Getting a Compass Rose on a generated chart

[Ken]

I'm here -- https://devgis.charttools.noaa.gov/pod/ working on generating some charts.  I'm getting the areas on want on charts and generating pdfs with the export function.  I have the compass rose option turned on but there  isn't a way to get a rose in the extent I generated.  I'm wondering if position a rose on the chart is a feature that is not implemented yet.

Attached is a screen shot of the application showing the extent I want to generate.  There's a partial rose in the picture but it's obviously not going to be in the extent.  Is the only solution to hack the chart after it's generated and copy and paste the rose into the image?

 

 

Edited May 27, 2022 by Ken

[Joseph Berkovitz]

The roses in the new NOAA POD tool are a work in progress. It is virtually impossible to control where and how they appear. 

At the moment I think it’s best to become adept at adding or subtracting 15 degrees as needed when calculating bearings using these charts. Mnemonic: Magnetic is More in Massachusetts and Maine.

Another choice which is graphically easier than creating your own compass roses is to overlay an array of vertical lines oriented to magnetic north. 

[Ken]

Thanks Joe.  I think you covered some of this stuff in the online sessions.  Tough to remember everything I try to learn last winter.

We're planning to paddle directly along the shore from Broad Cove to Mackworth island on Sunday.  So it isnt a must have this week.  I just want to keep learning so that I can get it when I need it.

-Ken

 

 

[Ken]

Being that I already printed the two pages I needed, I used the luddite tools of protractor and sharpie to add a couple 15* lines.  What's very coincidental is on a 25000 chart on 8.5x11" paper, the 15* lines line up within a whisker of where the 1 minute latitude lines meet the edge of the image.  So close that the next time I print something at this res, I don't think I'll even measure, just draw lines between those  points.  

 

Edited May 27, 2022 by Ken

[EEL]

You can buy compass roses printed on transparent plastic film with a sticky side to "tape" on a chart which might be a solution meeting your needs.  Usually they can be found at marine supply shops.  I have used them in the past to add a rose to a chart segment I had copied.

Ed Lawson

[David M]

Thanks Ed!

Here's a link.  Weems & Plath VEC Vecta Rose (pack of 6) (landfallnavigation.com)

I'm ordering a pack.

Edited May 28, 2022 by David M

[Nancy Hill]

Why does anyone need a compass rose on a chart? True north is up. Plot your course and add for variation (15 degrees for MA, about 17 degrees for mid-coast Maine). People now use variation and declination interchangeably (for historical reasons - nautically it was called variation because it was observed how the compass varied with location), but what ever you do don't call it deviation - that is something entirely different)

http://compassdude.com/i/declination-us.png

 

 

 

 

[Joseph Berkovitz]

While one doesn’t need a compass rose, it does provide something useful beyond noting what the numeric variation is.

I’ve found that for many map users the rose serves as a reference for the visual angle of a magnetic bearing - it shows directly what it looks like.  And of course there are procedures for using it to convert between true and magnetic that don’t involve adding or subtracting, which for some people is challenging or error prone. (Personally I do the math but I’m facile with numbers!)

 

[David M]

On 5/30/2022 at 9:26 AM, Nancy Hill said:

Why does anyone need a compass rose on a chart? True north is up. Plot your course and add for variation (15 degrees for MA, about 17 degrees for mid-coast Maine).  

 

 

 

 

For me, it's a matter of habit and preference. 

Early on in my sea kayak journey, with so many "first-time" experiences, I took the approach of minimizing the known from the unknown.   Transferring the magnetic north with an arrow often scaled to 1-5 nautical mile(s) provided a welcome reference when paddling with a folded chart in unfamiliar areas, especially in reduced visibility situations of fog or night paddling.

I further forged the habit when creating charts for extended international trips, where I will often cut and copy a chart into manageable pieces. 

For me, it's not unlike a spare paddle.  99 times out of 100 I don't need it, but when I do, I'm sure glad I brought it. 

[Pintail]

Joe and Nancy are quite right, of course: you don't need any compass rose on your charts -- and, in any case, variation changes all the time (well, every year or two).  Cleaner (on your charts) to make the correction in your head.  (Cadbury's Dairy Milk Very Tasty tells you which way to go to make the addition or subtraction, where Cadbury's = compass; Dairy = deviation (irrelevant here, since no electrical circuits or ferrous objects in your kayak); Milk = magnetic; Very = variation and Tasty = true).

As Nancy says, deviation is something quite different; but <was> relevant for me in a former life.  It can be quite alarming to see your magnetic compass change its apparent reading as you turn on some electrical circuit or other!

Edited June 7, 2022 by Pintail

[Ken]

Something I've been playing with.  I hate using drawing programs.  If I know I want a line 14.42* of so long, trying to get it exactly correct with a mouse drives me bat@#$% crazy.  So I wrote a little static webpage with a program in it do draw the lines I wanted.  It's design to fit charts generated at 25000:1 for 8.5"x11" paper in portrait mode from https://devgis.charttools.noaa.gov/pod/.  It's an overlay of a three mag north lines separated by .5 nautical miles.  (I need to double check the scale one more time but I think it's very close.)  The intent is that you either find a utility that can overlay an image on a .pdf or use the luddite solution of running a piece of paper through the printer twice.  Print the chart and the print this page on the same paper.

This was my lunch time project and I gotta get back to real life now.  But I'm going to look it over one more time and if everything looks good, do another one for landscape mode.

Disclaimer: it renders correctly for me in Chrome on my Mac.  On a different machine and/or with a different browser, it might scale differently.  The angle of the lines will likely remain correct but there's no guarantee on exact position of the lins.

If you try to use this, use the attached .html file, not the preview image.

 

What's in the file --

<!DOCTYPE html>
<html>
<body>

<canvas id="myCanvas" width="850" height="1100" style="border:0px solid #d3d3d3;">
Your browser does not support the canvas element.
</canvas>

<script>

// SET THIS BASED ON LOCATION -- magentic declination in degrees.
const deg = -14.42;

const canvas = document.getElementById("myCanvas");
const ctx = canvas.getContext("2d");

const rad = deg / 180 * Math.PI;
const lineTop = 110;  // Based on the canvas height and desired margin.
const lineBot = 950;  // Based on the canvs height and desired margin.
const centerX = 425;  // Based on center of the canvas width.
const lineYLen = lineBot-lineTop;
const lineSpacing = 151;  // Based on chart scale, how far apart parallel lines appear, determined emperically.
const intervalX = lineSpacing / Math.cos(rad);
const firstLineX = centerX - intervalX;
const lastLineX =  centerX + intervalX;

for (let lineX = firstLineX; lineX<=lastLineX; lineX=lineX+intervalX) {
  const lineTopX = lineX + lineYLen * Math.sin(rad) / 2;
  const lineBotX = lineX - lineYLen * Math.sin(rad) / 2;
  ctx.moveTo(lineBotX,lineBot);
  ctx.lineTo(lineTopX,lineTop);
  ctx.stroke();
}

</script>

</body>
</html>

 

linesOverlayPortrait.html