OpenLayers 3/4 加载 Baidu Maps 瓦片(偏移更小)

通过 OpenLayers 3/4 的 ol.layer.Tile 可以加载若干互联网底图服务，【扯淡大叔】的《OpenLayers 3 Primer》书中“万能瓦片地图加载秘籍”一节列举了各种地图的具体调用示例。但是百度地图的坐标系实在诡异，书中最终提供的调用方式仍然还是有些许偏移。

但是百度地图的制图质量还是很高的，很受欢迎，在 OpenLayers 的官方 Issues 里这样一个 Issue， Add Baidu layer #3522 ，还是有不少人有这个需求的。Issue 中 tschaub 给出的解决方法是根据 BD-09 坐标系生成了 baiduMercator 投影。

我将两种方式做了对比，对比结果显示，使用 baiduMercator 的地图偏移比直接使用 EPSG:3857 要小。

codepen

我对tschaub给出的代码做了些调整，主要是将js改为 ES5 版本。代码比较长：

var forEachPoint = function(func) {
  return function(input, opt_output, opt_dimension) {
    var len = input.length;
    var dimension = opt_dimension ? opt_dimension : 2;
    var output;
    if (opt_output) {
      output = opt_output;
    } else {
      if (dimension !== 2) {
        output = input.slice();
      } else {
        output = new Array(len);
      }
    }
    for (var offset = 0; offset < len; offset += dimension) {
      func(input, output, offset)
    }
    return output;
  };
};

var sphericalMercator = {}

var RADIUS = 6378137;
var MAX_LATITUDE = 85.0511287798;
var RAD_PER_DEG = Math.PI / 180;

sphericalMercator.forward = forEachPoint(function(input, output, offset) {
  var lat = Math.max(Math.min(MAX_LATITUDE, input[offset + 1]), -MAX_LATITUDE);
  var sin = Math.sin(lat * RAD_PER_DEG);

  output[offset] = RADIUS * input[offset] * RAD_PER_DEG;
  output[offset + 1] = RADIUS * Math.log((1 + sin) / (1 - sin)) / 2;
});

sphericalMercator.inverse = forEachPoint(function(input, output, offset) {
  output[offset] = input[offset] / RADIUS / RAD_PER_DEG;
  output[offset + 1] = (2 * Math.atan(Math.exp(input[offset + 1] / RADIUS)) - (Math.PI / 2)) / RAD_PER_DEG;
});


var baiduMercator = {}

var MCBAND = [12890594.86, 8362377.87,
    5591021, 3481989.83, 1678043.12, 0];

var LLBAND = [75, 60, 45, 30, 15, 0];

var MC2LL = [
    [1.410526172116255e-8, 0.00000898305509648872, -1.9939833816331,
        200.9824383106796, -187.2403703815547, 91.6087516669843,
        -23.38765649603339, 2.57121317296198, -0.03801003308653,
        17337981.2],
    [-7.435856389565537e-9, 0.000008983055097726239,
        -0.78625201886289, 96.32687599759846, -1.85204757529826,
        -59.36935905485877, 47.40033549296737, -16.50741931063887,
        2.28786674699375, 10260144.86],
    [-3.030883460898826e-8, 0.00000898305509983578, 0.30071316287616,
        59.74293618442277, 7.357984074871, -25.38371002664745,
        13.45380521110908, -3.29883767235584, 0.32710905363475,
        6856817.37],
    [-1.981981304930552e-8, 0.000008983055099779535, 0.03278182852591,
        40.31678527705744, 0.65659298677277, -4.44255534477492,
        0.85341911805263, 0.12923347998204, -0.04625736007561,
        4482777.06],
    [3.09191371068437e-9, 0.000008983055096812155, 0.00006995724062,
        23.10934304144901, -0.00023663490511, -0.6321817810242,
        -0.00663494467273, 0.03430082397953, -0.00466043876332,
        2555164.4],
    [2.890871144776878e-9, 0.000008983055095805407, -3.068298e-8,
        7.47137025468032, -0.00000353937994, -0.02145144861037,
        -0.00001234426596, 0.00010322952773, -0.00000323890364,
        826088.5]];

var LL2MC = [
    [-0.0015702102444, 111320.7020616939, 1704480524535203,
        -10338987376042340, 26112667856603880,
        -35149669176653700, 26595700718403920,
        -10725012454188240, 1800819912950474, 82.5],
    [0.0008277824516172526, 111320.7020463578, 647795574.6671607,
        -4082003173.641316, 10774905663.51142, -15171875531.51559,
        12053065338.62167, -5124939663.577472, 913311935.9512032,
        67.5],
    [0.00337398766765, 111320.7020202162, 4481351.045890365,
        -23393751.19931662, 79682215.47186455, -115964993.2797253,
        97236711.15602145, -43661946.33752821, 8477230.501135234,
        52.5],
    [0.00220636496208, 111320.7020209128, 51751.86112841131,
        3796837.749470245, 992013.7397791013, -1221952.21711287,
        1340652.697009075, -620943.6990984312, 144416.9293806241,
        37.5],
    [-0.0003441963504368392, 111320.7020576856, 278.2353980772752,
        2485758.690035394, 6070.750963243378, 54821.18345352118,
        9540.606633304236, -2710.55326746645, 1405.483844121726,
        22.5],
    [-0.0003218135878613132, 111320.7020701615, 0.00369383431289,
        823725.6402795718, 0.46104986909093, 2351.343141331292,
        1.58060784298199, 8.77738589078284, 0.37238884252424, 7.45]];


function getRange(v, min, max) {
  v = Math.max(v, min);
  v = Math.min(v, max);

  return v;
}

function getLoop(v, min, max) {
  var d = max - min;
  while (v > max) {
    v -= d;
  }
  while (v < min) {
    v += d;
  }

  return v;
}

function convertor(input, output, offset, table) {
  var px = input[offset];
  var py = input[offset + 1];
  var x = table[0] + table[1] * Math.abs(px);
  var d = Math.abs(py) / table[9];
  var y = table[2]
      + table[3]
      * d
      + table[4]
      * d
      * d
      + table[5]
      * d
      * d
      * d
      + table[6]
      * d
      * d
      * d
      * d
      + table[7]
      * d
      * d
      * d
      * d
      * d
      + table[8]
      * d
      * d
      * d
      * d
      * d
      * d;

  output[offset] = x * (px < 0 ? -1 : 1);
  output[offset + 1] = y * (py < 0 ? -1 : 1);
}

baiduMercator.forward = forEachPoint(function(input, output, offset) {
  var lng = getLoop(input[offset], -180, 180);
  var lat = getRange(input[offset + 1], -74, 74);

  var table = null;
  var j;
  for (j = 0; j < LLBAND.length; ++j) {
    if (lat >= LLBAND[j]) {
      table = LL2MC[j];
      break;
    }
  }
  if (table === null) {
    for (j = LLBAND.length - 1; j >= 0; --j) {
      if (lat <= -LLBAND[j]) {
        table = LL2MC[j];
        break;
      }
    }
  }
  output[offset] = lng;
  output[offset + 1] = lat;
  convertor(output, output, offset, table);
});

baiduMercator.inverse = forEachPoint(function(input, output, offset) {
  var y_abs = Math.abs(input[offset + 1]);

  var table = null;
  for (var j = 0; j < MCBAND.length; j++) {
    if (y_abs >= MCBAND[j]) {
      table = MC2LL[j];
      break;
    }
  }

  convertor(input, output, offset, table);
});

var gcj02 = {}

var PI = Math.PI;
var AXIS = 6378245.0;
var OFFSET = 0.00669342162296594323;  // (a^2 - b^2) / a^2

function delta(wgLon, wgLat) {
  var dLat = transformLat(wgLon - 105.0, wgLat - 35.0);
  var dLon = transformLon(wgLon - 105.0, wgLat - 35.0);
  var radLat = wgLat / 180.0 * PI;
  var magic = Math.sin(radLat);
  magic = 1 - OFFSET * magic * magic;
  var sqrtMagic = Math.sqrt(magic);
  dLat = (dLat * 180.0) / ((AXIS * (1 - OFFSET)) / (magic * sqrtMagic) * PI);
  dLon = (dLon * 180.0) / (AXIS / sqrtMagic * Math.cos(radLat) * PI);
  return [dLon, dLat];
}

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

function transformLat(x, y) {
  var 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) {
  var 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;
}

gcj02.toWGS84 = forEachPoint(function(input, output, offset) {
  var lng = input[offset];
  var lat = input[offset + 1];
  if (!outOfChina(lng, lat)) {
    var deltaD = delta(lng, lat);
    lng = lng - deltaD[0];
    lat = lat - deltaD[1];
  }
  output[offset] = lng;
  output[offset + 1] = lat;
});

gcj02.fromWGS84 = forEachPoint(function(input, output, offset) {
  var lng = input[offset];
  var lat = input[offset + 1];
  if (!outOfChina(lng, lat)) {
    var deltaD = delta(lng, lat);
    lng = lng + deltaD[0];
    lat = lat + deltaD[1];
  }
  output[offset] = lng;
  output[offset + 1] = lat;
});

var bd09 = {}

var PI = Math.PI;
var X_PI = PI * 3000 / 180;

function toGCJ02(input, output, offset) {
  var x = input[offset] - 0.0065;
  var y = input[offset + 1] - 0.006;
  var z = Math.sqrt(x * x + y * y) - 0.00002 * Math.sin(y * X_PI);
  var theta = Math.atan2(y, x) - 0.000003 * Math.cos(x * X_PI);
  output[offset] = z * Math.cos(theta);
  output[offset + 1] = z * Math.sin(theta);
  return output;
}

function fromGCJ02(input, output, offset) {
  var x = input[offset];
  var y = input[offset + 1];
  var z = Math.sqrt(x * x + y * y) + 0.00002 * Math.sin(y * X_PI);
  var theta = Math.atan2(y, x) + 0.000003 * Math.cos(x * X_PI);
  output[offset] = z * Math.cos(theta) + 0.0065;
  output[offset + 1] = z * Math.sin(theta) + 0.006;
  return output;
}

bd09.toWGS84 = function(input, opt_output, opt_dimension) {
  var output = forEachPoint(toGCJ02)(input, opt_output, opt_dimension);
  return gcj02.toWGS84(output, output, opt_dimension);
};

bd09.fromWGS84 = function(input, opt_output, opt_dimension) {
  var output = gcj02.fromWGS84(input, opt_output, opt_dimension);
  return forEachPoint(fromGCJ02)(output, output, opt_dimension);
};


var projzh = {}

projzh.smerc2bmerc = function(input, opt_output, opt_dimension) {
  var output = sphericalMercator.inverse(input, opt_output, opt_dimension);
  output = bd09.fromWGS84(output, output, opt_dimension);
  return baiduMercator.forward(output, output, opt_dimension);
};

projzh.bmerc2smerc = function(input, opt_output, opt_dimension) {
  var output = baiduMercator.inverse(input, opt_output, opt_dimension);
  output = bd09.toWGS84(output, output, opt_dimension);
  return sphericalMercator.forward(output, output, opt_dimension);
};

projzh.bmerc2ll = function(input, opt_output, opt_dimension) {
  var output = baiduMercator.inverse(input, opt_output, opt_dimension);
  return bd09.toWGS84(output, output, opt_dimension);
};

projzh.ll2bmerc = function(input, opt_output, opt_dimension) {
  var output = bd09.fromWGS84(input, opt_output, opt_dimension);
  return baiduMercator.forward(output, output, opt_dimension);
};

projzh.ll2smerc = sphericalMercator.forward;
projzh.smerc2ll = sphericalMercator.inverse;



var extent = [72.004, 0.8293, 137.8347, 55.8271];

var baiduMercatorProj = new ol.proj.Projection({
  code: 'baidu',
  extent: ol.extent.applyTransform(extent, projzh.ll2bmerc),
  units: 'm'
});

ol.proj.addProjection(baiduMercatorProj);
ol.proj.addCoordinateTransforms('EPSG:4326', baiduMercatorProj, projzh.ll2bmerc, projzh.bmerc2ll);
ol.proj.addCoordinateTransforms('EPSG:3857', baiduMercatorProj, projzh.smerc2bmerc, projzh.bmerc2smerc);

var bmercResolutions = new Array(19);
for (var i = 0; i < 19; ++i) {
  bmercResolutions[i] = Math.pow(2, 18 - i);
}
var baidu = new ol.layer.Tile({
  source: new ol.source.XYZ({
    projection: 'baidu',
    maxZoom: 18,
    tileUrlFunction: function(tileCoord) {
      var x = tileCoord[1];
      var y = tileCoord[2];
      var z = tileCoord[0];
      return "http://api0.map.bdimg.com/customimage/tile?x=" + x
        + "&y=" + y + "&z=" + z
        + "&udt=20170908&scale=2&ak=ZUONbpqGBsYGXNIYHicvbAbM"
        + "&styles=t%3Awater%7Ce%3Aall%7Cc%3A%23044161%2Ct%3Aland%7Ce%3Aall%7Cc%3A%23004981%2Ct%3Aboundary%7Ce%3Ag%7Cc%3A%23064f85%2Ct%3Arailway%7Ce%3Aall%7Cv%3Aoff%2Ct%3Ahighway%7Ce%3Ag%7Cc%3A%23004981%2Ct%3Ahighway%7Ce%3Ag.f%7Cc%3A%23005b96%7Cl%3A1%2Ct%3Ahighway%7Ce%3Al%7Cv%3Aoff%2Ct%3Aarterial%7Ce%3Ag%7Cc%3A%23004981%2Ct%3Aarterial%7Ce%3Ag.f%7Cc%3A%2300508b%2Ct%3Apoi%7Ce%3Aall%7Cv%3Aoff%2Ct%3Agreen%7Ce%3Aall%7Cv%3Aoff%7Cc%3A%23056197%2Ct%3Asubway%7Ce%3Aall%7Cv%3Aoff%2Ct%3Amanmade%7Ce%3Aall%7Cv%3Aoff%2Ct%3Alocal%7Ce%3Aall%7Cv%3Aoff%2Ct%3Aarterial%7Ce%3Al%7Cv%3Aoff%2Ct%3Aboundary%7Ce%3Ag.f%7Cc%3A%23029fd4%2Ct%3Abuilding%7Ce%3Aall%7Cc%3A%231a5787%2Ct%3Alabel%7Ce%3Aall%7Cv%3Aoff&t = 1505487396397";;
    },
    tileGrid: new ol.tilegrid.TileGrid({
      resolutions: bmercResolutions,
      origin: [0, 0],
      extent: ol.extent.applyTransform(extent, projzh.ll2bmerc),
      tileSize: [256, 256]
    })
  })
});

将 baidu 这个 layer 加载即可。

var map = new ol.Map(
  {
    layers : [ baidu ]
    view : new ol.View({
      center : ol.proj.transform([ 112.334403, 39.8 ], 'EPSG:4326', 'EPSG:3857'),
      zoom : 8
      projection : 'EPSG:3857'
    })
  });