Richard A. Becker, and Allan R. Wilks,
"Maps in S",
AT&T Bell Laboratories Statistics Research Report [93.2], 1993.
Richard A. Becker, and Allan R. Wilks,
"Constructing a Geographical Database",
AT&T Bell Laboratories Statistics Research Report [95.2], 1995.
US Department of Commerce, Census Bureau,
County Boundary File,
computer tape, available from Customer Services,
Bureau of the Census, Washingdon DC 20233.