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。

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;