The U.S. Census Bureau classifies as urban all territory, population, and housing units located within urbanized areas (UAs) and urban clusters (UCs). It delineates UA and UC boundaries to encompass densely settled territory, which generally consists of:
- A cluster of one or more block groups or census blocks each of which has a population density of at least 1,000 people per square mile at the time, and
- Surrounding block groups and census blocks each of which has a population density of at least 500 people per square mile at the time, and
- Less densely settled blocks that form enclaves or indentations, or are used to connect discontiguous areas with qualifying densities.
Rural consists of all territory, population, and housing units located outside of UAs and UCs. Geographic entities, such as metropolitan areas, counties, minor civil divisions (MCDs), and places, often contain both urban and rural territory, population, and housing units.
An urban cluster (UC) consists of densely settled territory that has at least 2,500 people but fewer than 50,000 people. (A UC can have 50,000 or more people if fewer than 35,000 people live in an area that is not part of a military reservation.)
The U.S. Census Bureau introduced the UC for Census 2000 to provide a more consistent and accurate measure of the population concentration in and around places. UCs replace the provision in the 1990 and previous censuses that defined as urban only those places with 2,500 or more people located outside of urbanized areas.
Urban Cluster Central Place
A central place functions as the dominant center of a UC. The U.S. Census Bureau identifies one or more central places for a UC, with a preference for the most populous incorporated place(s). (Some UCs do not have a central place.)
Urban Cluster Title and Code
The title of a UC may contain up to three incorporated place names, and will include the two-letter U.S. Postal Service abbreviation for each state into which the UC extends. However, if the UC does not contain an incorporated place, the UC title will include the single name of the geographic entity that occurs first from the following list: census designated place (CDP), minor civil division (MCD), or populated place recognized by the U.S. Geological Survey.
Each UC is assigned a five-digit census code in alphabetical sequence on a nationwide basis. The alphabetic assignment of codes for urban areas includes both urbanized areas (UAs) and UCs. A separate flag is included in data tabulation files to differentiate between UAs and UCs. In printed reports, this differentiation is included in the name.
An urbanized area (UA) consists of densely settled territory that contains 50,000 or more people. The U.S. Census Bureau delineates UAs to provide a better separation of urban and rural territory, population, and housing in the vicinity of large places. At least 35,000 people in a UA must live in an area that is not part of a military reservation.
For Census 2000, the UA criteria specify that the delineations be performed using a zero-based approach. Because of the more stringent density requirements and the less restrictive extended place criteria, some territory that was classified as urbanized for the 1990 census has been reclassified as rural. (Area that was part of a 1990 UA has not been automatically grandfathered into the 2000 UA.) In addition, some areas that were identified as UAs for the 1990 census have been reclassified as urban clusters.
Urbanized Area Central Place
The UA central place is an incorporated place or a census designated place (CDP) with the most population within the UA. Additional place(s) may become UA central places provided that:
- The places population within the UA exceeds 50,000 people; or
- The places population size is at least 2/3 of the most populous UA central place.
Urbanized Area Title and Code
The title of a UA may contain up to three incorporated place names, and will include the two-letter U.S. Postal Service abbreviation for each state into which the UA extends. However, if the UA does not contain an incorporated place, the UA title will include the single name of the geographic entity that occurs first from the following list: census designated place (CDP), minor civil division (MCD), or populated place recognized by the U.S. Geological Survey.
Each UA is assigned a five-digit census code in alphabetical sequence on a nationwide basis, interspersed with the codes for urban clusters (UCs), also in alphabetical sequence. For the 1990 census, the U.S. Census Bureau assigned a four-digit UA code based on the metropolitan area codes. For Census 2000, a separate flag is included in data tabulation files to differentiate between UAs and UCs. In printed reports, this differentiation is included in the name.
As a result of the urbanized area (UA) and urban cluster (UC) delineations, an incorporated place or census designated place may be partially within and partially outside of a UA or UC. Any place that is split by a UA or UC is referred to as an extended place.
For the 1990 and previous censuses, extended places were called extended cities. An extended city had to contain either 25 percent of the total land area or at least 25 square miles with an overall population density lower than 100 people per square mile. Such pieces of territory had to cover at least 5 square miles. This low-density area was classified as rural and the other, more densely settled portion of the incorporated place was classified as urban. For the 1970 and 1980 censuses, the U.S. Census Bureau identified extended cities only within UAs. For the 1990 census, the U.S. Census Bureau also applied this classification to qualifying incorporated places located outside UAs.
Documentation of the UA, UC, and extended place criteria is available from the Geographic Areas Branch, Geography Division, U.S. Census Bureau, Washington, DC 20233-7400; telephone 301-763-1099.
