For the benefit of the terminally obsessive (as well as the genuinely needy),
Thaddeus Vincenty devised formulae for calculating geodesic distances between
a pair of latitude/longitude points on the earths surface, using an accurate
ellipsoidal model of the earth.
Vincentys formula is accurate to within 0.5mm, or 0.000015", on the ellipsoid
being used. Calculations based on a spherical model, such as the (much simpler)
Haversine, are accurate to around 0.3% (which is still good enough for most
purposes, of course).
Note: the accuracy quoted by Vincenty applies to the theoretical ellipsoid
being used, which will differ (to varying degree) from the real earth geoid.
If you happen to be located in Colorado, 2km above msl, distances will be 0.03%
greater. In the UK, if you measure the distance from Lands End to John O
Groats using WGS-84, it will be 28m - 0.003% - greater than using the Airy
ellipsoid, which provides a better fit for the UK.
NOTE: This formula is still considered alpha quality in GIS::Distance as it
hasnt been tested all that much.
Normally this module is not used directly. Instead GIS::Distance
is used which in turn interfaces with the various formula classes.