II.   THE TIGER SYSTEM:   AN OVERVIEW


The U.S. Bureau of the Census, in cooperation with the U.S. Geological
Survey (USGS), developed a major new computer-readable geographic data base
for the United States and its possessions.  The primary sources of the data
base were the USGS 1:100,000-scale maps and the Census Bureau's 1980
GBF/DIME-Files.  The USGS compiled the 1:100,000-scale maps from
photographically reduced (to 1:100,000-scale) mosaicked 1:24,000-scale
maps.  The USGS 1:100,000-scale maps meet national map accuracy standards
and uses coordinates defined by the North American Datum, 1927 (see
reference McKenzie and LaMacchia, 1987).  For the lower 48 States, the
cartographic quality of most of the 1990 Census TIGER/Line files, in areas
outside the 1980 census GBF/DIME-File coverage and selected other large
metropolitan areas, compares favorably with the USGS 1:100,000-scale maps.
These geographic/cartographic files are excellent as a base for a
geographic information system (GIS) but are not recommended for detailed
applications requiring higher positional accuracy.  The Census Bureau's
TIGER System (which uses this new data base) automates the mapping and
related geographic activities required to support the census and sample
survey programs of the Census Bureau, starting with the 1990 Decennial
Census.  The TIGER System provides support for the following:

  -  Creation and maintenance of a digital geographic data base that
     includes complete coverage of the United States, Puerto Rico, the
     Virgin Islands of the United States, American Samoa, Guam, the
     Northern Mariana Islands, Palau, the other Pacific entities that were
     part of the Trust Territories of the Pacific areas (Marshall Islands
     and the Federated States of Micronesia), and the Midway Islands (to
     provide complete mapping within the boundaries of the State of
     Hawaii).
  
  -  Production of maps from the TIGER data base for all Census Bureau
     enumeration and publication programs.
  
  -  Ability to assign individual addresses to geographic area codes and to
     the polygons (census blocks) formed by features such as roads,
     streams, and so forth.

The design of the TIGER data base adapts the theories of topology, graph
theory, and associated fields of mathematics to provide a disciplined,
mathematical description for the geographic structure of the United States.
The TIGER data base, at the core of the TIGER System, relates the automated
description of the earth's surface with the boundary features that the
Census Bureau uses in preparing its data tabulations.  The topological
structure of the TIGER data base defines the location and relationship of
streets, rivers, and railroads to each other and to the numerous geographic
areas for which the Census Bureau tabulates data from its censuses and
sample surveys.  It is designed to assure no duplication of these features
or areas.  The building of the TIGER data base involved adapting a variety
of encoding techniques such as automated map scanning, manual map
"digitizing," standard data keying, and sophisticated computer file
matching.  The goal is to provide automated access to and retrieval of the
decennial census-relevant geographic information about the United States
and its possessions.

The geographic coverage for a 1990 Census TIGER/Line file is a county or
statistically equivalent entity (see Appendix A).  The files contain
January 1, 1990 geographic area codes for legal boundaries obtained in
response to the Census Bureau's Boundary and Annexation Survey (BAS).  The
1990 Census TIGER/Line files contain the final 1990 census tabulation
geographic area boundaries and codes.