根据经纬度求两点间距离

public static double DistanceOfTwoPoints(double lng1,double lat1,  double lng2, double lat2, GaussSphere gs)
{
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 * (gs == GaussSphere.WGS84 ? 6378137.0 : (gs == GaussSphere.Xian80 ? 6378140.0 : 6378245.0));
s = Math.Round(s * 10000) / 10000;
return s;
}

private static double Rad(double d)
{
return d * Math.PI / 180.0;
}

GaussSphere 为自定义枚举类型
/// <summary>
/// 高斯投影中所选用的参考椭球
/// </summary>
public enum GaussSphere
{
Beijing54,
Xian80,
WGS84,
}