OpenLayers学习笔记2——坐标转换问题

参照别人的添加marker的demo来改造时，发现无论怎样更改经纬度，都是停留在同一个位置。过了一两天突然想起可能是坐标参考的问题，尝试搜了一下，果然是这个问题。问题是这样子的：

WMTS中地图的坐标参考系是102100（具体是哪个不清楚），如下图所示：

而我在初始化地图时设置的参数如下图所示：projection属性设置的是‘EPSG：102100’，displayProjection属性设置的是‘EPSG：4326’，也即经纬度显示。

projection

{String} Set in the map options to specify the default projection for layers added to this map.  When using a projection other than EPSG:4326 (CRS:84, Geographic) or EPSG:3857 (EPSG:900913, Web Mercator), also set maxExtent, maxResolution or resolutions.  Default is “EPSG:4326”.  Note that the projection of the map is usually determined by that of the current baseLayer (see baseLayer and getProjectionObject).

displayProjection

{OpenLayers.Projection} Requires proj4js support for projections other than EPSG:4326 or EPSG:900913/EPSG:3857.  Projection used by several controls to display data to user.  If this property is set, it will be set on any control which has a null displayProjection property at the time the control is added to the map.

因此如果要利用经纬度进行定位标记的话，需要进行坐标转换，即EPSG：4326—>EPSG:102100，主要用到OpenLayers.LonLat的transform函数，代码如下：

//坐标转换
function corTransform(lon, lat) {
    var proj = new OpenLayers.Projection("EPSG:4326");
    var lonlat = new OpenLayers.LonLat(lon, lat);
    lonlat.transform(proj, this.map.getProjectionObject());
    return lonlat;

}

OpenLayers.LonLat的transform函数代码：

	/**
	 * APIMethod: transform
	 * Transform the LonLat object from source to dest. This transformation is
	 *    *in place*: if you want a *new* lonlat, use .clone() first.
	 *
	 * Parameters:
	 * source - {} Source projection.
	 * dest   - {} Destination projection.
	 *
	 * Returns:
	 * {} Itself, for use in chaining operations.
	 */
	transform : function (source , dest) {
		var point = OpenLayers.Projection.transform(
			{'x' : this.lon , 'y' : this.lat} , source , dest);
		this.lon = point.x;
		this.lat = point.y;
		return this;
	} ,

通过查看源代码会发现它调用的是OpenLayers.Projection的transform方法，有兴趣的可以一步一步看到转换的源码，这里就不赘述了