Add geodesic option for measure

src/ol/sphere/sphere.js

@@ -57,6 +57,36 @@ ol.Sphere.prototype.cosineDistance = function(c1, c2) {
 };
 
 
+/**
+ * Returns the geodesic area for a list of coordinates.
+ *
+ * [Reference](http://trs-new.jpl.nasa.gov/dspace/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.
+ * @return {number} Area.
+ * @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 += goog.math.toRadians(x2 - x1) *
+        (2 + Math.sin(goog.math.toRadians(y1)) +
+        Math.sin(goog.math.toRadians(y2)));
+    x1 = x2;
+    y1 = y2;
+  }
+  return area * this.radius * this.radius / 2.0;
+};
+
+
 /**
  * Returns the distance of c3 from the great circle path defined by c1 and c2.
  *
@@ -110,6 +140,7 @@ ol.Sphere.prototype.finalBearing = function(c1, c2) {
  * @param {ol.Coordinate} c1 Coordinate 1.
  * @param {ol.Coordinate} c2 Coordinate 2.
  * @return {number} Haversine distance.
+ * @api
  */
 ol.Sphere.prototype.haversineDistance = function(c1, c2) {
   var lat1 = goog.math.toRadians(c1[1]);