Earth is round (or so Copernicus will have us believe) and that sucks.
Why? Because we can't use c=Square root of (a2 +
b2) to figure the distance between two
points. So in order to perform this calculation we must use 3D
trigonometry. Linked below is a project that does just that. It
comes with an external C DLL. The reason the DLL was written in C is
because the calculations proved to be too slow in VB. The DLL is freely
distributable.
Download Distance.zip