accurate calculation of the centroid for small objects with large coordinates

lib/OpenLayers/Geometry/LinearRing.js

@@ -198,15 +198,23 @@ OpenLayers.Geometry.LinearRing = OpenLayers.Class(
             } else if (len > 2) {
                 var sumX = 0.0;
                 var sumY = 0.0;
-                for (var i = 0; i < this.components.length - 1; i++) {
+                var averX = 0.0;
+                var averY = 0.0;
+                for (var i = 0; i < len; i++) {
+                    averX += this.components[i].x;
+                    averY += this.components[i].y;
+                }
+                averX = averX / len;
+                averY = averY / len;
+                for (var i = 0; i < len - 1; i++) {
                     var b = this.components[i];
                     var c = this.components[i+1];
-                    sumX += (b.x + c.x) * (b.x * c.y - c.x * b.y);
+                    sumX += (b.x + c.x - 2 * averX) * ((b.x - averX) * (c.y - averY) - (c.x - averX) * (b.y - averY));
-                    sumY += (b.y + c.y) * (b.x * c.y - c.x * b.y);
+                    sumY += (b.y + c.y - 2 * averY) * ((b.x - averX) * (c.y - averY) - (c.x - averX) * (b.y - averY));
                 }
                 var area = -1 * this.getArea();
-                var x = sumX / (6 * area);
+                var x = averX + sumX / (6 * area);
-                var y = sumY / (6 * area);
+                var y = averY + sumY / (6 * area);
                 return new OpenLayers.Geometry.Point(x, y);
             } else {
                 return null;