400Km以上時計算方式：

/// <summary>  
/// 根據輸入的地點座標計算中心點  
/// </summary>  
/// <param name="geoCoordinateList"></param>  
/// <returns></returns>  
public GeoCoordinate GetCenterPointFromListOfCoordinates(List<GeoCoordinate> geoCoordinateList)  
{  
    int total = geoCoordinateList.Count;  
    double X = 0, Y = 0, Z = 0;  
    foreach (GeoCoordinate g in geoCoordinateList)  
    {  
        double lat, lon, x, y, z;  
        lat = g.Latitude * Math.PI / 180;  
        lon = g.Longitude * Math.PI / 180;  
        x = Math.Cos(lat) * Math.Cos(lon);  
        y = Math.Cos(lat) * Math.Sin(lon);  
        z = Math.Sin(lat);  
        X += x;  
        Y += y;  
        Z += z;  
    }  
    X = X / total;  
    Y = Y / total;  
    Z = Z / total;  
    double Lon = Math.Atan2(Y, X);  
    double Hyp = Math.Sqrt(X * X + Y * Y);  
    double Lat = Math.Atan2(Z, Hyp);  
    return new GeoCoordinate(Lat * 180 / Math.PI, Lon * 180 / Math.PI);  
}  
 

/// <summary>  
/// 根據輸入的地點座標計算中心點（適用於400km以下的場合）  
/// </summary>  
/// <param name="geoCoordinateList"></param>  
/// <returns></returns>  
public GeoCoordinate GetCenterPointFromListOfCoordinates(List<GeoCoordinate> geoCoordinateList)  
{  
    //以下為簡化方法（400km以內）  
    int total = geoCoordinateList.Count;  
    double lat = 0, lon = 0;  
    foreach (GeoCoordinate g in geoCoordinateList)  
    {  
        lat += g.Latitude * Math.PI / 180;  
        lon += g.Longitude * Math.PI / 180;  
    }  
    lat /= total;  
    lon /= total;  
    return new GeoCoordinate(lat * 180 / Math.PI, lon * 180 / Math.PI);  
}  
 

原文地址：http://blog.csdn.net/yl2isoft/article/details/16368397

在以下頁面包含有多種實現，大家可以參考。

http://stackoverflow.com/questions/6671183/calculate-the-center-point-of-multiple-latitude-longitude-coordinate-pairshttp://stackoverflow.com/questions/6671183/calculate-the-center-point-of-multiple-latitude-longitude-coordinate-pairs

 詳細的演算法說明，可以參考。

http://www.geomidpoint.com/calculation.html