American Community Survey (ACS) replaces the long form (sample questionnaire) of the decennial census from 2010. Collects statistics on detailed demographic and socio-economic characteristics. Statistics are available for some geographic areas from the ACS beginning in 2001.
Blocks (Census Blocks) are statistical areas bounded by visible features, such as streets, roads, streams, and railroad tracks, and by nonvisible boundaries, such as selected property lines and city, township, school district, and county limits and short line-of-sight extensions of streets and roads. Generally, census blocks are small in area; for example, a block in a city bounded on all sides by streets. Census blocks in suburban and rural areas may be large, irregular, and bounded by a variety of features, such as roads, streams, and transmission lines. In remote areas, census blocks may encompass hundreds of square miles. Census blocks cover the entire territory of the United States, Puerto Rico, and the Island Areas. Census blocks nest within all other tabulated census geographic entities and are the basis for all tabulated data.
Census Tracts are small, relatively permanent statistical subdivisions of a county or equivalent entity that are updated prior to each decennial census. The Census Bureau delineates census tracts in situations where no local participant existed or where state, 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 statistical data. Census tracts generally have a population size between 1,200 and 8,000 people, with an optimum size of 4,000 people. A census tract usually covers a contiguous area; however, the spatial size of census tracts varies widely depending on the density of settlement. Census tract boundaries are delineated with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. Census tracts occasionally are split due to population growth or merged as a result of substantial population decline.
Combined Statistical Areas (CSAs) consist of two or more adjacent CBSAs that have substantial employment interchange. The CBSAs that combine to create a CSA retain separate identities within the larger CSA. Because CSAs represent groupings of metropolitan statistical areas, they should not be ranked or compared with individual metropolitan and micropolitan statistical areas.
Consolidated Metropolitan Statistical Areas (CMSA) is created if the area meets the requirements to qualify as a metropolitan statistical area, has a population of 1,000,000 or more, if component parts are recognized as primary metropolitan statistical areas, and local opinion favors the designation. The CMSA concept was retired in 2003 with the introduction of the Core Based Statistical Area concepts.
Core Based Statistical Areas (CBSAs) consist of the county or counties or equivalent entities associated with at least one core (urbanized area or urban cluster) of at least 10,000 population, plus adjacent counties having a high degree of social and economic integration with the core as measured through commuting ties with the counties associated with the core. The general concept of a CBSA is that of a core area containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core.
Decennial Census (also known as the Census of Population and Housing) is conducted every 10 years beginning in 1790. From the 20th century it provides demographic and socio-economic statistics for population areas down to the block and neighborhood level.
Metropolitan Division is a smaller group of counties or equivalent entities defined within a metropolitan statistical area containing a single core with a population of at least 2.5 million. Not all metropolitan statistical areas of this size will contain metropolitan divisions. The concept was introduced in 2003.
Metropolitan Statistical Areas (MSA) are CBSAs that are associated with at least one urbanized area that has a population of at least 50,000. The metropolitan statistical area comprises the central county or counties or equivalent entities containing the core, plus adjacent outlying counties having a high degree of social and economic integration with the central county or counties as measured through commuting
Primary Metropolitan Statistical Areas (PMSA) are areas that meet the requirements to qualify as a metropolitan statistical area and has a population of one million or more, two or more PMSAs may be defined within it if statistical criteria are met and local opinion is in favor. A PMSA consists of one or more counties that have substantial commuting interchange. When two or more PMSAs have been recognized, the larger area of which they are components then is designated a consolidated metropolitan statistical area. This term was retired in 2003 and is replaced by Metropolitan Division.
Source: Geographic Terms and Concepts. U.S. Census Bureau. http://www.census.gov/geo/reference/pdfs/GTC_10.pdf