Saturday, November 12, 2022

Adding an image to a PDF

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. 

The NOAA Custom Charts (NCC) app sometimes has difficulty placing compass roses in best locations.  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.  See Instructions on adding an image to a PDF.

Preliminary Notes

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.

To remove all pages of the NCC except the chart (on page 1):

1) Open the chart in Google Chrome

2) Choose Print, top right icon

3) Under Destination choose Save as PDF 

4) Under which pages choose Custom and enter 1 in the field below it

5) Print, with the option to rename.

Owners of Adobe Acrobat Pro, with Mac or PC. This is the easiest solution.  

1) Open the PDF in Acrobat

2) Choose Tools tab and then Edit PDF

3) Choose Add Image, navigate to the rose PNG saved, press Open.

4) Resize with a corner control; drag to desired location

5) Click Close top right of the display.  Done

Mac computers use Preview App. (Easy… but involves an unusual trick!)

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.

2) In the rose window, click the screen to activate it, then do 

cmd+A to select all, then 

cmd+C to copy all, then 

cmd+V to paste what you copied right back on top of the image, 

then click the center of the image to activate it then

cmd+C to copy

3) go to the NCC window and click to activate it, then do cmd+V to paste the image onto the PDF

4) size as desired and more to desired position, then cmd+S to save and you are done.

PC computers install free graphics program called Inkscape and use it.

1) Open Inkscape, then menu File/Open and open the NCC PDF

2) accept all defaults they offer and choose OK

3) Position the window so you can see the full PDF

4) Do ctrl+A to select All

5) Right click image and choose Lock selected objects

6) Menu Layer / Layers to open the layers window on the right. 

7) Lock the top layer shown which is your NCC chart by clicking lock icon

8) Click + to add a layer; call it “compass rose”; Position “above current”, be sure it stays highlighted

9) Menu File/Open and open the rose PNG

10) Click the rose to activate it, do ctrl+A to select all, then ctrl+C to copy all.

11) Right click the screen and paste the rose

12) Position and size it as desired, then menu Save as, choose PDF format with name of choice, and done.

Wednesday, November 2, 2022

Slow Water Rule

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.

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.

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.

In this sketch, t3 is one third of the slack to peak times.  Alpha is the slow water estimate we propose.

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.

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.