具体参看网址： http://bbs.zdoz.net/thread-2-1-1.html
public static double getDistanceFromXtoY(double lat_a, double lng_a, double lat_b, double lng_b)
{
double pk = (double)(180 / 3.14169);
double a1 = lat_a / pk;
double a2 = lng_a / pk;
double b1 = lat_b / pk;
double b2 = lng_b / pk;
double t1 = Math.Cos(a1) * Math.Cos(a2) * Math.Cos(b1) * Math.Cos(b2);
double t2 = Math.Cos(a1) * Math.Sin(a2) * Math.Cos(b1) * Math.Sin(b2);
double t3 = Math.Sin(a1) * Math.Sin(b1);
double tt = Math.Acos(t1 + t2 + t3);
return 6366000 * tt;
}