# How to draw a specific arc in polar coordinate system

See the example below. I want to plot the r=5 arc in the polar coordinate system. I expected data like

``````0 5
60 5
``````

should be plotted along the r=5 arc, but actually, it doesn’t. I also tried the -A option but failed.

Is there a simple way to plot a specific arc in polar projection? Such plots are very useful when plotting a depth profile with Earth’s discontinuities like core-mantle boundary and inner core boundary.

``````gmt begin map
gmt basemap -R0/60/0/10 -JP10c -Baf
gmt plot -W2p,red << EOF
0  5
60 5
EOF
gmt end show
``````

This works:

``````gmt begin map png
gmt basemap -R0/60/0/10 -JP10c -Baf
echo 0 0 0 360  | gmt plot -SW10c -W2p,red
gmt end show
``````

It doesn’t solve my problem. Here is a real use case.

How can I plot the arc for the core-boundary boundary at the radius of 3480 km? I don’t think there is an easy way to calculate the correct size for -SW:

``````gmt begin map png
gmt basemap -R-60/60/0/6371 -JP10c+a -Baf
echo 0 0 -60 60 | gmt plot -SW10c -W2p,red
gmt end show
``````

When we did the -A stuff to control points and resampling we first only had lon,lat in mind (to follow great circles). Of course, in a polar coordinate system (r, theta) that translates into a straight line unless we let -A take new directives r|t for radius|theta. Then, we would resample along radius first, then theta, or the other way around. So in your case, I imagine that a -At selection and two records (-60, 3480), (60, 3480) would work. Perhaps turns this into a feature request?
Workaround via -SW would be to compute that diameter from your settings and look at the modifiers on -SW to not draw the radial line

See the feature request at