计算两点经纬度之间的距离

      /// <summary>
      /// distance (in km) between two points specified by latitude/longitude
      /// The Haversine formula, http://www.movable-type.co.uk/scripts/latlong.html
      /// </summary>
      /// <param name="p1"></param>
      /// <param name="p2"></param>
      /// <returns></returns>
      public double GetDistance(PointLatLng p1, PointLatLng p2)
      {
         double dLat1InRad = p1.Lat * (Math.PI / 180);
         double dLong1InRad = p1.Lng * (Math.PI / 180);
         double dLat2InRad = p2.Lat * (Math.PI / 180);
         double dLong2InRad = p2.Lng * (Math.PI / 180);
         double dLongitude = dLong2InRad - dLong1InRad;
         double dLatitude = dLat2InRad - dLat1InRad;
         double a = Math.Pow(Math.Sin(dLatitude / 2), 2) + Math.Cos(dLat1InRad) * Math.Cos(dLat2InRad) * Math.Pow(Math.Sin(dLongitude / 2), 2);
         double c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
         double dDistance = (Axis / 1000.0) * c;
         return dDistance;
      }

http://www.movable-type.co.uk/scripts/latlong.html 有详细的说明