Five key layers in the U.S. Census data model.

The portions of the U.S. Census data model [PDF-5.62 MB] illustrate the best practices and methods for deploying and using the U.S. Census Bureau's Topologically Integrated Geographic Encoding and Referencing (TIGER) database in ArcGIS 8.3 and beyond. The full model includes all themes and feature classes contained in the TIGER files for the United States.

Traditionally, the U.S. Census Bureau publishes its administrative units and street lines for use by many local, state, and federal government agencies. GIS users nationwide bring the TIGER geometry into their own GIS and subsequently enhance and maintain census administrative units for their jurisdiction in an integrated manner with other geographic layers in their GIS. These users require the ability to manage the sophisticated topological integration inherent in the TIGER database. The ArcGIS 8.3 release includes support for such topologies in a GIS. The purpose of this data model is to provide a common data model for the agencies and organizations that use the U.S. Census TIGER database.

Census geography and administrative units are excellent examples of the topological integration of various data layers. Census units, such as blocks, block groups, tracts, and county boundaries, are assembled out of topologically-related line work that represents features such as streets, railroads, hydro lines, and other linear features. Topological rules can be defined for maintaining the integrity and behavior of these highly integrated features. For example, all census data must abide by rules such as

• Census blocks cannot overlap and must nest within block groups.
• Block groups cannot overlap and must nest within census tracts.
• Census tracts cannot overlap and must nest within counties.

The set of thematic layers in the U.S. Census data model includes the set of census boundary line features and comprise the building blocks of all other census layers: streets and addresses; census administrative boundaries; and numerous other administrative units such as local, state, and national government districts for voting, legislation, statistical accounting, and other federal reporting. All of these layers are topologically integrated. The complete database contains more than 30 topologically integrated feature classes and 57 topology rules.

Esri has tested this data model for high performance and usability on databases covering

• The five-county area covering and surrounding Washington, D.C.
• The state of California (3.4 million features)
• The complete census geography for the United States and its territories (53.5 million features)

See the complete specification, including a sample database, at support.esri.com/datamodels.

Why Is a Data Model for U.S. Census TIGER Important?

The TIGER database for the United States can be collaboratively maintained by a network of local and state government agencies, the U.S. Census Bureau, and private contractors. A common data model that maintains topological integrity of the integrated data layers for the census is crucial. This U.S. Census data model represents a practical implementation that leverages today's GIS technology.