通过两个点的经纬度计算距离

http://panyee.cnblogs.com/archive/2006/07/04/442771.html
http://en.wikipedia.org/wiki/Great_circle_distance
从google maps的脚本里扒了段代码，没准啥时会用上。大家一块看看是怎么算的。

private const
double EARTH_RADIUS = 6378.137;

private static double rad(double d)

{

return
d * Math.PI / 180.0;

}

public static double GetDistance(double lat1, double lng1, double
lat2, double lng2)

{

double radLat1 = rad(lat1);

double
radLat2 = rad(lat2);

double a = radLat1 - radLat2;

double b =
rad(lng1) - rad(lng2);

double s = 2 *
Math.Asin(Math.Sqrt(Math.Pow(Math.Sin(a/2),2) +

Math.Cos(radLat1)*Math.Cos(radLat2)*Math.Pow(Math.Sin(b/2),2)));

s
= s * EARTH_RADIUS;

s = Math.Round(s * 10000) / 10000;

return
s;

}