球面座標的面積計算

     經過android gps採集的座標爲球面座標，若是此種座標造成的ploygon直接進行面積的計算是不可行的，計算出來的面積爲負數。因而須要將球面座標轉換成平面座標在進行計算方可。

public static double getArea(List<Point> 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));
	}