Census Tract, St. Clair County, Illinois

Census tracts are small, relatively permanent statistical subdivisions of a county or equivalent entity, and are reviewed and updated by local participants prior to each decennial census as part of the Census Bureau's Participant Statistical Areas Program. The Census Bureau updates census tracts in situations where no local participant existed or where local or tribal governments declined to participate. The primary purpose of census tracts is to provide a stable set of geographic units for the presentation of decennial census data.

Census tracts generally have a population size between 1,200 and 8,000 people with an optimum size of 4,000 people. The spatial size of census tracts varies widely depending on the density of settlement. Census tracts are delineated with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. However, physical changes in street patterns caused by highway construction, new development, and so forth, may require boundary revisions. In addition, census tracts occasionally are split due to population growth, or combined as a result of substantial population decline.

Census tract boundaries generally follow visible and identifiable features. They may follow legal boundaries such as minor civil division (MCD) or incorporated place boundaries in some states and situations to allow for census tract-to-governmental unit relationships where the governmental boundaries tend to remain unchanged between censuses. State and county boundaries always are census tract boundaries in the standard census geographic hierarchy.

In a few rare instances, a census tract may consist of discontiguous areas. These discontiguous areas may occur where the census tracts are coextensive with all or parts of legal entities that are themselves discontiguous.

Census Tract Codes and Numbers - Census tract numbers have up to a 4-digit basic number and may have an optional 2-digit suffix; for example, 1457.02. The census tract numbers (used as names) eliminate any leading zeroes and append a suffix only if required. The 6-character numeric census tract codes, however, include leading zeroes and have an implied decimal point for the suffix. Census tract codes range from 000100 to 998998 and are unique within a county or equivalent area. The Census Bureau reserved the census tract numbering range of 9400 to 9499 for use by American Indian area participants in situations where an American Indian entity crosses county or state lines. The Census Bureau assigned a default census tract code of 000000 to some coastal and Great Lakes water and territorial sea, rather than extend the census tract boundary into the Great Lakes or out to the three-mile limit. By closing off some census tracts along the shoreline or just offshore and assigning the default census tract to the offshore Current water areas, the Census Bureau provides complete census tract coverage of Current water areas in territorial seas and the Great Lakes. Because of updates since 2000, there are census tracts with code 000000 that now contain land. Census tract suffixes may range from .01 to .98.

The Census Bureau uses suffixes to help identify census tract changes for comparison purposes. Local participants have an opportunity to review the existing census tracts before each census. If local participants split a census tract, the split parts usually retain the basic number, but receive different suffixes. In a few counties, local participants request major changes to, and renumbering of, the census tracts. Changes to individual census tract boundaries usually do not result in census tract numbering changes.

Relationship to Other Geographic Entities - Within the standard census geographic hierarchy, census tracts never cross state or county boundaries, but may cross the boundaries of county subdivisions, places, urban areas, voting districts, congressional districts, traffic analysis districts, traffic analysis zones, and American Indian, Alaska Native, and Native Hawaiian areas.

Census Tract Numbers and Codes
> 0001 to 9989 - Basic number range for census tracts
> 0000 - Default basic number for census tracts
> 01 to 98 - Suffix codes for census tracts
> 00 - Suffix code for census tracts without a suffix ;