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.

GeographicLib 2.4 (released 2023-07-14)

GeographicLib is a fairly standard C++ library and CMake is a fairly standard way of compiling and installing such libraries. So I recommend that you first seek help on a C# forum on how to use C++ libraries with your C# application.

Yes, the method is now included in GeographicLib 2.3 (released July 2023). See the Intersect class and the IntersectTool utility.

There are three problems with your suggested scheme: The gnomonic projection is a bad starting point because it badly distorts areas. An equal area projection, e.g., Albers conic, is a better choice. But understand that this would only "work" if your polygon had a relatively small extent, say 100 km or less. Your proposed triangulation scheme then has the problem (a) that it will be slow, O(n^2), I think, and (b) that it's wrong since you don't weight the vertices of the succeeding triangles according...

GeographicLib 2.3 (released 2023-07-25)

Your .pro seems OK. But then I don't know the intricacies of .pro files. How does your application find bin/GeographicLib.dll? Is the bin directory in you PATH? Your problems are, most likely, nothing to do with GeographicLib but instead how you use any external library in your program. In this case, you need to seek help in a Qt-specific forum. Since your sample code is from ArcGis, maybe you can get help there. If you do resolve your problem, please post the solution to this forum so others can...

You don't give enough details to diagnose your problem. But it looks like the linker doesn't know the location of the GeographicLib library. That would be the place to look. Can you link any external library to your application? If not, then you need to seek help in a Qt-specific forum. I haven't messed with Qt's .pro files for many years. Instead, I build Qt projects with CMake. (Qt Creator can handle CMake builds.) In this case, you just use find_package(GeographicLib ...) to tell the system where...