      url: "http://t0.tianditu.com/DataServer?T=cva_w&x={x}&y={y}&l={z}&tk=" + tdk
                    })
                })
            );
            //实例化一个矢量图层Vector作为绘制层
            vectorSource = new ol.source.Vector({ wrapX: false });

            commonStyle = new ol.style.Style({
                    fill: new ol.style.Fill({
                        color: 'rgba(255, 255, 255, 0.2)'
                    }),
                    stroke: new ol.style.Stroke({
                