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Concepts & Definitions

Geography plays an important role in Census Bureau activities, including supporting the American Community Survey. It is the framework by which the Census Bureau collects and publishes demographic, social, housing, and economic data. Geography contributes to, and is involved in, ACS sampling, data collection, weighting, and data tabulation activities.

Key concepts to aid in your understanding include:

The nation is subdivided into two main types of geographic areas, legal and statistical.

Legal areas are defined specifically by law, and include state, local and tribal government units, as well as some specially defined administrative areas like congressional districts. Many, but not all, are represented by elected officials.

Example: New York State

Statistical areas are defined directly by the Census Bureau and state, regional or local authorities, and include census tracts and urban areas. The primary purpose of statistical areas is to tabulate and present census data.

Example: Metropolitan Statistical area - "Boston-Cambridge-Newton-MA-NH Metro area"

For more information on geographic areas, visit the Geography Glossary page.

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Geographic areas are organized in a geographic hierarchy. Larger units, like states, include smaller units, like counties and census tracts. This structure is derived from the legal, administrative, or areal relationships of the entities.

Hierarchy of Select Geographic Entities in the American Community Survey

ACS 5 Year Geographic Hierarchy

*    5-year estimates only
**   5-year estimates only, first release in 2012 for the 2007-2011 5-year estimates
This graphic does not represent the full set of entities for which the ACS publishes data.

This geographic hierarchy influences how the Census Bureau identifies geographic areas. A system of geocodes - numeric codes - are used to represent specific geographic areas.

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The Census Bureau has transitioned from the Federal Information Processing Standard (FIPS) codes (now referred to as Federal Information Processing Series codes) to using American National Standards Institute (ANSI) codes. The Census Bureau's Geography Division provides additional information on the transition from FIPS to ANSI. ANSI codes are currently used as geographic identifiers for ACS data.

To identify a geographic area in larger contexts like the nation, one or more higher level geocodes may be required. Census tracts are unique within counties and counties are unique within states. You need the full set of state, county, and tract geocodes to uniquely identify a census tract.

The unique geocode for the census tract 201 in Autauga County, Alabama is 01001020100.

Name of Geography State Code County Code Tract Code
Census Tract 201, Autauga County, Alabama 01 001 020100
Census Tract 201, La Paz County, Arizona 04 012 020100

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Changes to geographic area definitions include boundary changes to legal and statistical areas, name changes, and geographic coding changes.

Boundary Changes for Local Areas

The Census Bureau annually conducts the Boundary and Annexation Survey (BAS) to collect information about selected legally defined geographic areas. Information provided through the BAS is used to update legal boundaries and names of governmental units in the United States. The boundary information collected through the BAS is used by the Census Bureau to tabulate data for censuses and various surveys, including the American Community Survey. The ACS uses boundaries as of January 1 of the last year of the estimate period. For example, the 2014-2018 ACS 5-year uses boundaries as of January 1, 2018, as reported to the Census Bureau.

Example: Homerville City Annexations
Homerville City, a fictitious example, has annexed territory between 2005 and 2008 resulting in a boundary change. For the 2005-2009 ACS 5-year estimates, any survey responses collected from housing units in the annexed areas will be included in the Homerville City boundary because it is in the legal boundaries, as reported to the Census Bureau, on January 1, 2009.

Geography of Homerville

Boundary Changes for Statistical Areas

Statistical areas also experience boundary changes, most often in association with each decennial census and much less frequently than legal areas as a whole. Statistical areas are designed to have stable boundaries. There are several different statistical area definition programs associated with specific statistical area types. The Participant Statistical Areas Program (PSAP) addresses more familiar statistical areas like census tracts, block groups, Census Designated Places (CDPs) and tribal statistical areas.

Geographic Area Name and Code Changes

Occasionally a legal area will change its legal name, which also results in a change to the corresponding FIPS geographic code for the legal area. ANSI geographic codes are permanent and do not change with changes to the names of legal areas. For example, in 2006, Dover Township in Ocean County, NJ changed its name to Toms River Township. Its FIPS code changed from 18130 to 73125 in association with this name change, but its ANSI code remained 882074.

The American Community Survey does not update previously released estimates to reflect subsequent boundary changes.

Learn more about tools to help identify geographic and boundary changes for geographic areas. Check out our geographic boundary change notes to find information on boundary changes at the Census Bureau.

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Learn more about the definitions of geographic terms and concepts, such as Minor Civil Division, Place, Metropolitan Statistical Area, and more, by visiting the Geography Glossary page.

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Page Last Revised - March 15, 2023
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