and Table 1.5-2.

• Safety depth zones can be defined to match the vessel. See Figures 2.4-1a and 2.4-1b.

Special Value of ENC

• More info on the chart. Objects have more specifications, and this will just improve with time, plus they effectively include all Light List data.

• Alarms on obstructions and depths are automatic,  whereas if needed on RNC we have to define boundaries.

• Cleaner view of some areas, with options to "over zoom" in productive ways.

• US ENC one of the few that do the vertical datum contour correctly. Can use it to read MHW. See Figure 2.8-2.  The renowned UKHO ENC do not do this properly!  Canadian ENC are even worse on this detail, though both are otherwise fine sources of ENC.

• File size for storage and update delivery is much smaller than RNC. (Once activated in your nav program this is less of a factor, because the ENC size gets doubled when creating the necessary SENC files, but even doubled they are notably smaller.)

• Looking ahead, the new S-412 weather overlays planned by the NWS will be so powerful that we will be forced to use ENC just to access that program.  See discussion in our new book Modern Marine Weather, 3rd ed.

Wednesday, May 9, 2018

Ocean Rowboat Polar Diagram

Ocean rowing is a growing sport. This year the Great Pacific Rowing Race will be sharing the race course to Hawaii with both the Pacific Cup and Vic-Maui sailboat fleets. The dynamics of rowing are of course very different from sailing, but it occurs to me that we might do some form of optimum routing for the rowboats as we do routinely for sailboats.

Figure 1. Start of the 2014 Great Pacific Rowing Race. Photo by Ellen Hoke Photography. This shows the type of boats we are discussing, sometimes called "classic fours." Crew of four, rowing two at a time, about 30 ft long, about 3,000 lbs fully loaded for a crossing.

As it turns out, wind is as important to ocean rowboats as it is to sailboats, and ocean currents are proportionally more important. We have all the tools in place from the sailboat technology to compute an optimum route, providing we can come up with some reasonable set of rowboat polar diagrams.

I have been working with my friend Jordan Hanson and his OAR Northwest rowing projects for many years, starting with assisting them on their record-setting 2006 transatlantic victory, NYC to Falmouth, England. They also rowed around Vancouver Is, which is frankly as challenging on the outside leg as being in the North Atlantic—actually more so, because there is a rocky coast on one side of them. In 2013, he and his team rowed from Dakar, Senegal to just north of Puerto Rico, where an unusual back to back sequence of big waves flipped the boat during a watch change when a hatch was open. All were rescued, and in another feat of seamanship and perseverance Jordan followed up with an air search and subsequent tugboat rescue of the boat itself.

He and his team have also rowed around the Olympic Peninsula, a complete circumnavigation by row
boat. Yes, there is indeed water on all sides of this large section of our state... something that we at Starpath confirmed years ago.

What we have below is a compilation of his thoughts on this matter for a classic fours as we discussed it. We propose this as a starting point to build from. These days with all the sophisticated data logging possible, we should be able to home in on this fairly well. The upcoming row race that starts on Jun 2 will be online, tracked by YellowBrick. We also have Jordan's Logbook from the 2006 trip and the training for it. That includes wind speed and direction, discussion of waves, along with COG and SOG, although actual numbers from that data were not used here—that is on the list to do.

We make these assumptions:

(1) We consider two rowing speed limits. A boat that can row steadily, over long periods in flat water and no wind, with an average speed of 1.5 kts and a boat that can average 2.5 kts in these same conditions. Most boats will be somewhere between these two limits.

(2) We are predicting the average speed made good (SMG) over a long period, several hours or more. These boats can surf down big waves at up to 10 kts or so, but these are just very short bursts.

(3) We assume here that the wind has been blowing long enough at the given speeds that the seas have built to their typical potential for each wind speed. This won't necessarily be fully developed seas, but just typical of what you might see. (This is a different approach from sailboat routing where we consider effect of wind and waves separately.)

(4) We assume the wind and waves are in the same direction. It is likely fair to assume that the SMG will be lower when these two differ by 45º or more.

(5) We assume for now that we can base the full range of wind angles on vector components of the estimates for head winds and tail winds alone.

And we come up with Figure 2.

Figure 2. Estimates of rowing speeds vs. wind speeds.

Sailors will have to pause a moment when looking at this!  It looks a lot different than what we are used to.  Boats that row 1.5 to 2.5 kts in flat water will be assisted by tail winds quite notably. The issue here is momentum. The boats weigh some 3,000 lbs, so it is work to get them going, but once moving, they can be kept moving with less effort, which is notably assisted with wind behind them. With a sustained wind behind them of just 15 kts, the average speed, with about the same effort, goes up some 40%. This gain increases with wind speed up till there is a physical limit on steering the boat. At some point around 20-25 kts, it can be more efficient to give up, set out a sea anchor, and just drift with this tail wind in the right direction.

The amount they slow down on the sea anchor depends on the diameter of the anchor. Jordan's experience is mostly with a 9-ft model, which he says really stops the boat, leaving it drifting at some 1 kt or so. A smaller sea anchor would let them drift faster, which we have estimated in the diagram above.  This is just one aspect of this preliminary analysis that we await more input on.  We assume that by 25 kts most boats would opt for the extra rest rather than the fight to steer the boat.

Rowing into the wind, they get stopped at lower wind speeds.  Here we guess that some boats might fight it up to 17-20 kts, but at some point, again, the effort is not worth the value of rest.

With wind on the nose, when you stop rowing and set the sea anchor, you start drifting backwards, so your SMG is negative. In this case the bigger the sea anchor the less you lose. (Do boats carry sea anchors of different sizes?)

With these starting point estimates, we then look at wind at other angles, using the logic of Figure 3.

Figure 3. Estimating effect of TWS at various TWA based on head-wind and tail-wind values. For example, with a TWS of 10 kts at a TWA = 45, we would read the effect on the boat from Figure 2 using a TWS of 7 kts. 

We break the wind up into either a head-wind or tail-wind component and a leeway component, which in this first analysis, we assume does not affect forward speed.  With this assumption, a beam wind does not slow down forward progress, but just makes it harder to row. On closer look, this might end up being a negative number near the beam, if the rowers can only keep one oar in the water at a time. Again, we wait for feedback on this.

Figure 4. Wind on the beam. Jordan's 2006 crossing.

This simple model implies that if with 15 kts of wind on the stern you make good 3.5 kts, then with 15 kts on the quarter (relative bearing 135º) you would enter Figure 2 with 0.7x15 = 10.5 kts and find a SMG of about 2.8 kts.

Likewise, if 15 kts on the nose has slowed you to 1.0 kts, then 15 kts of wind on the bow (relative bearing 45º), enter the table with 10.5 kts to get a SMG of about 1.5 kts.

If this logic makes sense, then we can create the polars for all TWA and TWS.  Each boat would then have a set of curves for SMG vs TWS and TWA depending on their starting point at TWS = 0. But before getting out the spread sheets, we wait for comments here to confirm or propose changes to this crude  model.

Ocean rowboat routes are largely downwind (Figure 5), which can be seen by looking at the Yellowbrick tracks of past races along with the prevailing weather maps at the time. But there will certainly be times in a race where one might want to work toward a desired route, and indeed if this analysis works out, we should be able to compute when that is best done.  If the polars are right, the routing programs will tell us that.

Figure 5. July COGOW scatterometer winds as presented by Pitufa, overlaid with the 2014 routes from the Great Pacific Row Race site. The 2016 routes are about the same. The goal at hand is to figure how much a rower should strive for the best route when it does not follow the winds around the Pacific High route.

Needless to say, we face the same challenge of sailors in that the forecasts are good for a few days but then get progressively less reliable.  We can, for example, get 16 days of GFS wind forecasts, which would make an interesting test for the rowboat routing computation.  The wind forecasts are not dependable after 4 or 5 days, but they will be something to start with, and the amount the forecasts drift off of correct is more important to sailboats than to rowboats. So this too adds some confidence to this approach.

The working procedure with these tactics is you run the optimization computation every 6 hr when the new forecasts come out. We cover the ways to evaluate these computations while doing them in our newly released Modern Marine Weather, 3rd ed. The routes compute in seconds, so we can try lots of variations.  There are several high quality routing programs for iPad or Android tablets, and the wind data can be downloaded by satphone while underway. Predictwind offers a service of computing the routes for you based on your polar diagrams stored with your account info. You send the request (departure, destination, and start time) by email and they return the route by email.

Standing by...