Block Assignment File

The file format for many of the geographic products that the Census Bureau provides to states and other data users containing the small area census data necessary for legislative redistricting. Contains census tabulation block codes and geographic area codes for a specific geographic entity type.


The extent or limit of a geographic area such as a block, census tract, county, or place. A boundary may or may not follow a visible geographic physical feature and may be legal or statistical.

Census Block

A statistical area 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 districts, and county boundaries. A block is the smallest geographic unit for which the Census Bureau tabulates decennial census data. Many blocks correspond to individual city blocks bounded by streets, but blocks – especially in rural areas – may include many square miles and may have some boundaries that are not streets. The Census Bureau established blocks covering the entire nation for the first time in 1990. Previous censuses back to 1940 had blocks established only for part of the nation. Over 8 million blocks were identified for the 2000 Census and over 11 million blocks were identified for the 2010 Census.

Census Block Group

A statistical division of census tracts and the smallest geographic unit for which the Census Bureau tabulates sample data; are generally defined to contain between 600 and 3,000 people and are used to present data and control block numbering. A block group consists of clusters of blocks within the same census tract that have the same first digit of their four-digit census block number. For example, blocks 3001, 3002, 3003, . . ., 3999 in census tract 1210.02 belong to BG 3 in that census tract. Most BGs were delineated by local participants in the Census Bureau's Participant Statistical Areas Program. The Census Bureau delineated BGs only where a local or tribal government declined to participate, and a regional organization or State Data Center was not available to participate. A BG usually covers a contiguous area. Each census tract contains at least one BG, and BGs are uniquely numbered within the census tract. Within the standard census geographic hierarchy, BGs never cross state, county, or census tract boundaries but may cross the boundaries of any other geographic entity. Tribal census tracts and tribal BGs are separate and unique geographic areas defined within federally recognized American Indian reservations and can cross state and county boundaries. The tribal census tracts and tribal block groups may be completely different from the census tracts and block groups defined by state and county.

Census geography

The geographic units for which census information is tabulated and reported with several hierarchies; the most basic Census geographies in order from smallest to largest are census block, census block groups, census tracts, counties, and states.

Census tract

A small, relatively permanent statistical subdivision of a county delineated by a local committee of census data users for the purpose of presenting data. Census tracts nest within counties, and their boundaries normally follow visible features, but may follow legal geography boundaries and other non-visible features in some instances. Census tracts ideally contain about 4,000 people and 1,600 housing units.

County and Equivalent Entity

The primary legal subdivision of most states. In Louisiana, these subdivisions are known as parishes. The Census Bureau treats the following entities as equivalents of counties for purposes of data presentation:

  • Boroughs, city and boroughs, municipalities, and census areas in Alaska
  • Municipios in Puerto Rico
  • Districts and islands in American Samoa
  • Municipalities in the Commonwealth of the Northern Mariana Islands
  • Islands in the U.S. Virgin Islands

The District of Columbia and Guam have no primary divisions, and the entire area is considered equivalent to a county for statistical purposes. In four states (Maryland, Missouri, Nevada, and Virginia), there are one or more cities that are independent of any county and thus constitute primary subdivisions of their states.


The measure of how much a given district or plan varies from the ideal population, however defined, per district. Deviation can be expressed as an absolute number or as a percentage. Serves as the standard by which courts review districts for compliance with the “one person, one vote” constitutional principle.

Disaggregated Data

Refers to numerical or non-numerical information that has been (1) collected from multiple sources and/or on multiple measures, variables, or individuals; (2) compiled into aggregate data—i.e., summaries of data—typically for the purposes of public reporting or statistical analysis; and (3) broken down into component parts or smaller units of data.

Equal Population

A constitutional requirement derived from the “one person, one vote” principle that is imposed upon all congressional districts and legislative districts. Congressional districts within the same state must have more or less the same number of people in each with next to no population deviations. While legislative districts within the same state must also have equal population, courts have construed this requirement as permitting larger population deviations amongst them. A state must have a rational state policy to justify their creation of a district with more or less people than the ideal population.

Geographic Information System (GIS)

A computer system that captures, stores, checks, and displays data related to positions on Earth’s surface. By relating seemingly unrelated data, GIS helps individuals and organizations better understand spatial patterns and relationships. GIS technology is a crucial part of spatial data infrastructure, which the White House defines as “the technology, policies, standards, human resources, and related activities necessary to acquire, process, distribute, use, maintain, and preserve spatial data.” GIS can use any information that includes location. The location can be expressed in many different ways, such as latitude and longitude, address, or ZIP code. Many different types of information can be compared and contrasted using GIS. The system can include data about people, such as population, income, or education level. It can include information about the landscape, such as the location of streams, different kinds of vegetation, and different kinds of soil. It can include information about the sites of factories, farms, and schools, or storm drains, roads, and electric power lines.

Natural Boundaries

Boundaries that include or adhere to natural geographic features such as bodies of water, mountains, etc.

Overall Range

The difference in population between the largest and smallest districts in a districting plan in either absolute (persons) or relative (percentage) terms.

PL 94-171

Public Law (P.L.) 94-171, enacted by Congress in December 1975, requiring the Census Bureau to provide states with the data necessary to conduct legislative redistricting and an opportunity to identify the small area geography for which they will need such data. The law also requires the U.S. Census Bureau to deliver this data no later than one year from Census day.

Population Estimates

The calculated number of people living in an area as of a specified point in time, usually July 1st. The estimated population is calculated using a component of change model that incorporates information on natural increase (births, deaths) and net migration (net domestic migration, net international migration) that has occurred in an area since the latest decennial census.

Population Projections

Estimates of the population for future dates. They illustrate plausible courses of future population change based on assumptions about future births, deaths, international migration, and domestic migration. Projections are based on an estimated population consistent with the most recent decennial census as enumerated. While projections and estimates may appear similar, there are some distinct differences between the two measures. Estimates usually are for the past, while projections typically are for future dates. Estimates generally use existing data, while projections must assume what demographic trends will be in the future. For dates when both population estimates and projections are available, population estimates are the preferred data.

Redistricting Data Program

A decennial census program that provides state officials the opportunity to identify the small area geographies they will need to successfully meet their state or local redistricting requirements. Traditionally this has been done through the Block Boundary Suggestion Project, where they select the map features they want as census tabulation block boundaries, and through the Voting District Project, where they define specific areas representing the geographies used to conduct elections such as election precincts, polls, wards, or election districts. For the 1990 Census, the Census Bureau developed the generic term voting district to identify all such areas in the TIGER database. The Redistricting Program also serves as the liaison to the state legislatures and elected officials responsible for statewide redistricting. This program is also responsible for developing the data dissemination plan of the initial release of the decennial census data to official recipients as required by P.L. 94-171.


A shapefile stores nontopological geometry and attribute information for the spatial features in a data set. The geometry for a feature is stored as a shape comprising a set of vector coordinates.

Standard Deviation

A measure which shows the average variability in population from the mean.

Definitions were compiled from the Census Bureau Glossary, NCSL, National Geographic, ESRI, and the Education Glossary. All terms are linked to their respective source.