//地球半径，单位米
       private const double EARTH_RADIUS = 6378137;
       /// <summary>
       /// 计算两点位置的距离，返回两点的距离，单位 米
       /// 该公式为GOOGLE提供，误差小于0.2米
       /// </summary>
       /// <param name="lat1">第一点纬度</param>
       /// <param name="lng1">第一点经度</param>
       /// <param name="lat2">第二点纬度</param>
       /// <param name="lng2">第二点经度</param>
       /// <returns></returns>
       public static double GetDistance(double lat1, double lng1, double lat2, double lng2)
       {
           double radLat1 = Rad(lat1);
           double radLng1 = Rad(lng1);
           double radLat2 = Rad(lat2);
           double radLng2 = Rad(lng2);
           double a = radLat1 - radLat2;
           double b = radLng1 - radLng2;
           double result = 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))) * EARTH_RADIUS;
           return result;
       }
 
       /// <summary>
       /// 经纬度转化成弧度
       /// </summary>
       /// <param name="d"></param>
       /// <returns></returns>
       private static double Rad(double d)
       {
           return (double)d * Math.PI / 180d;
       }