�示信息
            if (sketch) {
                var geom = (sketch.getGeometry());
                if (geom instanceof ol.geom.Polygon) {
                    helpMsg = continuePolygonMsg; //绘制多边形时提示相应内容
                } else if (geom instanceof ol.geom.LineString) {
                    helpMsg = continueLineMsg; //绘制线时提示相应内容
                }
            }
            if(helpTooltipElement)
            {
                helpTooltipElement.innerHTML = helpMsg