Distance bewteen GPS coordinates

We need to be able to calculate the distance between two (3D) GPS coordinates. It is easy to include the altitude, when available, since that is provided in meters. However, we need a function that can determine the distance between two longitude/latitude pairs, in meters.

This is rather simple to implement. We just need to know how much distance do we travel when coordinates change, ie. how much is it in meters, when coordinates change 0.001? Google will probably help... I'll see to this in the next few days.

...guess we'll need to make/find a formula to calculate a distance along a line going across earth surface, or something to that direction.

http://www.codecodex.com/wiki/Calculate_distance_between_two_points_on_a_globe#JavaScript
and
http://en.wikipedia.org/wiki/Haversine_formula

That doesn't include altitude, but I would consider it accurate enough without altitude. Of course we could approximately add altitude difference with something like: c = (a^2 + b^2)^(1/2), where a = distance and b = altitude difference (Pythagorean theorem...)

That looks very usable, good work :)