team_tar
Navicacher
Group: Members
Posts: 14
Joined: Nov. 2003 

Posted: Nov. 18 2003,6:03 am 

Quote (Volvo Man @ Nov. 17 2003,7:58 pm)  how's the distance between 2 sets of coordinates calculated? is it as follows:
(difference in latitude) squared + (difference in longitude)squared = distance squared 
Well, I don't think it's so simple. sqrt(lat^2+lon^2) is valid on SHORT distances, as it assumes a flat surface, while the earth is (approximated in coordinate system as) an triaxial ellipsoid with some given parameters (whic in fact define the ellipsoid used in your map datum). In fact, what your GPS (or the computer programs) does when calculating the distance is using some kind of algorythm to integrate along the shortest path (pseudomeridian, it seems to me is called) along the elipsoidic surface. The way surface is flattened is one of the core problems of mapping (while a map is flat). I don't work in geodesy so are unable to give you a figure of what "short distance" means... if you want to preserve the precision of the GPS measurement, which in general is in the order of 10 m, you probably could not go over 100km, likely less... in some book (or site) on geodesy and mapping you may likely find exact figures on this regard, and also the formulae to calculate distances along given elipsoids... which at the end is merely a geometrical problem, but likely in a specialized site or book you will find them in the best form to be applied to geographical coords.
About the length of a minute in lat/lon, we live in a better world because we have Joe Mehaffey ! Go to: http://home.online.no/%7Esigurdhu/Grid_1deg.htm As you see, the metric of a arc minute changes depending of the position...
Stop! Joe is even better... found also the formulae: http://gpsinformation.net/main/transformation.htm
Enjoy...
