Problems generating a small circle: Arctic Circle

Q&A
1

On one of my previous plots, I’ve generated a segment of the Arctic Circle with:

gmt project -C-40/66.5629444444 -E40/66.5629444444 -G0.01/23.4370555556

When I tried to plot the complete Arctic Circle, i.e. 0 - 360, I got nothing; I guess it due to this: Note, when using -C and -E to generate a circle that goes through the center and end point, the center and end point cannot be farther apart than 2|colat|.

Searching the forum, I found this nice list of what I might try.

project -Z ([…] to create the coordinates of a full small circle) seems to be the appropriate thing to do. My attempt:

gmt project -Z2604.2361 -C0/90 -G0.1
             |           |      |
             1           2      3
  1. Difference in latitude from north pole, (90) to arctic circle (66°33′49.5″), expressed as km. I want a circle, so major axis = minor axis.
  2. Origin, north pole, (0,90).
  3. Output sample density.

This produces a circle that is too far north (red). A segment of the Arctic Circle that is correct is seen in blue. See script and figure below. What am I doing wrong?

gmt begin arctic-circle png

gmt basemap -JS0/90/20c -Bxafg -Bya5f1g5 -R0/360/60/90
gmt coast -W -Dc
gmt project -Z2604.2361 -C0/90 -G0.1 | gmt plot -Wthick,red,- -l"polar circle with project -Z"
gmt project -C-40/66.5629444444 -E40/66.5629444444 -G0.01/23.4370555556 | gmt plot -Wthick,blue,- -l"polar circle with project -C -E"

gmt end show

arctic-circle
arctic-circle2496×2495 311 KB

2

Would overlay a cartesian grid do the trick?

3

Not sure, or tested. But I would really like to find out why I don’t get the expected result in my initial post.

4

Semi-axis versus full axis

5

And there we have it; double the distance and it lines up with the ‘other’ (correct) arctic circle.

So the distance specified in -Z is full axis, and not semi?
Is this something that wise people just know, or could it be mentioned in the docs?

gmt begin arctic-circle png

gmt basemap -JS0/90/20c -Bxafg -Bya5f1g5 -R0/360/60/90
gmt coast -W -Dc
gmt project -Z5208.4722 -C0/90 -G0.1 | gmt plot -Wthick,red,- -l"polar circle with project -Z"
gmt project -C-40/66.5629444444 -E40/66.5629444444 -G0.01/23.4370555556 | gmt plot -Wthick,blue,- -l"polar circle with project -C -E"

gmt end show

arctic-circle
arctic-circle2496×2495 315 KB

1 Like
6

Hah. Well, I just read project -Z documentation and it talks about major and minor axis, not semi-axis, so at least the documentation is correct. And yes, wise people just know these things (after reading the docs!).

1 Like
7

Thanks Paul. The confusion just hits the roof sometimes.

8

Isn’t semi-axis the most common term used in gmt to specify these kind of things? I may have assumed that this was (implicitly) the case here too. No excuse though. You may force feed me lutefisk.

9

The ellipses in plot are specified the same way (full diameter if circle) so I think we are consistent

1 Like