通过android gps采集的坐标为球面坐标，如果此种坐标形成的ploygon直接进行面积的计算是不可行的，计算出来的面积为负数。于是需要将球面坐标转换成平面坐标在进行计算方可。

public static double getArea(List
    
      list, MapView mapView) {        DecimalFormat dfAreaR = new DecimalFormat("0.00");		Polygon polygon = new Polygon();		for (int i = 0; i < list.size(); i++) {			Point pt = list.get(i);			if (i == 0) {				polygon.startPath(pt);			} else {				polygon.lineTo(pt);			}		}		polygon.closeAllPaths();		SpatialReference webMercator = SpatialReference.create(2362);        //project from wgs84		Polygon newPoly = (Polygon) GeometryEngine.project(polygon, mapView.getSpatialReference(),webMercator);		double area = newPoly.calculateArea2D();		return Double.valueOf(dfAreaR.format(area));	}