WGS-84 coordinate system: use of GPS, Beidou, etc

GCJ-02 coordinate system: used in China, offset from WGS-84

BD-09 coordinate system: dedicated to Baidu, offset from GCJ-02

(PS: due to the needs of the project, I originally wanted to read the longitude and latitude of the picture displayed on Baidu offline map. Later, it was found that the positioning deviation was too large. After careful consideration, it turned out that the picture was different from the coordinate system used by Baidu.

Calculation conversion part

    public class GPSChange
    {
        private const double pi = 3.14159265358979324;
        private const double x_pi = 3.14159265358979324 * 3000.0 / 180.0;

        //Parameters of krasotsky ellipsoid
        private const double a = 6378245.0;
        //First eccentricity
        private const double ee = 0.00669342162296594323;


        /// <summary>
        /// GCJ-02 Transformation BD-09
        /// </summary>
        /// <param name="gg_lat">latitude</param>
        /// <param name="gg_lon">longitude</param>
        /// <returns></returns>
        public static GPSPoint GCJ02_to_BD09(double gg_lat, double gg_lon)
        {
            GPSPoint point = new GPSPoint();
            double x = gg_lon, y = gg_lat;
            double z = Math.Sqrt(x * x + y * y) + 0.00002 * Math.Sin(y * x_pi);
            double theta = Math.Atan2(y, x) + 0.000003 * Math.Cos(x * x_pi);
            double bd_lon = z * Math.Cos(theta) + 0.0065;
            double bd_lat = z * Math.Sin(theta) + 0.006;
            point.SetLat(bd_lat);
            point.SetLng(bd_lon);
            return point;
        }

        /// <summary>
        /// BD-09 Transformation GCJ-02
        /// </summary>
        /// <param name="bd_lat">latitude</param>
        /// <param name="bd_lon">longitude</param>
        /// <returns></returns>
        public static GPSPoint BD09_to_GCJ02(double bd_lat, double bd_lon)
        {
            GPSPoint point = new GPSPoint();
            double x = bd_lon - 0.0065, y = bd_lat - 0.006;
            double z = Math.Sqrt(x * x + y * y) - 0.00002 * Math.Sin(y * x_pi);
            double theta = Math.Atan2(y, x) - 0.000003 * Math.Cos(x * x_pi);
            double gg_lon = z * Math.Cos(theta);
            double gg_lat = z * Math.Sin(theta);
            point.SetLat(gg_lat);
            point.SetLng(gg_lon);
            return point;
        }



        /// <summary>
        /// WGS-84 Transformation GCJ-02
        /// </summary>
        /// <param name="wgLat">latitude</param>
        /// <param name="wgLon">longitude</param>
        /// <returns></returns>
        public static GPSPoint WGS84_to_GCJ02(double wgLat, double wgLon)
        {
            GPSPoint point = new GPSPoint();
            if (OutOfChina(wgLat, wgLon))
            {
                point.SetLat(wgLat);
                point.SetLng(wgLon);
                return point;
            }
            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);
            double lat = wgLat + dLat;
            double lon = wgLon + dLon;
            point.SetLat(lat);
            point.SetLng(lon);
            return point;
        }


        public static void Transform(double wgLat, double wgLon, double[] latlng)
        {
            if (OutOfChina(wgLat, wgLon))
            {
                latlng[0] = wgLat;
                latlng[1] = 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);
            latlng[0] = wgLat + dLat;
            latlng[1] = wgLon + dLon;
        }

        private static bool 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;
        }

        private 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;
        }

        private 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;
        }

    }

Basic class

    public class GPSPoint
    {
        private double lat;// latitude
        private double lng;// longitude

        public GPSPoint()
        {
        }

        public GPSPoint(double lng, double lat)
        {
            this.lng = lng;
            this.lat = lat;
        }

        
        public double GetLat()
        {
            return lat;
        }
        public void SetLat(double lat)
        {
            this.lat = lat;
        }
        public double GetLng()
        {
            return lng;
        }
        public void SetLng(double lng)
        {
            this.lng = lng;
        }
        

    }