GetDistance between two points in Windows Phone SDK

DistanceCalc.cs

public static class DistanceCalc
{
    /// <summary>
    /// 長半径(赤道半径) at WGS84 (GPS)
    /// </summary>
    private const double equationalRaius = 6378137.000;
    /// <summary>
    /// 短半径(極半径) at WGS84 (GPS)
    /// </summary>
    private const double polarRadius = 6356752.314245;
    /// <summary>
    /// 第一離心率の二乗
    /// </summary>
    private const double eSquared = (equationalRaius * equationalRaius - polarRadius * polarRadius)
                                        / (equationalRaius * equationalRaius);
    /// <summary>
    /// 子午線曲率半径の分子
    /// </summary>
    private const double numerator = equationalRaius * (1 - eSquared);
 
    public static double Calc(double lon1, double lat1, double lon2, double lat2)
    {
        //度からラジアンに変換
        var lonRad1 = lon1 * Math.PI / 180;
        var latRad1 = lat1 * Math.PI / 180;
        var lonRad2 = lon2 * Math.PI / 180;
        var latRad2 = lat2 * Math.PI / 180;
        var deltaLat = latRad1 - latRad2;
        var deltaLon = lonRad1 - lonRad2;
        var avgLat = (latRad1 + latRad2) / 2;
        var sin = Math.Sin(avgLat);
        var doubleu = Math.Sqrt(1 - eSquared * sin * sin);
        //子午線曲率半径
        var meridian = numerator / (doubleu * doubleu * doubleu);
        //卯酉線曲率半径
        var primeVertical = equationalRaius / doubleu;
        var deltaY = deltaLat * meridian;
        var deltaX = deltaLon * primeVertical * Math.Cos(avgLat);
        return Math.Sqrt(deltaX * deltaX + deltaY * deltaY);
    }
}

上のサンプルはHubeny の公式を使った計算処理。

GeoCoordinate.GetDistanceTo メソッドもあるけど、これは地球を真球と仮定するHaversine　の式で出している。
http://msdn.microsoft.com/ja-jp/library/system.device.location.geocoordinate.getdistanceto%28v=vs.92%29.aspx