Brewer, C. & Suchan, T., (2001). Mapping census 2000: The geography of U. S. diversity. U. S. Census Bureau, Census Special Reports, Series CENSR/01-1. Washington, D. C.: U.S. Government Printing Office.
Chrisman, N. (1997). Exploring geographic information systems. New York: John Wiley & Sons, Inc.
Chrisman, N. (2002). Exploring geographic information systems. (2nd ed.). New York: John Wiley & Sons, Inc.
Goodchild, M. (1992). Geographical information science. International Journal of Geographic Information Systems 6:1, 31-45.
National Decision Systems. A zip code can make your company lots of money! Retrieved on July 6, 1999, from http://laguna.natdecsys.com/lifequiz (since retired).
Steger, T. D. (1986). Topographic maps. Washington D.C.: U.S. Government Printing Office.
Stevens, S.S. (1946). On the theory of scales of measurement. Science, 103, 677-680.1
U.S. Geological Survey (2012) Topographic Map Symbols, ISBN 0-607-96942-3; available at: https://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf
Worboys, M. F. (1995). GIS: A computing perspective. London: Taylor and Francis.