Structure and algorithms Index node explicitly maintains both   Bounding sphere   Bounding rectangle Center is weighted centroid of child nodes Radius is minimum of maximum distances to   Bounding spheres of child nodes   Bounding rectangles of child nodes Radius is tighter than SS-tree When searching, minimum distance of query point to a node is maximum of minimum distances to   Bounding sphere of the node   Bounding rectangle of the node Similarly, maximum distance is minimum of maximum's   Better lower and upper bounds