TREE STRUCTURES FOR REGION REPRESENTATION The support of the Defense Advanced Research Projects Agency and the U.S. Army Night Vision Laboratory under Contract DAAG-53-76C-0138 (DARPA Order 3206) is gratefully acknowledged, as is the help of Kathryn Riley in preparing this paper. The material in this paper is based on a series of reports by H. Samet, as well as two reports by C. R. Dyer and M. Shneier.