Function for getting spherical area

src/ol/sphere.js

@@ -52,18 +52,7 @@ ol.Sphere = function(radius) {
  * @api
  */
 ol.Sphere.prototype.geodesicArea = function(coordinates) {
-  var area = 0, len = coordinates.length;
-  var x1 = coordinates[len - 1][0];
-  var y1 = coordinates[len - 1][1];
-  for (var i = 0; i < len; i++) {
-    var x2 = coordinates[i][0], y2 = coordinates[i][1];
-    area += ol.math.toRadians(x2 - x1) *
-        (2 + Math.sin(ol.math.toRadians(y1)) +
-        Math.sin(ol.math.toRadians(y2)));
-    x1 = x2;
-    y1 = y2;
-  }
-  return area * this.radius * this.radius / 2.0;
+  return ol.Sphere.getArea_(coordinates, this.radius);
 };
 
 
@@ -117,7 +106,7 @@ ol.Sphere.DEFAULT_RADIUS = 6371008.8;
  * the sum of all rings.  For points, the length is zero.  For multi-part
  * geometries, the length is the sum of the length of each part.
  * @param {ol.geom.Geometry} geometry A geometry.
- * @param {olx.SphereLengthOptions} opt_options Options for the length
+ * @param {olx.SphereMetricOptions} opt_options Options for the length
  *     calculation.  By default, geometries are assumed to be in 'EPSG:3857'.
  *     You can change this by providing a `projection` option.
  * @return {number} The spherical length (in meters).
@@ -206,3 +195,92 @@ ol.Sphere.getDistance_ = function(c1, c2, radius) {
       Math.cos(lat1) * Math.cos(lat2);
   return 2 * radius * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
 };
+
+
+/**
+ * Get the spherical area of a geometry.  This is the area (in meters) assuming
+ * that polygon edges are segments of great circles on a sphere.
+ * @param {ol.geom.Geometry} geometry A geometry.
+ * @param {olx.SphereMetricOptions} opt_options Options for the area
+ *     calculation.  By default, geometries are assumed to be in 'EPSG:3857'.
+ *     You can change this by providing a `projection` option.
+ * @return {number} The spherical area (in meters).
+ */
+ol.Sphere.getArea = function(geometry, opt_options) {
+  var options = opt_options || {};
+  var radius = options.radius || ol.Sphere.DEFAULT_RADIUS;
+  var projection = options.projection || 'EPSG:3857';
+  geometry = geometry.clone().transform(projection, 'EPSG:4326');
+  var type = geometry.getType();
+  var area = 0;
+  var coordinates, coords, i, ii, j, jj;
+  switch (type) {
+    case ol.geom.GeometryType.POINT:
+    case ol.geom.GeometryType.MULTI_POINT:
+    case ol.geom.GeometryType.LINE_STRING:
+    case ol.geom.GeometryType.MULTI_LINE_STRING:
+    case ol.geom.GeometryType.LINEAR_RING: {
+      break;
+    }
+    case ol.geom.GeometryType.POLYGON: {
+      coordinates = /** @type {ol.geom.Polygon} */ (geometry).getCoordinates();
+      area = Math.abs(ol.Sphere.getArea_(coordinates[0], radius));
+      for (i = 1, ii = coordinates.length; i < ii; ++i) {
+        area -= Math.abs(ol.Sphere.getArea_(coordinates[i], radius));
+      }
+      break;
+    }
+    case ol.geom.GeometryType.MULTI_POLYGON: {
+      coordinates = /** @type {ol.geom.SimpleGeometry} */ (geometry).getCoordinates();
+      for (i = 0, ii = coordinates.length; i < ii; ++i) {
+        coords = coordinates[i];
+        area += Math.abs(ol.Sphere.getArea_(coords[0], radius));
+        for (j = 1, jj = coords.length; j < jj; ++j) {
+          area -= Math.abs(ol.Sphere.getArea_(coords[j], radius));
+        }
+      }
+      break;
+    }
+    case ol.geom.GeometryType.GEOMETRY_COLLECTION: {
+      var geometries = /** @type {ol.geom.GeometryCollection} */ (geometry).getGeometries();
+      for (i = 0, ii = geometries.length; i < ii; ++i) {
+        area += ol.Sphere.getArea(geometries[i], opt_options);
+      }
+      break;
+    }
+    default: {
+      throw new Error('Unsupported geometry type: ' + type);
+    }
+  }
+  return area;
+};
+
+
+/**
+ * Returns the spherical area for a list of coordinates.
+ *
+ * [Reference](https://trs-new.jpl.nasa.gov/handle/2014/40409)
+ * Robert. G. Chamberlain and William H. Duquette, "Some Algorithms for
+ * Polygons on a Sphere", JPL Publication 07-03, Jet Propulsion
+ * Laboratory, Pasadena, CA, June 2007
+ *
+ * @param {Array.<ol.Coordinate>} coordinates List of coordinates of a linear
+ * ring. If the ring is oriented clockwise, the area will be positive,
+ * otherwise it will be negative.
+ * @param {number} radius The sphere radius.
+ * @return {number} Area (in meters).
+ */
+ol.Sphere.getArea_ = function(coordinates, radius) {
+  var area = 0, len = coordinates.length;
+  var x1 = coordinates[len - 1][0];
+  var y1 = coordinates[len - 1][1];
+  for (var i = 0; i < len; i++) {
+    var x2 = coordinates[i][0], y2 = coordinates[i][1];
+    area += ol.math.toRadians(x2 - x1) *
+        (2 + Math.sin(ol.math.toRadians(y1)) +
+        Math.sin(ol.math.toRadians(y2)));
+    x1 = x2;
+    y1 = y2;
+  }
+  return area * radius * radius / 2.0;
+};