OpenLayers學習筆記2——坐標轉換問題

參照別人的添加marker的demo來改造時，發現無論怎樣更改經緯度，都是停留在同一個位置。過了一兩天突然想起可能是坐標參考的問題，嘗試搜了一下，果然是這個問題。問題是這樣子的：

WMTS中地圖的坐標參考系是102100（具體是哪個不清楚），如下圖所示：

而我在初始化地圖時設置的參數如下圖所示：projection屬性設置的是‘EPSG：102100’，displayProjection屬性設置的是‘EPSG：4326’，也即經緯度顯示。

 

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.

 

因此如果要利用經緯度進行定位標記的話，需要進行坐標轉換，即EPSG：4326—>EPSG:102100，主要用到OpenLayers.LonLat的transform函數，代碼如下：

 

//坐標轉換
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函數代碼：

 

 

/**
 * 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;
} ,

通過查看源代碼會發現它調用的是OpenLayers.Projection的transform方法，有興趣的可以一步一步看到轉換的源碼，這里就不贅述了

 

 

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