True vs. grid north

[letmaik]

I am a bit confused by the azimuth output of geographiclib. I use the Python implementation, but I guess that doesn't make a difference.

>>> from geographiclib.geodesic import Geodesic
>>> lat1, lon1 = 85, 0
>>> lat2, lon2 = 90, 0
>>> Geodesic.WGS84.Inverse(lat1, lon1, lat2, lon2)['azi2']
0.0
>>> lat1, lon1 = 85, 0
>>> lat2, lon2 = 90, 10
>>> Geodesic.WGS84.Inverse(lat1, lon1, lat2, lon2)['azi2']
10.0

I am certainly not an expert, but from what I understood, true-north azimuth would in both cases be the same. To me this looks like grid azimuth, on a plate carree grid maybe. I couldn't find anything in the docs except "The azimuth is the heading measured clockwise from north." which lacks some details.

Could you shed a light on this? Maybe I'm just misunderstanding something.

Thanks
Maik

[Charles Karney]

Sorry to be so late answering this...

The documentation in the C++ library explains how to interpret the azimuth at one of the poles:

If either point is at a pole, the azimuth is defined by keeping the longitude fixed, writing latitude = ±(90° − ε), and taking the limit ε → 0+.

Let me know of you have further questions.