code: function (polygon) {
          var sum = 0;
  
          for (var i = 0; i < polygon.length; i++) {
              var ring = polygon[i];
  
              for (var j = 0, len = ring.length, k = len - 1; j < len; k = j++) {
                  sum += (ring[j][0] - ring[k][0]) * (ring[j][1] + ring[k][1]) * (i ? -1 : 1);
              }
          }
  
          return (Math.abs(sum) / 2) * this.kx * this.ky;
      }
docs: Given a polygon (an array of rings, where each ring is an array of points), returns the area.
  
  @param {Array<Array<Array<number>>>} polygon
  @returns {number} area value in the specified units (square kilometers by default)
  @example
  var area = ruler.area([[
  [-67.031, 50.458], [-67.031, 50.534], [-66.929, 50.534],
  [-66.929, 50.458], [-67.031, 50.458]
  ]]);
  //=area