code: function(mapBounds) {
          if(!mapBounds) {
              mapBounds = this.getMapBounds();
          }
          var center = mapBounds.getCenterLonLat();
          var dataWidth = mapBounds.getWidth() * this.ratio;
          var dataHeight = mapBounds.getHeight() * this.ratio;
          this.bounds = new OpenLayers.Bounds(
              center.lon - (dataWidth / 2),
              center.lat - (dataHeight / 2),
              center.lon + (dataWidth / 2),
              center.lat + (dataHeight / 2)
          );
      }
docs: Method: calculateBounds
  
  Parameters:
  mapBounds - {<OpenLayers.Bounds>} the current map extent, will be
  retrieved from the map object if not provided