code: function() {
          var mapExtent = this.map.getExtent();
          var maxExtent = this.map.getMaxExtent();
          var testExtent = new OpenLayers.Bounds(
                                  Math.max(mapExtent.left, maxExtent.left),
                                  Math.max(mapExtent.bottom, maxExtent.bottom),
                                  Math.min(mapExtent.right, maxExtent.right),
                                  Math.min(mapExtent.top, maxExtent.top));        
  
          if (this.ovmap.getProjection() != this.map.getProjection()) {
              testExtent = testExtent.transform(
                  this.map.getProjectionObject(),
                  this.ovmap.getProjectionObject() );
          }
  
          var resRatio = this.ovmap.getResolution() / this.map.getResolution();
          return ((resRatio > this.minRatio) &&
                  (resRatio <= this.maxRatio) &&
                  (this.ovmap.getExtent().containsBounds(testExtent)));
      }
docs: Method: isSuitableOverview
  Determines if the overview map is suitable given the extent and
  resolution of the main map.