Distance between two points!

[Larz60+]

Yes, latitude and longitude.

The following is a really good writeup on the various methods of calculating distance
using latitude and longitude: http://www.movable-type.co.uk/scripts/latlong.html.

AussieSusan is exactly correct.

I was involved in a project that controlled spin and distance traveled by a stratospheric balloon,
and we had to take curvature and altitude into account. Calculating distance taken from an
on-board GPS. The paper for this is still on the web somewhere.