Pygmt Project default projection surface?

Hi. I’m trying to use the project function to compare data along tracks, but I’ve realized that even trying to project a track to itself gives me a pretty big deviation.

What I mean by this is, say I have my data in a list, using data[0] as the projection center, data[-1] as the endpoint, and data as the input data for the project function, the mean value of the perpendicular coordinate I get is about 100m.

I don’t know if it’s because my data doesn’t fit well along a great circle, or because the reference surface pygmt uses is far enough from the real shape of the earth that it’s causing those deviations. I couldn’t find the surface information in the documentation, so if anybody knows it would be very appreciated. I know I can define a projection ellipse through axis lengths, but I’d hoped the default setting would be more accurate than that.

Thanks a lot!

If you need an exact geodesic and not a great circle then GMT project is not going to do that; it is using a spherical calculation. What is the geographic extent of your data and what options are you giving to project?