^* This work was partially supported by the National Science Foundation (NSF) for National Center for Geographic Information and Analysis (NCGIA) under grant number SBE-8810917. Max Egenhofer's work is further supported by NSF grant IRI-9309230, and grants from Intergraph Corporation, Space Imaging Inc., Environmental Systems Research Institute, and the Scientific and Environmental Affairs Division of the North Atlantic Treaty Organization.

[1] A poem by Waddington (1993) used the same term in a different context.