function addQualityTileOverlay() {
          var factory = OverlayFactory();
          if(!map.qualityTileOverlay){
              map.qualityTileOverlay = new factory.CanvasTileLayerOverlay(map, "mylayer");
              map.addOverlay(map.qualityTileOverlay);
          }
        }

function removeQualityTileOverlay() {
          if(map.qualityTileOverlay){
              map. removeOverlay(map.qualityTileOverlay);
              delete map.qualityTileOverlay;
          }
        }