# Problems generating a small circle: Arctic Circle

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
``````

Would overlay a cartesian grid do the trick?

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

Semi-axis versus full axis

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
``````

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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!).

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Thanks Paul. The confusion just hits the roof sometimes.

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.

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

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