The Census 2010 release from the Census Bureau includes updated demographic data as well as updated geographic boundaries. An example is shown in the following screen shot:
Change is inevitable with any geographic area. Statistical areas, such as block groups and census tracts, are defined by the Census Bureau to collect and report data. These areas change every ten years, with each new census.
Political areas, such as cities or townships, change whenever local governments elect to revise their boundaries. Larger political areas, such as counties, change less often than other places, but boundary revisions are not uncommon. ZIP Codes, which are defined solely to expedite mail delivery, can change monthly or whenever the U.S. Postal Service revises delivery routes. Metropolitan areas are usually revised annually, although the Office of Management and Budget overhauls the definitions decennially, with data from the census.
Even if boundaries do not change, the Census Bureau can renumber geographic codes for areas. It is essential that users be cognizant of geographic differences between 2000 and 2010 when analyzing trends.
U.S. standard geographies
A census block is a component of a block group, identified by a four-digit code. Blocks are small in area, in general, especially in cities. However, blocks in rural or remote areas may cover hundreds of square miles. A block code that starts with a zero indicates a water-only block. There are 11,078,297 blocks in Census 2010 geography, as compared to 8.2 million in Census 2000.
A block group (BG) is a collection of one or more blocks and a statistical division of a census tract, identified by a one-digit code. Block groups do not cross census tract, county, or state boundaries. In general, a BG is comprised of 600 to 3,000 residents. A zero BG code indicates a water-only BG. Boundary changes and code restructuring are reflected in the Census 2010 release. There are 217,740 BGs in the Census 2010 geography.
Census tracts are small statistical subdivisions of a county, with 1,200 to 8,000 residents typically. The boundaries are usually delineated by local committees, and do not cross county or state lines. Tracts are identified by a six-digit code, with an implied decimal between the fourth and fifth digit. Boundary changes and code restructuring are reflected in the Census 2010 release. There are 73,057 tracts in the Census 2010 geography.
Counties are the primary legal divisions of a state, identified by a two-digit state FIPS code and a three-digit county FIPS code. Boundary changes, as well as code and name changes, occurred in Alaska. Specifically, three codes were dropped and five new codes and names were added. There was also a slight change to a county name in Illinois and New Mexico. There are 3,143 counties in the Census 2010 geography.
County subdivisions (CSDs) are the primary divisions of counties and include census county divisions (CCDs), minor civil divisions (MCDs), census subareas, and unorganized territories. CCDs exist in 20 states, MCDs are in 29 states, census subareas exist only in Alaska, and unorganized territories are in 9 states. CSDs can be uniquely identified using a two-digit state FIPS code, three-digit county FIPS code, and five-digit CSD FIPS code. Boundary changes, as well as code and name changes, are reflected in the Census 2010 release. There are 35,703 county subdivisions in the Census 2010 geography.
Places include incorporated places (usually cities, towns, villages, or boroughs), census designated places, and balance portions of consolidated cities. Places are uniquely identified using a two-digit state FIPS code and five-digit place FIPS code. Boundary changes, as well as code and name changes, are reflected in the Census 2010 release. There are 29,261 places in the Census 2010 geography, compared to 25,150 in Census 2000.
Core Based Statistical Areas
Core Based Statistical Areas (CBSAs), which include metropolitan and micropolitan statistical areas, are comprised of one or more counties and are defined by the U.S. Office of Management and Budget (OMB). A metropolitan statistical area is affiliated with at least one urbanized area of 50,000 or more inhabitants. A micropolitan statistical area is associated with at least one urban cluster of at least 10,000 people, but less than 50,000.
Designated Market Areas
Designated Market Areas (DMAs) are television markets defined by The Nielsen Company, revised on an annual basis. The majority of DMAs are comprised of one or more whole counties, although a few include parts of counties.
Congressional districts (CDs) are the areas from which individuals are elected to the U.S. House of Representatives. Once the apportionment of congressional seats is made based on census population counts within a state, each state will establish CDs to elect representatives. A congressional district is uniquely identified using a two-digit state FIPS code and two-digit CD FIPS code.
Residential ZIP Codes
Created by the U.S. Postal Service to deliver the mail, ZIP Codes do not represent standard census geographic areas for data reporting. Because ZIP Code boundaries are not contiguous with census geographic areas or stable over time, data estimated for ZIP Codes is also subject to change. Residential ZIP Code data is estimated from block group data, using a correspondence file created by assigning Census 2010 block points to ZIP Code boundaries from HERE.