GPS獲取座標 顯示Google map偏差計算

用手機獲取GPS座標 顯示在手機地圖偏差大約在100-200米左右，我把座標放在 Maps.google.com 搜尋座標定位則相當精確。

可能是.....為了安全吧故意加的偏差

不過可以計算偏差使位置精確

程式碼如下

public class EvilTransform {
     static double  pi = 3.14159265358979324;
     static double a = 6378245.0;
     static double ee = 0.00669342162296594323;
     public static double mgLat;
     public static double mgLon;
    public static void transform( double wgLat, double wgLon)
    {
        if (outOfChina(wgLat, wgLon))
        {
            mgLat = wgLat;
            mgLon = wgLon;
            return;
        }
        double dLat = transformLat(wgLon - 105.0, wgLat - 35.0);
        double dLon = transformLon(wgLon - 105.0, wgLat - 35.0);
        double radLat = wgLat / 180.0 * pi;
        double magic = Math.sin(radLat);
        magic = 1 - ee * magic * magic;
        double sqrtMagic = Math. sqrt(magic);
        dLat = (dLat * 180.0) / (( a * (1 - ee)) / (magic * sqrtMagic) * pi);
        dLon = (dLon * 180.0) / ( a / sqrtMagic * Math.cos(radLat) * pi);
        mgLat = wgLat + dLat;
        mgLon = wgLon + dLon;
    }

    static Boolean outOfChina( double lat, double lon)
    {
        if (lon < 72.004 || lon > 137.8347)
            return true;
        if (lat < 0.8293 || lat > 55.8271)
            return true;
        return false;
    }

    static double transformLat( double x, double y)
    {
        double ret = -100.0 + 2.0 * x + 3.0 * y + 0.2 * y * y + 0.1 * x * y + 0.2 * Math.sqrt(Math.abs(x));
        ret += (20.0 * Math. sin(6.0 * x * pi) + 20.0 * Math.sin(2.0 * x * pi)) * 2.0 / 3.0;
        ret += (20.0 * Math. sin(y * pi) + 40.0 * Math.sin(y / 3.0 * pi)) * 2.0 / 3.0;
        ret += (160.0 * Math. sin(y / 12.0 * pi) + 320 * Math.sin(y * pi / 30.0)) * 2.0 / 3.0;
        return ret;
    }

    static double transformLon( double x, double y)
    {
        double ret = 300.0 + x + 2.0 * y + 0.1 * x * x + 0.1 * x * y + 0.1 * Math.sqrt(Math.abs(x));
        ret += (20.0 * Math. sin(6.0 * x * pi) + 20.0 * Math.sin(2.0 * x * pi)) * 2.0 / 3.0;
        ret += (20.0 * Math. sin(x * pi) + 40.0 * Math.sin(x / 3.0 * pi)) * 2.0 / 3.0;
        ret += (150.0 * Math. sin(x / 12.0 * pi) + 300.0 * Math.sin(x / 30.0 * pi)) * 2.0 / 3.0;
        return ret;
    }
}