openlayers3中应用proj4js

要在openlayers3中应用proj4js，需要在html中引用proj4js，然后在引用所需要的projection的js定义，如 http://epsg.io/21781-1753.js

然后在openlayers中就会支持这种EPSG:21781的坐标转换。

 <script src="http://cdnjs.cloudflare.com/ajax/libs/proj4js/2.2.1/proj4.js" type="text/javascript"></script>
    <script src="http://epsg.io/21781-1753.js" type="text/javascript"></script>

http://epsg.io/21781-1753.js会返回一个js，这个js一旦执行就会给proj4添加一个支持的projection，如下：

proj4.defs("EPSG:21781","+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 +x_0=600000 +y_0=200000 +ellps=bessel +towgs84=660.077,13.551,369.344,2.484,1.783,2.939,5.66 +units=m +no_defs");

下面是openlayer的实现

/**
 * Fetches a Projection object for the code specified.
 *
 * @param {ol.proj.ProjectionLike} projectionLike Either a code string which is
 *     a combination of authority and identifier such as "EPSG:4326", or an
 *     existing projection object, or undefined.
 * @return {ol.proj.Projection} Projection object, or null if not in list.
 * @api stable
 */
ol.proj.get = function(projectionLike) {
  var projection;
  if (projectionLike instanceof ol.proj.Projection) {
    projection = projectionLike;
  } else if (goog.isString(projectionLike)) {
    var code = projectionLike;
    var projections = ol.proj.projections_;
    projection = projections[code];

//判断proj4js被引入进来了

if (ol.ENABLE_PROJ4JS && !goog.isDef(projection) && typeof proj4 == 'function') { 
      //如果需要的code=EPSG被引入进来了，就会proj4.defs（code）返回定义，并加入到openlayers中
      var def = proj4.defs(code);
      if (goog.isDef(def)) {
        var units = def.units;
        if (!goog.isDef(units)) {
          if (goog.isDef(def.to_meter)) {
            units = def.to_meter.toString();
            ol.proj.METERS_PER_UNIT[units] = def.to_meter;
          }
        }
        projection = new ol.proj.Projection({
          code: code,
          units: units,
          axisOrientation: def.axis
        });
        ol.proj.addProjection(projection);
        var currentCode, currentDef, currentProj, proj4Transform;
        for (currentCode in projections) {
          currentDef = proj4.defs(currentCode);
          if (goog.isDef(currentDef)) {
            currentProj = ol.proj.get(currentCode);
            if (currentDef === def) {
              ol.proj.addEquivalentProjections([currentProj, projection]);
            } else {
              proj4Transform = proj4(currentCode, code);
              ol.proj.addCoordinateTransforms(currentProj, projection,
                  proj4Transform.forward, proj4Transform.inverse);
            }
          }
        }
      } else {
        goog.asserts.assert(goog.isDef(projection));
        projection = null;
      }
    }
  } else {
    projection = null;
  }
  return projection;