[shittyadmin]

If you need a library to do this, check out GeographicLib. It's written by C. F. F. Karney the same guy who developed an algorithm and wrote a scientific paper about improving Vincenty's formulae for performance and fixing issues at nearly antipodal points.

It has good implementations in C++, C# and Javascript.

https://geographiclib.sourceforge.io/html/

[damian2000]

Minor point but I know c# has a GeoCoordinate class into the framework Dlls. It also contains methods to calculate distance between points. Many .net Devs I've worked with didn't know about it. I think it came out first in .net 4.0.