That's a pretty good reference. It relies on the Mean Earth radius, that is,

as the earth is an ellipsoid (actually a geoid), it takes the average radius

of the earth, which yields results that are more or less accurate. To get a

more accurate measurement, you can use an ellipsoidal model, or a geodetic

model, both of which are a good bit more accurate. The only problem with the

geodetic model is that there are so many variations, called "geodetic

datums" that are all similar but slightly different, to choose from. In most

cases, the ellipsoidal model will give a highly accurate result. Here is

another reference which may be helpful:

http://www.ga.gov.au/geodesy/datums/calcs.jsp
You may also find some good information from the Open Geospatial Consortium,

a fairly new organization which is working on standardizing many GIS-related

technologies:

http://www.opengeospatial.org/
--

HTH,

Kevin Spencer

Microsoft MVP

Professional Numbskull

Hard work is a medication for which

there is no placebo.

"Jacob" <ja***@reimers.dk> wrote in message

news:11*********************@u72g2000cwu.googlegro ups.com...

This is a good site for the formulas:

http://www.movable-type.co.uk/scripts/LatLong.html

If your GPS points are close you may be able to get by with a straight

line calculation using Pythagoras.

I also developed a .NET control for Google Maps, included in it is a

.NET class for geographic calculations (distance, bearing etc.) you can

download it at http://www.reimers.dk