Interfacing U.S. census map files with statistical graphics software: application and use in epidemiology

In 1990, the United States Bureau of the Census released detailed geographic map files known as TIGER/Line (Topologically Integrated Geographic Encoding and Referencing). The TIGER files, accessible through purchase or federal repository libraries, contain 24 billion characters of data describing various geographic features including coastlines, hydrography, transportation networks, political boundaries, etc. for the entire United States. Many of these physical features are of potential interest in epidemiological case studies. Unfortunately, the TIGER data base only provides raw alphanumeric data; no utility software, graphical or otherwise, is included. Recently, the S statistical software package has been extended to include a map display function. The map function augments S's high-level approach towards statistical analysis and graphical display of data. Coupling this statistical software with the map data base developed for U.S. Census data collection will facilitate epidemiological research. We discuss the technical background necessary to utilize the TIGER data base for mapping with S. Two types of S maps, segment-based and polygon-based, are discussed along with methods to construct them from TIGER data. Polygon-based maps are useful for displaying regional statistical data, such as disease rates or incidence at the census tract level. Segment-based maps are easier to assemble and are appropriate when the data are not regionalized. Census tract data of AIDS incidence in San Francisco and lung cancer case locations relative to petrochemical refinery sites in Contra Costa County are used to illustrate the methods and potential uses of interfacing the TIGER data base with S.