On Thu, Nov 02, 2006 at 09:11:58PM +0530, Sandeep Kumar Jakkaraju wrote: > Like We have used the Haversine formula ...given on most of the websites ... > to calculate the distance between to lat/lon points .. You can examine the PostGIS source code to see how distance_sphere() works; the underlying function is LWGEOM_distance_sphere() in lwgeom/lwgeom_spheroid.c. The algorithm appears to be Haversine. > which doesnt match with the distance given by distance_sphere() method Could you post an example showing how you're calling distance_sphere(), the output you get, and the output you get from other Haversine functions? Several possibilities might explain the discrepancy: 1. If the difference is small then distance_sphere() might be using a different value for the earth's radius than some other Haversine function. The earth isn't a perfect sphere so simple functions use approximations of the earth's radius and different functions might use different values. distance_sphere() uses 6370986.884258304, which is a reasonable (if overly precise) approximation of the earth's radius in meters. 2. Make sure you're comparing the same units. A Haversine function that returns distance in feet or miles won't return the same distance as distance_sphere(), which returns meters. 3. Make sure your PostGIS geometries have coordinates in the correct order: (X Y) which is (longitude latitude). This is a common cause of problems for people accustomed to thinking (latitude longitude) and who therefore create geometries as (Y X). -- Michael Fuhr
