OpenLayers学习笔记2——坐标转换问题

分手后的思念是犯贱 2022-08-05 02:47 481阅读 0赞

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：

![Center][]

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.

![Center 1][]

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 - {<OpenLayers.Projection>} Source projection.
    	 * dest   - {<OpenLayers.Projection>} Destination projection.
    	 *
    	 * Returns:
    	 * {<OpenLayers.LonLat>} 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

[Center]: /images/20220805/40ec2c994b2b4b289a2f3e0a8e22c7b6.png
[baseLayer]: http://localhost:63342/JSLearing/QDWebgis--new/olapi/doc/apidocs/files/OpenLayers/Map-js.html#OpenLayers.Map.baseLayer
[getProjectionObject]: http://localhost:63342/JSLearing/QDWebgis--new/olapi/doc/apidocs/files/OpenLayers/Map-js.html#OpenLayers.Map.getProjectionObject
[OpenLayers.Projection]: http://localhost:63342/JSLearing/QDWebgis--new/olapi/doc/apidocs/files/OpenLayers/Projection-js.html#OpenLayers.Projection
[Center 1]: /images/20220805/81612b9e9a3f41218ed43277a658c1fb.png