code: function(cluster, feature) {
          var cc = cluster.geometry.getBounds().getCenterLonLat();
          var fc = feature.geometry.getBounds().getCenterLonLat();
          var distance = (
              Math.sqrt(
                  Math.pow((cc.lon - fc.lon), 2) + Math.pow((cc.lat - fc.lat), 2)
              ) / this.resolution
          );
          return (distance <= this.distance);
      }
docs: Method: shouldCluster
  Determine whether to include a feature in a given cluster.
  
  Parameters:
  cluster - {<OpenLayers.Feature.Vector>} A cluster.
  feature - {<OpenLayers.Feature.Vector>} A feature.
  
  Returns:
  {Boolean} The feature should be included in the cluster.