docs: function(extent, res) {
          var upperLeft = new OpenLayers.Geometry.Point(extent.left, extent.top);
          var bottomRight = new OpenLayers.Geometry.Point(extent.right, extent.bottom);
          var start = this.getContainingTileCoords(upperLeft, res);
          var end = this.getContainingTileCoords(bottomRight, res);
          var lod = {
              resolution: res,
              startTileCol: start.x,
              startTileRow: start.y,
              endTileCol: end.x,
              endTileRow: end.y
          };
          return this.calculateMaxExtentWithLOD(lod);
     }