【百度地图】获取某个覆盖物的总面积

//参数是一个百度类型的多边形覆盖物
function getArea(polygon){

  // 检查类型：既不是百度类型的范围又不是数组类型的数据，直接返回0
  if (!(polygon instanceof BMap.Polygon) && !(polygon instanceof Array)) {
          return 0;
  }

   //如果是百度类型的，得到点集合，不是的话，另说。
   var pts = new Array();
   if (polygon instanceof BMap.Polygon) {
        pts = polygon.getPath();
   }

   //判断数组的长度，如果是小于3的话，不构成面，无法计算面积
   if (pts.length < 3) {
          return 0;
   }

   var totalArea = 0;// 初始化总面积
   var LowX = 0.0; 
   var LowY = 0.0; 
   var MiddleX = 0.0;
   var MiddleY = 0.0;
   var HighX = 0.0;
   var HighY = 0.0;
   var AM = 0.0;
   var BM = 0.0;
   var CM = 0.0;
   var AL = 0.0;
   var BL = 0.0;
   var CL = 0.0;
   var AH = 0.0;
   var BH = 0.0;
   var CH = 0.0;
   var CoefficientL = 0.0;
   var CoefficientH = 0.0;
   var ALtangent = 0.0;
   var BLtangent = 0.0;
   var CLtangent = 0.0;
   var AHtangent = 0.0;
   var BHtangent = 0.0;
   var CHtangent = 0.0;
   var ANormalLine = 0.0;
   var BNormalLine = 0.0;
   var CNormalLine = 0.0;
   var OrientationValue = 0.0;
   var AngleCos = 0.0;
   var Sum1 = 0.0;
   var Sum2 = 0.0;
   var Count2 = 0;
   var Count1 = 0;
   var Sum = 0.0;
   var Radius = 6378137.0;// ,WGS84椭球半径
   var Count = pts.length;
    for ( var i = 0; i < Count; i++) {
          if (i == 0) {
              LowX = pts[Count - 1].lng * Math.PI / 180;
              LowY = pts[Count - 1].lat * Math.PI / 180;
              MiddleX = pts[0].lng * Math.PI / 180;
              MiddleY = pts[0].lat * Math.PI / 180;
              HighX = pts[1].lng * Math.PI / 180;
              HighY = pts[1].lat * Math.PI / 180;
          } else if (i == Count - 1) {
              LowX = pts[Count - 2].lng * Math.PI / 180;
              LowY = pts[Count - 2].lat * Math.PI / 180;
              MiddleX = pts[Count - 1].lng * Math.PI / 180;
              MiddleY = pts[Count - 1].lat * Math.PI / 180;
              HighX = pts[0].lng * Math.PI / 180;
              HighY = pts[0].lat * Math.PI / 180;
          } else {
              LowX = pts[i - 1].lng * Math.PI / 180;
              LowY = pts[i - 1].lat * Math.PI / 180;
              MiddleX = pts[i].lng * Math.PI / 180;
              MiddleY = pts[i].lat * Math.PI / 180;
              HighX = pts[i + 1].lng * Math.PI / 180;
              HighY = pts[i + 1].lat * Math.PI / 180;
          }
          AM = Math.cos(MiddleY) * Math.cos(MiddleX);
          BM = Math.cos(MiddleY) * Math.sin(MiddleX);
          CM = Math.sin(MiddleY);
          AL = Math.cos(LowY) * Math.cos(LowX);
          BL = Math.cos(LowY) * Math.sin(LowX);
          CL = Math.sin(LowY);
          AH = Math.cos(HighY) * Math.cos(HighX);
          BH = Math.cos(HighY) * Math.sin(HighX);
          CH = Math.sin(HighY);
          CoefficientL = (AM * AM + BM * BM + CM * CM)/ (AM * AL + BM * BL + CM * CL);
          CoefficientH = (AM * AM + BM * BM + CM * CM)/ (AM * AH + BM * BH + CM * CH);
          ALtangent = CoefficientL * AL - AM;
          BLtangent = CoefficientL * BL - BM;
          CLtangent = CoefficientL * CL - CM;
          AHtangent = CoefficientH * AH - AM;
          BHtangent = CoefficientH * BH - BM;
          CHtangent = CoefficientH * CH - CM;
          AngleCos = (AHtangent * ALtangent + BHtangent * BLtangent + CHtangent* CLtangent)/ (Math.sqrt(AHtangent * AHtangent + BHtangent* BHtangent + CHtangent * CHtangent) * Math.sqrt(ALtangent * ALtangent + BLtangent* BLtangent + CLtangent * CLtangent));
          AngleCos = Math.acos(AngleCos);
          ANormalLine = BHtangent * CLtangent - CHtangent * BLtangent;
          BNormalLine = 0 - (AHtangent * CLtangent - CHtangent* ALtangent);
          CNormalLine = AHtangent * BLtangent - BHtangent * ALtangent;
          if (AM != 0)
              OrientationValue = ANormalLine / AM;
          else if (BM != 0)
              OrientationValue = BNormalLine / BM;
          else
              OrientationValue = CNormalLine / CM;
          if (OrientationValue > 0) {
              Sum1 += AngleCos;
              Count1++;
          } else {
              Sum2 += AngleCos;
              Count2++;
          }
      }

      var tempSum1, tempSum2;
      tempSum1 = Sum1 + (2 * Math.PI * Count2 - Sum2);
      tempSum2 = (2 * Math.PI * Count1 - Sum1) + Sum2;
      if (Sum1 > Sum2) {
          if ((tempSum1 - (Count - 2) * Math.PI) < 1)
              Sum = tempSum1;
          else
              Sum = tempSum2;
      } else {
          if ((tempSum2 - (Count - 2) * Math.PI) < 1)
              Sum = tempSum2;
          else
              Sum = tempSum1;
      }
      totalArea = (Sum - (Count - 2) * Math.PI) * Radius * Radius;

      return totalArea; // 返回总面积
}