In decennial census data products, geographic entities usually are presented in an hierarchical arrangement or as an inventory listing.

There are both legal and statistical American Indian, Alaska Native, and native Hawaiian entities for which the U.S. 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.
In all cases, these areas are mutually exclusive in that no American Indian, Alaska Native, or Hawaiian home land can overlap another tribal entity, except for tribal subdivisions, which subdivide some American Indian entities, and Alaska Native village statistical areas, which exist within Alaska Native Regional Corporations. In some cases where more than one tribe claims jurisdiction over an area, the U.S. Census Bureau creates a joint use area as a separate entity to define this area of dual claims. The following provides more detail about each of the various American Indian areas, Alaska Native areas, and Hawaiian home lands.

Area measurement data provide the size, in square units (metric and nonmetric) of geographic entities for which the U.S. Census Bureau tabulates and disseminates data. Area is calculated from the specific boundary recorded for each entity in the U.S. Census Bureau's geographic database (see TIGER® database). These area measurements are recorded as whole square meters. (To convert square meters to square kilometers, divide by 1,000,000; to convert square kilometers to square miles, divide by 2.58999; to convert square meters to square miles, divide by 2,589,988.) The U.S. Census Bureau provides area measurement data for both land area and total water area. The water area figures include inland, coastal, Great Lakes, and territorial water. (For the 1990 census, the U.S. Census Bureau provided area measurements for land and total water; water area for each of the four water classifications was available in the Geographic Identification Code Scheme (GICS) product only.) " Inland water" consists of any lake, reservoir, pond, or similar body of water that is recorded in the U.S. Census Bureau's geographic database. It also includes any river, creek, canal, stream, or similar feature that is recorded in that database as a twodimensional feature (rather than as a single line). The portions of the oceans and related large embayments (such as the Chesapeake Bay and Puget Sound), the Gulf of Mexico, and the Caribbean Sea that belong to the United States and its territories are classified as " coastal" and " territorial" waters; the Great Lakes are treated as a separate water entity. Rivers and bays that empty into these bodies of water are treated as " inland water" from the point beyond which they are narrower than one nautical mile across. Identification of land and inland, coastal, territorial, and Great Lakes waters is for data presentation purposes only and does not necessarily reflect their legal definitions.

Land and water area measurements may disagree with the information displayed on U.S. Census Bureau maps and in the TIGER® database because, for area measurement purposes, features identified as " intermittent water" and " glacier" are reported as land area. For this reason, it may not be possible to derive the land area for an entity by summing the land area of its component census blocks. In addition, the water area measurement reported for some geographic entities includes water that is not included in any lower-level geographic entity. Therefore, because water is contained only in a higher-level geographic entity, summing the water measurements for all the component lower-level geographic entities will not yield the water area of that higher-level entity. This occurs, for example, where water is associated with a county but is not within the legal boundary of any minor civil division. Crews-of-vessels entities (see CENSUS TRACT and CENSUS BLOCK) do not encompass territory and, therefore, have no area measurements.

The accuracy of any area measurement data is limited by the accuracy inherent in (1) the location and shape of the various boundary information in the TIGER® database, (2) the location and shapes of the shorelines of water bodies in that database, and (3) rounding affecting the last digit in all operations that compute and/or sum the area measurements.

A block group (BG) is a cluster of census blocks having the same first digit of their four-digit identifying numbers within a census tract. For example, block group 3 (BG 3) within a census tract includes all blocks numbered from 3000 to 3999. BG's generally contain between 600 and 3,000 people, with an optimum size of 1,500 people. BG's on American Indian reservations, offreservation trust lands, and special places must contain a minimum of 300 people. (Special places include correctional institutions, military installations, college campuses, workers dormitories, hospitals, nursing homes, and group homes.)

Most BG's were delineated by local participants as part of the U.S. Census Bureau's Participant Statistical Areas Program. The U.S. Census Bureau delineated BG's only where a local, state, or tribal government declined to participate or where the U.S. Census Bureau could not identify a potential local or tribal participant.

BG's never cross the boundaries of states, counties, or statistically equivalent entities, except for a BG delineated by American Indian tribal authorities, and then only when tabulated within the American Indian hierarchy (see TRIBAL BLOCK GROUP). BG's never cross the boundaries of census tracts, but may cross the boundary of any other geographic entity required as a census block boundary (see CENSUS BLOCK).

In decennial census data tabulations, a BG may be split to present data for every unique combination of American Indian area, Alaska Native area, Hawaiian home land, congressional district, county subdivision, place, voting district, or other tabulation entity shown in the data products. For example, if BG 3 is partly in a city and partly outside the city, there are separate tabulated records for each portion of BG 3. BG's are used in tabulating data nationwide, as was done for the 1990 census, for all block-numbered areas in the 1980 census, and for selected areas in the 1970 census. For data presentation purposes, BG's are a substitute for the enumeration districts (EDs) used for reporting data in many parts of the United States for the 1970 and 1980 censuses and in all areas before 1970. Also, BG's are the lowest level of the geographic hierarchy for which the U.S. Census Bureau tabulates and presents sample data.

Many of the legal and statistical entities for which the U.S. Census Bureau tabulates decennial census data have had boundary changes between the 1990 census and Census 2000; that is, between January 2, 1990, and January 1, 2000. Boundary changes to legal entities result from:

1. Annexations to or detachments from legally established governmental units.
2. Mergers or consolidations of two or more governmental units.
3. Establishment of new governmental units.
4. Disincorporations or disorganizations of existing governmental units.
5. Changes in treaties or executive orders, and governmental action placing additional lands in trust.
6. Decisions by federal, state, and local courts.
7. Redistricting for congressional districts or county subdivisions that represent single-member districts for election to a county governing board.


Statistical entity boundaries generally are reviewed by local, state, or tribal governments and can have changes to adjust boundaries to visible features to better define the geographic area each encompasses or to account for shifts and changes in the population distribution within an area.

The historical counts shown for counties, county subdivisions, places, and American Indian, Alaska Native, and Native Hawaiian areas are not updated for such changes, and thus reflect the population and housing units in each entity as delineated at the time of each decennial census. Boundary changes are not reported for some entities, such as census designated places and block groups.

Changes to the boundaries for census tracts and, for the first time, for census blocks are available in relationship files, which are only available in computer-readable form. The census tract relationship files feature the relationship of census tracts/block numbering areas at the time of the 1990 census to census tracts for Census 2000, and vice versa, including partial relationships. For the first time, the census tract relationship files show a measure of the magnitude of change using the proportion of the length of roads and sides of roads contained in partial census tracts. This information can be used to proportion the data for the areas where census tracts have changed.

The census block relationship files, which are available only in computer-readable form, present relationships of the 1990 census and Census 2000 blocks on the basis of whole blocks or part blocks (" P" ). The following relationships can be derived:



	1990 census block	2000 census block
One to one	601	1017
One to many	101P	3028
	101P	2834
Many to one	410	2554P
	503	2554P
Many to many	404	1007P
	501P	1007P
	502P	1008P


Block relationship files are available to compare the following sets of census blocks:

• 1990 tabulation block to 2000 collection block,
• 2000 collection block to 2000 tabulation block, and
• 1990 tabulation block to 2000 tabulation block.

Census tract relationship files and block relationship files are not geographic equivalency files. For a true areal comparison between the census tracts/block numbering areas and blocks used for the 1990 census and the census tracts and blocks used for Census 2000 (as well as other geographic areas), it is necessary to use the 2000 TIGER/Line® files. The 2000 TIGER/Line files will contain 1990 and 2000 boundaries for counties and statistically equivalent entities, county subdivisions, places, American Indian areas, Alaska Native village statistical areas, census tracts, census blocks, and by derivation from the census blocks, block groups.

Census blocks are areas bounded on all sides by visible features, such as streets, roads, streams, and railroad tracks, and by invisible boundaries, such as city, town, township, and county limits, property lines, and short, imaginary extensions of streets and roads. Generally, census blocks are small in area; for example, a block bounded by city streets. However, census blocks in sparsely settled areas may contain many square miles of territory.

All territory in the United States, Puerto Rico, and the Island Areas has been assigned block numbers, as was the case for the 1990 census. To improve operational efficiency and geographic identifications, the U.S. Census Bureau has introduced different numbering systems for tabulation blocks used in decennial census data products, and for collection blocks, used in administering the census. (In 1990, there generally was a single numbering system.) Collection block numbers are available only in the TIGER/Line® data products; the U.S. Census Bureau does not tabulate data for collection blocks.

Many tabulation blocks, used in decennial census data products, represent the same geographic area as the collection blocks used in the Census 2000 enumeration process. Where the collection blocks include territory in two or more geographic entities, each unique piece required for data tabulation is identified as a separate tabulation block with a separate block number. It is possible for two or more collection blocks to be combined into a single tabulation block. This situation can occur when a visible feature established as a collection block boundary is deleted during the field update operation. Tabulation blocks do not cross the boundaries of any entity for which the U.S. Census Bureau tabulates data, including American Indian areas, Alaska Native areas, Hawaiian home lands, census tracts, congressional districts, counties, county subdivisions, places, state legislative districts, urban and rural areas, school districts, voting districts, and ZIP Code® tabulation areas. Tabulation blocks also generally do not cross the boundaries of certain landmarks, including military installations, national parks, and national monuments.

Tabulation blocks are identified uniquely within census tract by means of a four-digit number. (The 1990 census block numbers had three digits, with a potential alphabetic suffix.) The Census 2000 collection blocks are numbered uniquely within county (or statistically equivalent entity), and consist of four or five digits. For its Census 2000 data tabulations, the U.S. Census Bureau created a unique set of census block numbers immediately before beginning the tabulation process. These are the census block numbers seen in the data presentations. For the 1990 census, the U.S. Census Bureau created a separate block with a suffix of " Z" to identify crews-of-vessels population. For Census 2000, crews-of-vessels population is assigned to the land block identified by the U.S. Census Bureau as associated with the home port of the vessel.

Participants in certain U.S. Census Bureau-sponsored programs were able to request that line features in the TIGER® database be held as tabulation block boundaries, provided that these conformed to U.S. Census Bureau criteria. This option was available to participants in the Census 2000 Redistricting Data Program (the Block Boundary Suggestion Project), American Indian and Alaska Native Area Tribal Review (Block Definition Project), and the District of Columbia and the Puerto Rico Block Boundary Definition Project.
The U.S. Census Bureau introduced a different method for identifying the water areas of census blocks. For the 1990 census, water was not uniquely identified within a census block; instead, all water area internal to a block group was given a single block number ending in " 99" (for example, in block group 1, all water was identified as block 199). A suffix was added to each water block number where the block existed in more than one tabulation entity within its block group. For Census 2000, water area located completely within the boundary of a single land tabulation block has the same block number as that land block. Water area that touches more than one land block is assigned a unique block number not associated with any adjacent land block. The water block numbers begin with the block group number followed by " 999" and proceed in descending order (for example, in block group 3, the numbers assigned to water areas that border multiple land blocks are 3999, 3998, etc.). In some block groups, the numbering of land blocks might use enough of the available numbers to reach beyond the 900 range within the block group. For this reason, and because some land blocks include water (ponds and small lakes), no conclusions about whether a block is all land or all water can be made by looking at the block number. The land/water flag, set at the polygon level in the TIGER® database and shown in TIGER/Line® and statistical data tabulation files, is the only way to know if a block is all water when viewing the computer files. On maps, water areas are shown with a screen symbol.

Census divisions are groupings of states and the District of Columbia that are subdivisions of the four census regions. There are nine census divisions, which the U.S. Census Bureau established in 1910 for the presentation of census data. Each census division is identified by a one-digit census code; the same number appears as the first digit in the two-digit census state code (see STATE).

Puerto Rico and the Island Areas are not part of any census region or census division. For a list of all census regions, census divisions, and their constituent states, see Figure A-3.

Census regions are groupings of states and the District of Columbia that subdivide the United States for the presentation of census data. There are four census regions-Northeast, Midwest, South, and West. Each of the four census regions is divided into two or more census divisions. Before 1984, the Midwest region was named the North Central region. From 1910, when census regions were established, through the 1940s, there were three census regions-North, South, and West. Each census region is identified by a single-digit census code.

Puerto Rico and the Island Areas are not part of any census region or census division. For a list of all census regions, census divisions, and their constituent states, see Figure A-3.

Census tracts are small, relatively permanent statistical subdivisions of a county or statistically equivalent entity delineated by local participants as part of the U.S. Census Bureau's Participant Statistical Areas Program. The U.S. Census Bureau delineated census tracts where no local participant existed or where a local or tribal government declined to participate. The primary purpose of census tracts is to provide a stable set of geographic units for the presentation of decennial census data. This is the first decennial census for which the entire United States is covered by census tracts. For the 1990 census, some counties had census tracts and others had block numbering areas (BNAs). For Census 2000, all BNAs were replaced by census tracts, which may or may not represent the same areas.

Census tracts in the United States, Puerto Rico, and the Virgin Islands of the United States generally have between 1,500 and 8,000 people, with an optimum size of 4,000 people. For American Samoa, the Northern Mariana Islands, and Guam, the optimum size is 2,500 people. Counties and statistically equivalent entities with fewer than 1,500 people have a single census tract. Census tracts on American Indian reservations, off-reservation trust lands, and special places must contain a minimum of 1,000 people. (Special places include correctional institutions, military installations, college campuses, workers dormitories, hospitals, nursing homes, and group homes.) When first delineated, census tracts are designed to be relatively homogeneous with respect to population characteristics, economic status, and living conditions. 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 many decades so that statistical comparisons can be made from decennial census to decennial census. However, physical changes in street patterns caused by highway construction, new developments, and so forth, may require occasional boundary revisions. In addition, census tracts occasionally are split due to population growth or combined as a result of substantial population decline.

Census tracts are identified by a four-digit basic number and may have a two-digit numeric suffix; for example, 6059.02. The decimal point separating the four-digit basic tract number from the two-digit suffix is shown in the printed reports and on census maps. In computer-readable files, the decimal point is implied. Many census tracts do not have a suffix; in such cases, the suffix field is either left blank or is zero-filled. Leading zeros in a census tract number (for example, 002502) are shown only in computer-readable files. Census tract suffixes may range from .01 to .98. For the 1990 census, the .99 suffix was reserved for census tracts/block numbering areas (BNAs) that contained only crews-of-vessels population; for Census 2000, the crews-of-vessels population is included with the related census tract.

Census tract numbers range from 1 to 9999 and are unique within a county or statistically equivalent entity. The U.S. Census Bureau reserves the basic census tract numbers 9400 to 9499 for census tracts delineated within or to encompass American Indian reservations and offreservation trust lands that exist in multiple states or counties (see TRIBAL CENSUS TRACTS). The number 0000 in computer-readable files identifies a census tract delineated to provide complete coverage of water area in territorial seas and the Great Lakes.

Congressional districts (CDs) are the 435 areas from which people are elected to the U.S. House of Representatives. After the apportionment of congressional seats among the states, based on census population counts, each state is responsible for establishing CDs for the purpose of electing representatives. Each CD is to be as equal in population to all other CDs in the state as practicable. The CDs in effect at the time of Census 2000 are those of the 106th Congress, whose session began in January 1999. The CDs of the 103rd Congress (January 1993 to 1995) were the first to reflect redistricting based on the 1990 census. These CD boundaries and numbers remained in effect until after Census 2000, except where a state initiative or a court-ordered redistricting had required a change. Six states redistricted for the 104th Congress (Georgia, Louisiana, Maine, Minnesota, South Carolina, and Virginia), five states redistricted for the 105th Congress (Florida, Georgia, Kentucky, Louisiana, and Texas), and three states (New York, North Carolina, and Virginia) redistricted for the 106th Congress. The 108th Congress will be the first to reflect reapportionment and redistricting based on Census 2000 data.

CDs are identified with a two-digit Federal Information Processing Standards (FIPS) code. The code "00" is used for states with a single representative.

American Samoa, Guam, the Virgin Islands of the United States, and the District of Columbia are represented in the House of Representatives by a delegate, and Puerto Rico by a resident commissioner, all of whom may not vote on the floor of the House of Representatives, but may vote on legislation as it is considered by committees to which they have been named. In computerreadable data products that display a congressional district field, the two-digit FIPS code " 98" is used to identify such representational areas. The Northern Mariana Islands does not have representation in Congress. The FIPS code " 99" identifies areas with no representation in Congress.

The primary legal divisions of most states are termed " counties." In Louisiana, these divisions are known as parishes. In Alaska, which has no counties, the statistically equivalent entities are census areas, city and boroughs (as in Juneau City and Borough), a municipality (Anchorage), and organized boroughs. Census areas are delineated cooperatively for data presentation purposes by the state of Alaska and the U.S. Census Bureau. In four states (Maryland, Missouri, Nevada, and Virginia), there are one or more incorporated places that are independent of any county organization and thus constitute primary divisions of their states; these incorporated places are known as " independent cities" and are treated as equivalent to counties for data presentation purposes. (In some data presentations, they may be treated as county subdivisions and places.) The District of Columbia has no primary divisions, and the entire area is considered equivalent to a county for data presentation purposes. In American Samoa, the primary divisions are districts and islands; in the Northern Mariana Islands, municipalities; in the Virgin Islands of the United States, the principal islands of St. Croix, St. John, and St. Thomas. Guam has no primary divisions, and the entire area is considered equivalent to a county for data presentation purposes.

Each county and statistically equivalent entity is assigned a three-digit Federal Information Processing Standards code that is unique within state. These codes are assigned in alphabetical order of county or county equivalent within state, except for the independent cities, which are assigned codes higher than and following the listing of counties.

County subdivisions are the primary divisions of counties and statistically equivalent entities for data presentation purposes. They include census county divisions, census subareas, minor civil divisions (MCDs), unorganized territories, and incorporated places that are independent of any MCD.

Each county subdivision is assigned a five-digit Federal Information Processing Standards (FIPS) code in alphabetical order within each state.

Geographic codes are shown primarily in computer-readable data products, such as computer tape and CD-ROM/DVD media, including data tabulations and data tables associated with computer-readable boundary files, but they also are shown on some U.S. Census Bureau maps. Census codes are used only if there is no Federal Information Processing Standards (FIPS) code for the same geographic entity or if the FIPS code is not adequate for data presentation. A code that is not identified as either " census" or " FIPS" is usually a census code for which there is no FIPS equivalent. Entities that use only FIPS codes in U.S. Census Bureau products are congressional district, county and statistically equivalent entity, county subdivision, subbarrio, Alaska Native Regional Corporation, metropolitan area (that is, metropolitan statistical area, consolidated metropolitan statistical area, primary metropolitan statistical area, and New England county metropolitan area), place, and state. (A census code exists for each state, but was not assigned in alphabetical sequence and serves to organize the states by census region and census division.)

Census codes are assigned for a variety of geographic entities, including American Indian area, Alaska Native village statistical area, Hawaiian home land, census division, census region, urbanized area, urban cluster, state legislative district, school district, urban growth area, and voting district. The structure, format, and meaning of census codes used in U.S. Census Bureau data products appear in the appropriate technical documentation.

An internal point is a set of geographic coordinates (latitude and longitude) that is located within a specified geographic entity. A single point is identified for each entity; for many entities, this point represents the approximate geographic center of that entity. If the shape of the entity causes this point to be located outside the boundary of the entity or in a water body, it is relocated to land area within the entity. In computer-readable products, internal points are shown to six decimal places; the decimal point is implied.

The first character of the latitude or longitude is a plus (+) or a minus (-) sign. A plus sign in the latitude identifies the point as being in the Northern Hemisphere, while a minus sign identifies a location in the Southern Hemisphere. For longitude, a plus sign identifies the point as being in the Eastern Hemisphere, while a minus sign identifies a location in the Western Hemisphere.

The Island Areas of the United States are American Samoa, Guam, the Commonwealth of the Northern Mariana Islands (Northern Mariana Islands), and the Virgin Islands of the United States. The U.S. Census Bureau treats the Island Areas as entities that are statistically equivalent to states for data presentation purposes. Geographic definitions specific to the Island Areas are shown in the appropriate publications and documentation that accompany the data products for the Island Areas.

Sometimes the Island Areas are referred to as " Island Territories" or " Insular Areas." For the 1990 and previous censuses, the U.S. Census Bureau referred to the entities as " Outlying Areas." The term " U.S. Minor Outlying Islands" refers to certain small islands under U.S. jurisdiction in the Caribbean and Pacific: Baker Island, Howland Island, Jarvis Island, Johnston Atoll, Kingman Reef, Midway Islands, Navassa Island, Palmyra Atoll, and Wake Island.

The general concept of a metropolitan area (MA) is one of 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.

The MAs and the central cities within an MA are designated and defined by the federal Office of Management and Budget, following a set of official standards that are published in a Federal Register Notice. These standards were developed by the interagency Federal Executive Committee on Metropolitan Areas, with the aim of producing definitions that are as consistent as possible for all MAs nationwide.

Each MA must contain either a place with a minimum population of 50,000 or a U.S. Census Bureau-defined urbanized area and a total MA population of at least 100,000 (75,000 in New England). An MA contains one or more central counties. An MA also may include one or more outlying counties that have close economic and social relationships with the central county. An outlying county must have a specified level of commuting to the central counties and also must meet certain standards regarding metropolitan character, such as population density, urban population, and population growth. In New England, MAs consist of groupings of cities and county subdivisions (mostly towns) rather than whole counties.
The territory, population, and housing units in MAs are referred to as " metropolitan." The metropolitan category is subdivided into " inside central city" and " outside central city." The territory, population, and housing units located outside territory designated " metropolitan" are referred to as " nonmetropolitan." The metropolitan and nonmetropolitan classification cuts across the other hierarchies; for example, generally there are both urban and rural territory within both metropolitan and nonmetropolitan areas.

To meet the needs of various users, the standards provide for a flexible structure of metropolitan definitions that classify each MA either as a metropolitan statistical area (MSA) or as a consolidated metropolitan statistical area divided into primary metropolitan statistical areas. In New England, there also is an alternative county-based definition of MSAs known as the New England County Metropolitan Areas. (See definitions below.) Documentation of the MA standards and how they are applied is available from the Population Distribution Branch, Population Division, U.S. Census Bureau, Washington, DC 20233-8800, telephone 301-457-2419.

Places, for the reporting of decennial census data, include census designated places, consolidated cities, and incorporated places. Each place is assigned a five-digit Federal Information Processing Standards (FIPS) code, based on the alphabetical order of the place name within each state. If place names are duplicated within a state and they represent distinctly different areas, a separate code is assigned to each place name alphabetically by primary county in which each place is located, or if both places are in the same county, alphabetically by their legal description (for example, " city" before " village" ).

Population and housing unit density are computed by dividing the total population or number of housing units within a geographic entity (for example, United States, state, county, place) by the land area of that entity measured in square kilometers or square miles. Density is expressed as both " people (or housing units) per square kilometer" and " people (or housing units) per square mile" of land area.

A public use microdata area (PUMA) is a decennial census area 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. These extracts are referred to as " public use microdata sample (PUMS)" files. Since 1960, data users have been using these files to create their own statistical tabulations and data summaries.

For Census 2000, state, District of Columbia, Puerto Rico, and Island Area participants, following U.S. Census Bureau criteria, delineated two types of PUMAs within their states. PUMAs of one type comprise areas that contain at least 100,000 people. The PUMS files for these PUMAs contain a 5-percent sample of the long-form records. The other type of PUMAs, super-PUMAs, comprise areas of at least 400,000 people. The sample size is 1 percent for the PUMS files for super-PUMAs. PUMAs cannot be in more than one state or statistically equivalent entity. The larger 1-percent PUMAs are aggregations of the smaller 5-percent PUMAs. PUMAs of both types, wherever the population size criteria permit, comprise areas that are entirely within or outside metropolitan areas or the central cities of metropolitan areas.

The U.S. Census Bureau treats the Commonwealth of Puerto Rico as the statistical equivalent of a state for data presentation purposes. Each state and statistically equivalent entity is assigned a two-digit Federal Information Processing Standards (FIPS) code in alphabetical order by state name, followed in alphabetical order by Puerto Rico and the Island Areas. Each state and statistically equivalent entity also is assigned the two-letter FIPS/U.S. Postal Service code.

School districts are geographic entities within which state, county, or local officials or the Department of Defense provide public educational services for the areas residents. The U.S. Census Bureau obtains the boundaries and names for school districts from state officials. The U.S. Census Bureau first provided data for school districts in conjunction with the 1970 census. For Census 2000, the U.S. Census Bureau tabulated data for three types of school districts: elementary, secondary, and unified.

Each school district is assigned a five-digit code that is unique within state. School district codes are assigned by the Department of Education and are not necessarily in alphabetical order by school district name.

States are the primary governmental divisions of the United States. The District of Columbia is treated as a statistical equivalent of a state for data presentation purposes. For Census 2000, the U.S. Census Bureau also treats a number of entities that are not legal divisions of the United States as statistically equivalent to a state: American Samoa, the Commonwealth of the Northern Mariana Islands, Guam, Puerto Rico, and the Virgin Islands of the United States.

Each state and statistically equivalent entity is assigned a two-digit numeric Federal Information Processing Standards (FIPS) code in alphabetical order by state name, followed in alphabetical order by Puerto Rico and the Island Areas. Each state and statistically equivalent entity also is assigned a two-letter FIPS/U.S. Postal Service code and a two-digit census code. The census code is assigned on the basis of the geographic sequence of each state within each census division; the first digit of the code identifies the respective division, except for Puerto Rico and the Island Areas, which are not assigned to any region or division. The census regions, census divisions, and their component states are listed in Figure A-3.

State legislative districts (SLDs) are the areas from which members are elected to state legislatures. The SLDs embody the upper (senate) and lower (house) chambers of the state legislature. (Nebraska has a unicameral legislature that the U.S. Census Bureau treats as an upper-chamber legislative area for data presentation purposes. There are, therefore, no data by lower chamber.) A unique census code of up to three characters, identified by state participants, is assigned to each SLD within state. The code " ZZZ" identifies parts of a county in which no SLDs were identified. As an option in the Census 2000 Redistricting Data Program (Public Law 94-171), participating states receive P.L. 94-171 census data for their SLDs (see VOTING DISTRICT (VTD)). Not all states delineated SLDs for the purpose of presenting Census 2000 data, in which case the entire state is treated as a single SLD coded with blanks at both levels.

TIGER® is an acronym for the Topologically Integrated Geographic Encoding and Referencing (System or database). It is a digital (computer-readable) geographic database that automates the mapping and related geographic activities required to support the U.S. Census Bureau's census and survey programs. The U.S. Census Bureau developed the TIGER System to automate the geographic support processes needed to meet the major geographic needs of the 1990 census: producing the cartographic products to support data collection and map presentations, providing the geographic structure for tabulation and dissemination of the collected statistical data, assigning residential and employer addresses to the correct geographic location and relating those locations to the geographic entities used for data tabulation, and so forth. The content of the TIGER database is undergoing continuous updates and is made available to the public through a variety of TIGER/Line® files that may be obtained free of charge from the Internet or packaged on CD-ROM or DVD from Customer Services, U.S. Census Bureau, Washington, DC 20233-1900; telephone 301-457-4100; Internet http://www.census.gov/geo/www/tiger

A traffic analysis zone (TAZ) is a statistical entity delineated by state and/or local transportation officials for tabulating traffic-related census data-especially journey-to-work and place-of-work statistics. A TAZ usually consists of one or more census blocks, block groups, or census tracts. For the 1990 census, TAZs were defined as part of the Census Transportation Planning Package (CTPP). The U.S. Census Bureau first provided data for TAZs in conjunction with the 1980 census, when it identified them as " traffic zones."
Each TAZ is identified by a six-character alphanumeric code that is unique within county or statistically equivalent entity. For the 1990 census, TAZ codes were unique within CTPP area, which generally conformed to a metropolitan area.

A tribal block group (BG) is a cluster of census blocks having the same first digit of their fourdigit identifying numbers and are within a single tribal census tract. For example, tribal BG 3 consists of all blocks within tribal tract 9406 numbered from 3000 to 3999. Where a federally recognized American Indian reservation and/or off-reservation trust land crosses county and/or state lines, the same tribal BG may be assigned on both sides of the state/county boundary within a tribal census tract that is numbered from 9400 to 9499. The optimum size for a tribal BG is 1,000 people; it must contain a minimum of 300 people. (See also BLOCK GROUP (BG).)

The difference between a tribal BG and a nontribal BG is in the hierarchical presentation of the data. A tribal BG is part of the American Indian hierarchy; that is, the tribal BG is within a tribal census tract that is within a federally recognized American Indian reservation and/or offreservation trust land. (See INTRODUCTION-GEOGRAPHIC PRESENTATION OF DATA.)

Tribal census tracts are small, relatively permanent statistical subdivisions of a federally recognized American Indian reservation and/or off-reservation trust land. The optimum size for a tribal census tract is 2,500 people; it must contain a minimum of 1,000 people. Where a federally recognized American Indian reservation or off-reservation trust land crosses county or state lines, the same tribal census tract number may be assigned on both sides of the state/county boundary. The U.S. Census Bureau uses the census tract numbers 9400 to 9499 for tribal census tracts that cross state/county boundaries and are within or encompassing American Indian reservations and off-reservation trust land. (See also Census Tract.)

The difference between a tribal census tract and a nontribal census tract is in the hierarchical presentation of the data. A tribal census tract is part of the American Indian hierarchy; that is, the tribal census tract is within a federally recognized American Indian reservation and/or offreservation trust land. (See Introduction-Geographic Presentation of Data.)

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

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 growth area (UGA) is a legally defined geographic entity in Oregon that the U.S. Census Bureau includes in the TIGER® database in agreement with the state. UGAs, which are defined around incorporated places, are used to control urban growth. UGA boundaries, which need not follow visible features, are delineated cooperatively by state and local officials and then confirmed in state law. UGAs are a new geographic entity for Census 2000.

Each UGA is identified by a five-digit census code, which generally is the same as the Federal Information Processing Standards (FIPS) code for the incorporated place for which the UGA is named. The codes are assigned alphabetically within Oregon.

Voting district (VTD) is the generic name for geographic entities, such as precincts, wards, and election districts, established by state, local, and tribal governments for the purpose of conducting elections. States participating in the Census 2000 Redistricting Data Program as part of Public Law 94-171 (1975) may provide boundaries, codes, and names for their VTDs to the U.S. Census Bureau. The U.S. Census Bureau first reported data for VTDs following the 1980 census. Because the U.S. Census Bureau requires that VTDs follow boundaries of census blocks, participating states often adjusted the boundaries of the VTDs they submit to conform to census block boundaries for data presentation purposes. If requested by the participating state, the U.S. Census Bureau identifies the VTDs that have not been adjusted as an " A" for actual in the VTD indicator field of the PL data file. The VTD indicator for all other VTDs is shown as " P" for pseudo.

For Census 2000, each VTD is identified by a one- to six-character alphanumeric census code that is unique within county. The code " ZZZZZZ" identifies parts of a county in which no VTDs were identified. For a state or county that did not participate in the VTD project, the code fields are blank.

A ZIP Code® tabulation area (ZCTA™) is a statistical geographic entity that approximates the delivery area for a U.S. Postal Service five-digit or three-digit ZIP Code. ZCTAs are aggregations of census blocks that have the same predominant ZIP Code associated with the residential mailing addresses in the U.S. Census Bureau's Master Address File. Three-digit ZCTA codes are applied to large contiguous areas for which the U.S. Census Bureau does not have five-digit ZIP Code information in its Master Address File. ZCTAs do not precisely depict ZIP Code delivery areas, and do not include all ZIP Codes used for mail delivery. The U.S. Census Bureau has established ZCTAs as a new geographic entity similar to, but replacing, data tabulations for ZIP Codes undertaken in conjunction with the 1990 and earlier censuses.





Figure A-3. Census Regions, Census Divisions, and Their Constituent States

Northeast Region

New England Division:
Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut

Middle Atlantic Division:
New York, New Jersey, Pennsylvania

Midwest Region

East North Central Division:
Ohio, Indiana, Illinois, Michigan, Wisconsin

West North Central Division:
Minnesota, Iowa, Missouri, North Dakota, South Dakota, Nebraska, Kansas

South Region

South Atlantic Division:
Delaware, Maryland, District of Columbia, Virginia, West Virginia, North Carolina, South Carolina, Georgia, Florida

East South Central Division:
Kentucky, Tennessee, Alabama, Mississippi

West South Central Division:
Arkansas, Louisiana, Oklahoma, Texas

West Region

Mountain Division:
Montana, Idaho, Wyoming, Colorado, New Mexico, Arizona, Utah, Nevada

Pacific Division:
Washington, Oregon, California, Alaska, Hawaii

Adjacent New England City and Town Areas (see glossary term New England City and Town Area) that have a high degree of employment interchange may be combined to form Combined New England City and Town Areas (or CNECTAs). New England City and Town Areas that are part of a CNECTA retain their separate identities.

Defined by the federal Office of Management and Budget for use by federal statistical agencies according to the Census data, adjacent metropolitan and micropolitan statistical areas may be combined to form a CSA if the areas have a high degree of employment interchange. Each metropolitan or micropolitan statistical area retains its own identity.

Core Based Statistical Area is a collective term for both metropolitan and micropolitan areas.

A metropolitan statistical area with population greater than 2.5 million may be divided into smaller groups known as metropolitan divisions.

Metropolitan and micropolitan statistical areas (metro and micro areas) are geographic entities defined by the U.S. Office of Management and Budget for use by Federal statistical agencies in collecting, tabulating, and publishing Federal statistics. A metro area contains a core urban area of 50,000 or more population, and a micro area contains an urban core of at least 10,000 (but less than 50,000) population. Each metro or micro area consists of one or more counties and includes the counties containing the core urban area, as well as any adjacent counties that have a high degree of social and economic integration (as measured by commuting to work) with the urban core.

is a geographic entity defined by the U.S. Office of Management and Budget for use by Federal statistical agencies in describing aspects of the New England region in the United States. NECTA is based on the concept of a core area with a large population nucleus, plus adjacent communities having a high degree of economic and social integration with that core. Similar to the metropolitan and micropolitan statistical areas, a metropolitan NECTA has an urban core of at least 50,000 population, while a micropolitan NECTA has an urban core of at least 10,000 inhabitants but less than 50,000.

A New England City and Town Area with population greater than 2.5 million may be divided into smaller groups known as NECTA Divisions.

Statistical entities for state recognized American Indian tribes that do not have a state recognized land base (reservation). SDTSAs are identified and delineated for the U.S. Census Bureau by a state liaison identified by the governor's office in each state; formerly known as State Designated American Indian Statistical Area (SDAISA).