openlay 的测量误差

openlayer的测量有两种，如上图ol.Shere.getLength().....比较符合实际（使用大圆弧线进行计算）必须是4326的，

官网例子地址http://openlayers.org/en/latest/examples/measure.html

var formatArea = function (polygon) {
   
   //这个geometry得是4326的
   var geom=polygon.clone();
   var geom4326=geom.transform("EPSG:3857","EPSG:4326");
        var coordinates = geom4326.getLinearRing(0).getCoordinates();
        var wgs84Sphere = new ol.Sphere(6378137);
        var area = Math.abs(wgs84Sphere.geodesicArea(coordinates));