OpenLayers学习笔记2——坐标转换问题

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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 - {<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

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