census geography

Census geography provides the framework for interpreting, analyzing, and understanding census data. The Census Bureau classifies all geographic entities into two broad categories: legal and administrative entities, and statistical entities.

Legal/Administrative entities generally originate from charters, laws, treaties, resolutions, or court decisions. They include:

• Congressional District. One of the 435 areas from which people are elected to the U.S. House of Representatives.

• County. The primary legal division of every state except Alaska and Louisiana. A number of geographic entities are not legally designated as a county, but are recognized by the Census Bureau as equivalent to a county for data presentation purposes. These include the boroughs, city and boroughs, municipalities, and census areas in Alaska; parishes in Louisiana; and cities that are independent of any county in Maryland, Missouri, Nevada, and Virginia. They also include the municipios in Puerto
  Rico, districts and islands in American Samoa, municipalities in the Northern Mariana Islands, and islands in the Virgin Islands of the United States. Because they contain no primary legal divisions, the Census Bureau treats the District of Columbia and Guam each as equivalent to a county (as well as equivalent to a state) for data presentation purposes. In American Samoa, a county is a minor civil division.

• Incorporated Place (cities, towns, villages, etc.). A type of governmental unit, incorporated under state law as a city, town (except in New England, New York, and Wisconsin), borough (except in Alaska and New York), or village, generally to provide a wide array of specific governmental services for a concentration of people within legally prescribed boundaries. New for Census 2000 are “city and borough” and “municipality,” which serve as both place- and county-level entities in Alaska.

• Minor Civil Division (MCD). The primary governmental or administrative division of a county or statistically equivalent entity in many states and statistically equivalent entities. MCDs are identified by a variety of terms, such as township, town (in 8 states), or district. The Census Bureau recognizes MCDs in 28 states, the District of Columbia, Puerto Rico, and the Island Areas. In 20 states and American Samoa, all or many MCDs are active general-purpose governmental units. Many MCDs are not general-purpose governmental units, and therefore do not have elected officials to carry out legal functions; instead, they serve as nonfunctioning administrative entities.

• State. A primary governmental division of the United States. The Census Bureau treats the District of Columbia as the equivalent of a state for data presentation purposes. It also treats a number of entities that are not legal 5 divisions of the United States (e.g. Island Areas) as the equivalent of states for data presentation purposes.

• United States. The 50 states and the District of Columbia.

• Voting District (VTD). The generic name for a geographic entity - such as an election district, precinct, or ward - established by state, local, and tribal governments for the purpose of conducting elections. Some reviewing officials adjusted the boundaries of the voting districts (VTDs) they submitted to conform to census block boundaries for data presentation purposes, and therefore a VTD for which Census 2000 provides data might not exactly represent the legal entity; the Census Bureau refers to such VTDs as pseudovoting districts (pseudo-VTDs). Such VTDs, as well as any others for which state officials did not specify a status, are identified by a “P” in the VTD indicator field of the Public Law (PL) data file.

Statistical entities usually evolve from practice, custom, usage, or need, and generally the Census Bureau develops criteria and guidelines for their dentification and delineation. They include:

• Block Group (BG). A statistical subdivision of a census tract. A BG consists of all tabulation blocks whose numbers begin with the same digit in a census tract. BGs generally contain between 300 and 3,000 people, with an optimum size of 1,500 people. The BG is the lowestlevel geographic entity for which the Census Bureau tabulates sample data from a decennial census.

• Census Block. An area bounded on all sides by visible and/or nonvisible features shown on a map prepared by the Census Bureau. A block is the smallest geographic entity for which the Census Bureau tabulates decennial census data. The census blocks were completely renumbered for Census 2000 using 4- digit numbers.

• Census County Divisions (CCD). A statistical subdivision of a county, established and delineated cooperatively by the Census Bureau and state, local, and tribal officials for data presentation purposes. CCDs have been established in 21 states.

• Census Designated Place (CDP). A geographic entity that serves as the statistical counterpart of an incorporated place for the purpose of presenting census data for an area with a concentration of population, housing, and commercial structures that is identifiable by name, but is not within an incorporated place. CDPs usually are defined cooperatively with state, local, and tribal officials based on Census Bureau guidelines. For Census 2000, for the first time, CDPs did not have to meet minimum population threshold to qualify for the tabulation of census data. Note: A CDP in Puerto Rico is called a comunidad or zona urbana.

• Census Regions and Divisions. The 50 states and the District of Columbia have been grouped into four regions, each containing two or three divisions.

• Census Tract. A small, relatively permanent statistical subdivision of a county or statistically equivalent entity, delineated for data presentation purposes by a local group of census data users or the geographic staff of a regional census center in accordance with Census Bureau guidelines. Census tracts generally contain between 1,000 and 8,000 people. Census tract boundaries are delineated with the intention of being stable over many decades, so they generally follow relatively permanent visible features. However, they may follow governmental unit boundaries and other invisible features in some instances; the boundary of a state or county is always a census tract boundary. Block Numbering Areas (BNAs) are now called census tracts.

• Metropolitan Area (MA). A large population nucleus, together with adjacent communities that have a high degree of economic and social integration with that nucleus. (Some MAs are defined around two or more nuclei.) MA is a collective term, established by the federal Office of Management and Budget in 1990, to refer to metropolitan statistical areas (MSAs), consolidated metropolitan statistical areas (CMSAs), and primary metropolitan statistical areas (PMSAs). Metropolitan Statistical Area (MSA). An MSA is a metropolitan area (MA) that is not closely associated with another MA. An MSA consists of one or more 6 counties, except in New England, where MSAs are defined in terms of county subdivisions (primarily cities and towns). Primary Metropolitan Statistical Area (PMSA). If an area that qualifies as a metropolitan area (MA) has a population of 1,000,000 or more, two or more primary metropolitan statistical areas (PMSAs) may be defined within it if they meet official standards and local governments favor that designation. When PMSAs are established within an MA, that MA is designated a consolidated metropolitan statistical area. Consolidated Metropolitan Statistical Area (CMSA). An area becomes a CMSA if it qualifies as a metropolitan area, has a population of 1,000,000 or more, has component parts that qualify as primary metropolitan statistical areas (PMSAs) based on official standards, and local governments favor the designation. CMSAs consist of whole counties except in New England, where they consist of county subdivisions (primarily cities and towns).

• Public Use Microdata Area (PUMA). A geographic entity for which the U.S. Census Bureau provides specially selected extracts of raw data from a small sample of long-form census records that are screened to protect confidentiality of census records. The extract files are referred to as public use microdata samples (PUMS). Public use microdata areas (PUMAs), which must have a minimum census population of 100,000 and cannot cross a state line, receive a 5-percent sample of the longform records; these records are presented in state files. These PUMAs are aggregated into super-PUMAs, which must have a minimum census population of 400,000 and receive a 1-percent sample in a national file. PUMAs and super-PUMAs are mutually exclusive, that is, they use different records to create each sample. Data users can use these files to create their own statistical tabulations and data summaries.
• Rural. All territory, population, and housing units located outside of urbanized areas and urban clusters.

• Urban. All territory, population, and housing units located within urbanized areas and urban clusters.

• Urbanized Area (UA). Densely settled area that has a census population of at least 50,000. The geographic core of block groups or blocks must have a population density of at least 1,000 people per square mile, and adjacent block groups and blocks with at least 500 people per square mile.

• Urban Cluster (UC). Consists of a geographic core of block groups or blocks must have a population density of at least 1,000 people per square mile, and adjacent block groups and blocks with at least 500 people per square mile that together encompass a population of at least 2,500 people, but fewer than 50,000 people.

• ZIP Code Tabulation Area (ZCTA). A statistical entity developed by the Census Bureau to approximate the delivery area for a U.S. Postal Service five-digit or three-digit ZIP Code in the U.S. and Puerto Rico. A ZCTA is an aggregation of census blocks that have the same predominant ZIP Code associated with the mailing addresses in the Census Bureau’s Master Address File. Thus, the Postal Service’s delivery areas have been adjusted to encompass whole census blocks so that the Census Bureau can tabulate census data for the ZCTAs. ZCTAs do not include all ZIP Codes used for mail delivery.

There are both legal and statistical American Indian, Alaska Native, and native Hawaiian entities for which the Census Bureau provides data for Census 2000. The legal entities consist of federally recognized American Indian reservations and off-reservation trust land areas, the tribal subdivisions that can divide these entities, state recognized American Indian reservations, Alaska Native Regional Corporations, and Hawaiian home lands. The statistical entities are Alaska Native village statistical areas, Oklahoma tribal statistical areas, tribal designated statistical areas, and state designated American Indian statistical areas. Tribal subdivisions can exist within the statistical Oklahoma tribal statistical areas.

For more information on these areas, as well as other census geography, see the Geographic Changes for Census 2000 + Glossary, which provides good background for Census 2000 users including changes from 1990 geography and new terms. The Geographic Areas Reference Manual (1994) has excellent background and user aids.

The geographic areas may work in a hierarchical fashion, with smaller areas nested within larger ones (e.g., blocks in block groups, in census tracts in counties, etc.). Some areas, like ZIP Code Tabulation Areas, are given only as subsets of the nation (see Figure).

One easy way to understand the relationships of the geographic areas is to use the American FactFinder (AFF). An address search shows a series “Geographies Containing” that address. Users can click on block, block group, census tract, etc., and press the “map it” button to explore the different levels of geography.

When you look up the “National Zoo,” AFF displays the census geographies for that area. After selecting the tract number and clicking on the “map it” button, the map is displayed.

The labels show you the city, county, and state (or in this case, the equivalents), tract numbers, and more.