wgs2mars/lib/wgs2mars.js

a little function to transform longitude and latitude from WGS-84 To GCJ-02

/**
 *   Usage: let gcjloc = transformFromWGSToGCJ(lng,lat);
 *   Source: https://github.com/hiwanz/wgs2mars.js.git
 */
import chinaBorderData from './border-data';

function transformFromWGSToGCJ(wgLon, wgLat) {

  // const PI
  let PI = 3.14159265358979324;
  // Krasovsky 1940
  //
  // a = 6378245.0, 1/f = 298.3
  // b = a * (1 - f)
  // ee = (a^2 - b^2) / a^2;
  let a = 6378245.0;
  let ee = 0.00669342162296594323;

  function isInChina(point, vs) {
    // ray-casting algorithm based on
    // http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html

    let x = point[0], y = point[1];
    let inside = false;

    for (let i = 0, j = vs.length - 1; i < vs.length; j = i++) {
      let xi = vs[i][0], yi = vs[i][1];
      let xj = vs[j][0], yj = vs[j][1];
      let intersect = ((yi > y) !== (yj > y)) && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);

      if (intersect) inside = !inside;
    }

    return inside;
  };

  function transformLat(x, y) {
    let 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;
  }

  function transformLon(x, y) {
    let 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;
  }

  // World Geodetic System ==> Mars Geodetic System
  function transform(wgLon, wgLat) {
    let mgLoc = {};

    if (!isInChina([wgLon, wgLat], chinaBorderData)) {
      mgLoc = {
        lat: wgLat,
        lng: wgLon
      };
      return mgLoc;
    }
    let dLat = transformLat(wgLon - 105.0, wgLat - 35.0);
    let dLon = transformLon(wgLon - 105.0, wgLat - 35.0);
    let radLat = wgLat / 180.0 * PI;
    let magic = Math.sin(radLat);

    magic = 1 - ee * magic * magic;

    let sqrtMagic = Math.sqrt(magic);

    dLat = (dLat * 180.0) / ((a * (1 - ee)) / (magic * sqrtMagic) * PI);
    dLon = (dLon * 180.0) / (a / sqrtMagic * Math.cos(radLat) * PI);
    mgLoc = {
      lat: wgLat + dLat,
      lng: wgLon + dLon
    };
    return mgLoc;
  }

  return transform(wgLon, wgLat);
}

export default transformFromWGSToGCJ;