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# /中国地图偏移算法.md

Created Feb 22, 2017

# 中国地图偏移算法

WGS-84 坐标系，是 GPS 系统所采用的坐标系。一切正常工作的 GPS 或 GPS 芯片所返回的坐标值都是这个坐标系下的数值。Google 地图采用的卫星图也是按照这个坐标系摆放的。

## 源文件C

```//
//  ChinaMapShift.h
//  ChinaMapShift
//
//  Most code created by someone anonymous.
//  transformFromGCJToWGS() added by Fengzee (fengzee@fengzee.com).
//

#ifndef ChinaMapShift_ChinaMapShift_h
#define ChinaMapShift_ChinaMapShift_h

typedef struct {
double lng;
double lat;
} Location;

Location transformFromWGSToGCJ(Location wgLoc);
Location transformFromGCJToWGS(Location gcLoc);
Location bd_encrypt(Location gcLoc);
Location bd_decrypt(Location bdLoc);

#endif```
```//
//  ChinaMapShift.mm
//  ChinaMapShift
//
//  Most code created by someone anonymous.
//  transformFromGCJToWGS() added by Fengzee (fengzee@fengzee.com).
//

#include "ChinaMapShift.h"
#include <math.h>

inline Location
LocationMake(double lng, double lat) {
Location loc; loc.lng = lng, loc.lat = lat; return loc;
}

///
///  Transform WGS-84 to GCJ-02 (Chinese encrypted coordination system)
///

const double pi = 3.14159265358979324;

//
// Krasovsky 1940
//
// a = 6378245.0, 1/f = 298.3
// b = a * (1 - f)
// ee = (a^2 - b^2) / a^2;
const double a = 6378245.0;
const double ee = 0.00669342162296594323;

int outOfChina(double lat, double lon)
{
if (lon < 72.004 || lon > 137.8347)
return 1;
if (lat < 0.8293 || lat > 55.8271)
return 1;
return 0;
}

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 * sqrt(x > 0 ? x:-x);
ret += (20.0 * sin(6.0 * x * pi) + 20.0 *sin(2.0 * x * pi)) * 2.0 / 3.0;
ret += (20.0 * sin(y * pi) + 40.0 * sin(y / 3.0 * pi)) * 2.0 / 3.0;
ret += (160.0 * sin(y / 12.0 * pi) + 320 * sin(y * pi / 30.0)) * 2.0 / 3.0;
return ret;
}

double transformLon(double x, double y)
{
double ret = 300.0 + x + 2.0 * y + 0.1 * x * x + 0.1 * x * y + 0.1 * sqrt(x > 0 ? x:-x);
ret += (20.0 * sin(6.0 * x * pi) + 20.0 * sin(2.0 * x * pi)) * 2.0 / 3.0;
ret += (20.0 * sin(x * pi) + 40.0 * sin(x / 3.0 * pi)) * 2.0 / 3.0;
ret += (150.0 * sin(x / 12.0 * pi) + 300.0 * sin(x / 30.0 * pi)) * 2.0 / 3.0;
return ret;
}

Location transformFromWGSToGCJ(Location wgLoc)
{
Location mgLoc;
if (outOfChina(wgLoc.lat, wgLoc.lng))
{
mgLoc = wgLoc;
return mgLoc;
}
double dLat = transformLat(wgLoc.lng - 105.0, wgLoc.lat - 35.0);
double dLon = transformLon(wgLoc.lng - 105.0, wgLoc.lat - 35.0);
double radLat = wgLoc.lat / 180.0 * pi;
magic = 1 - ee * magic * magic;
double sqrtMagic = sqrt(magic);
dLat = (dLat * 180.0) / ((a * (1 - ee)) / (magic * sqrtMagic) * pi);
dLon = (dLon * 180.0) / (a / sqrtMagic * cos(radLat) * pi);
mgLoc.lat = wgLoc.lat + dLat;
mgLoc.lng = wgLoc.lng + dLon;

return mgLoc;
}

///
///  Transform GCJ-02 to WGS-84
///  Reverse of transformFromWGSToGC() by iteration.
///
///  Created by Fengzee (fengzee@fengzee.com) on 13-11-9.
///
Location transformFromGCJToWGS(Location gcLoc)
{
Location wgLoc = gcLoc;
Location currGcLoc, dLoc;
while (1) {
currGcLoc = transformFromWGSToGCJ(wgLoc);
dLoc.lat = gcLoc.lat - currGcLoc.lat;
dLoc.lng = gcLoc.lng - currGcLoc.lng;
if (fabs(dLoc.lat) < 1e-7 && fabs(dLoc.lng) < 1e-7) {  // 1e-7 ~ centimeter level accuracy
// Result of experiment:
//   Most of the time 2 iterations would be enough for an 1e-8 accuracy (milimeter level).
//
return wgLoc;
}
wgLoc.lat += dLoc.lat;
wgLoc.lng += dLoc.lng;
}

return wgLoc;
}

///
///  Transform GCJ-02 to BD-09
///
const double x_pi = 3.14159265358979324 * 3000.0 / 180.0;
Location bd_encrypt(Location gcLoc)
{
double x = gcLoc.lng, y = gcLoc.lat;
double z = sqrt(x * x + y * y) + 0.00002 * sin(y * x_pi);
double theta = atan2(y, x) + 0.000003 * cos(x * x_pi);
return LocationMake(z * cos(theta) + 0.0065, z * sin(theta) + 0.006);
}

///
///  Transform BD-09 to GCJ-02
///
Location bd_decrypt(Location bdLoc)
{
double x = bdLoc.lng - 0.0065, y = bdLoc.lat - 0.006;
double z = sqrt(x * x + y * y) - 0.00002 * sin(y * x_pi);
double theta = atan2(y, x) - 0.000003 * cos(x * x_pi);
return LocationMake(z * cos(theta), z * sin(theta));
}```