Estimate point resolution for Proj4js projections

src/ol/projection.js

@@ -8,6 +8,7 @@ goog.require('goog.object');
 goog.require('ol.Coordinate');
 goog.require('ol.Extent');
 goog.require('ol.TransformFunction');
+goog.require('ol.sphere.NORMAL');
 
 
 /**
@@ -129,10 +130,49 @@ ol.Proj4jsProjection_ = function(code, proj4jsProj) {
    */
   this.proj4jsProj_ = proj4jsProj;
 
+  /**
+   * @private
+   * @type {?ol.TransformFunction}
+   */
+  this.toEPSG4326_ = null;
+
 };
 goog.inherits(ol.Proj4jsProjection_, ol.Projection);
 
 
+/**
+ * @inheritDoc
+ */
+ol.Proj4jsProjection_.prototype.getPointResolution =
+    function(resolution, point) {
+  if (this.getUnits() == ol.ProjectionUnits.DEGREES) {
+    return resolution;
+  } else {
+    // Estimate point resolution by transforming the center pixel to EPSG:4326,
+    // measuring its width and height on the normal sphere, and taking the
+    // average of the width and height.
+    if (goog.isNull(this.toEPSG4326_)) {
+      this.toEPSG4326_ = ol.projection.getTransform(
+          this, ol.projection.getProj4jsProjectionFromCode_('EPSG:4326'));
+    }
+    var vertices = [
+      point.x - resolution / 2, point.y,
+      point.x + resolution / 2, point.y,
+      point.x, point.y - resolution / 2,
+      point.x, point.y + resolution / 2
+    ];
+    vertices = this.toEPSG4326_(vertices, vertices, 2);
+    var width = ol.sphere.NORMAL.haversineDistance(
+        new ol.Coordinate(vertices[0], vertices[1]),
+        new ol.Coordinate(vertices[2], vertices[3]));
+    var height = ol.sphere.NORMAL.haversineDistance(
+        new ol.Coordinate(vertices[4], vertices[5]),
+        new ol.Coordinate(vertices[6], vertices[7]));
+    return (width + height) / 2;
+  }
+};
+
+
 /**
  * @return {Proj4js.Proj} Proj4js projection.
  */

test/spec/ol/projection.test.js

@@ -144,6 +144,36 @@ describe('ol.projection', function() {
       expect(point.y).toRoughlyEqual(200146.976, 1);
     });
 
+    it('numerically estimates point scale at the equator', function() {
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      expect(googleProjection.getPointResolution(1, new ol.Coordinate(0, 0))).
+          toRoughlyEqual(1, 1e-1);
+    });
+
+    it('numerically estimates point scale at various latitudes', function() {
+      var epsg3857Projection = ol.projection.getFromCode('EPSG:3857');
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      var point, y;
+      for (y = -20; y <= 20; ++y) {
+        point = new ol.Coordinate(0, 1000000 * y);
+        expect(googleProjection.getPointResolution(1, point)).toRoughlyEqual(
+            epsg3857Projection.getPointResolution(1, point), 1e-1);
+      }
+    });
+
+    it('numerically estimates point scale at various points', function() {
+      var epsg3857Projection = ol.projection.getFromCode('EPSG:3857');
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      var point, x, y;
+      for (x = -20; x <= 20; ++x) {
+        for (y = -20; y <= 20; ++y) {
+          point = new ol.Coordinate(1000000 * x, 1000000 * y);
+          expect(googleProjection.getPointResolution(1, point)).toRoughlyEqual(
+              epsg3857Projection.getPointResolution(1, point), 1e-1);
+        }
+      }
+    });
+
   });
 
   describe('ol.projection.getTransform()', function() {