public static class GeoCodeCalc
    {
        public const double EarthRadiusInMiles = 3956.0;
        public const double EarthRadiusInKilometers = 6367.0;
        public static double ToRadian(double val) { return val * (Math.PI / 180); }
        public static double DiffRadian(double val1, double val2) { return ToRadian(val2) - ToRadian(val1); }
        /// <summary>
        /// Calculate the distance between two geocodes. Defaults to using Miles.
        /// </summary>
        public static double CalcDistance(double lat1, double lng1, double lat2, double lng2)
        {
            return CalcDistance(lat1, lng1, lat2, lng2, GeoCodeCalcMeasurement.Miles);
        }
        /// <summary>
        /// Calculate the distance between two geocodes.
        /// </summary>
        public static double CalcDistance(double lat1, double lng1, double lat2, double lng2, GeoCodeCalcMeasurement m)
        {
            double radius = GeoCodeCalc.EarthRadiusInMiles;
            if (m == GeoCodeCalcMeasurement.Kilometers) { radius = GeoCodeCalc.EarthRadiusInKilometers; }
            return radius * 2 * Math.Asin(Math.Min(1, Math.Sqrt((Math.Pow(Math.Sin((DiffRadian(lat1, lat2)) / 2.0), 2.0) + Math.Cos(ToRadian(lat1)) * Math.Cos(ToRadian(lat2)) * Math.Pow(Math.Sin((DiffRadian(lng1, lng2)) / 2.0), 2.0)))));
        }
    }
    public enum GeoCodeCalcMeasurement : int
    {
        Miles = 0,
        Kilometers = 1
    }