gps transform

gps.js

const pi = Math.PI;
const a = 6378245.0;
const ee = 0.006693421622965943;

function gps2Mars(lng, lat) {
  if (outOfChina(lng, lat)) {
    return {
      lng: lng,
      lat: lat
    };
  }

  let dLat = transformLat(lng - 105.0, lat - 35.0);
  let dLng = transformLng(lng - 105.0, lat - 35.0);
  let radLat = lat / 180.0 * pi;
  let magic = Math.sin(radLat);
  let sqrtMagic;
  let mgLat;
  let mgLng;

  magic = 1 - ee * magic * magic;
  sqrtMagic = Math.sqrt(magic);
  dLat = (dLat * 180.0) / ((a * (1 - ee)) / (magic * sqrtMagic) * pi);
  dLng = (dLng * 180.0) / (a / sqrtMagic * Math.cos(radLat) * pi);
  mgLat = lat + dLat;
  mgLng = lng + dLng;

  return {
    lng: mgLng,
    lat: mgLat
  };
}

function mars2Gps(lng, lat) {
  const position = gps2Mars(lng, lat);

  return {
    lng: lng * 2 - position.lng,
    lat: lat * 2 - position.lat
  };
}

function outOfChina(lng, lat) {
  if (lng < 72.004 || lng > 137.8347) {
    return true;
  }
  if (lat < 0.8293 || lat > 55.8271) {
    return true;
  }
  return false;
}

function transformLng(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;
}

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;
}