计算2个坐标（经纬度）之间的距离，.Net

.Net 中计算2个点之间的距离的方法，或者叫计算2个坐标，2个经纬度之间的距离。

代码如下：

/// <summary>
    /// Calculates the distance between two geographics locations
    /// </summary>
    public static class GeoDistance
    {
        public static double Between(double latitude1, double longitude1, double latitude2, double longitude2,
            DistanceUnits units = DistanceUnits.Kilometers)
        {
            double a1 = latitude1 * RadianConversionFactor;
            double b1 = longitude1 * RadianConversionFactor;
            double a2 = latitude2 * RadianConversionFactor;
            double b2 = longitude2 * RadianConversionFactor;

            double distance =
                Math.Acos(Math.Cos(a1) * Math.Cos(b1) * Math.Cos(a2) * Math.Cos(b2) +
                    Math.Cos(a1) * Math.Sin(b1) * Math.Cos(a2) * Math.Sin(b2) + Math.Sin(a1) * Math.Sin(a2)) * DistanceConstants[units];

            return distance;
        }

        private const double RadianConversionFactor = Math.PI / 180;

        private static readonly Dictionary<DistanceUnits, double> DistanceConstants = new Dictionary<DistanceUnits, double>(3) {
            { DistanceUnits.Miles, 3963.1 },
            { DistanceUnits.NauticalMiles, 3443.9 },
            { DistanceUnits.Kilometers, 6378 }
        };
    }

    public enum DistanceUnits
    {
        Miles,
        NauticalMiles,
        Kilometers,
    }

代码很简单，但是费了不少劲。就是还没测试效率怎么样。如果有高效的算法，请不要吝啬指教！！！