This web page documents and summarizes geographic entities used in the 2007 edition of the MABLE database, accessible via the Geocorr 2000 web application.
The geographic coverages are presented here in the same order as they appear on the Geocorr 2000 select lists for source and target geocodes. The majority of codes are based on the 2000 decennial census, specifically the geographic header files that came as part of Summary File 1. As such, they are vintage January 1, 2000. But we do have geocodes that are more recent, and these will be explicitly identified.
The Missouri State Census Data Center and OSEDA maintain a library of geographic code modules in the form of SAS format codes. These modules have special application for SAS software users, since they allow codes to be readily converted to their corresponding names. Sometimes format modules are used not to provide names, but rather to link codes to other entities as a kind of table lookup. Note that although these modules are, technically, "code" you do not have to be a programmer or know any SAS to use these as codebook files to look up a geographic code.
Most of these geographic codes comprise numeric digits, but they have no numeric significance. They are stored in the MABLE database as character strings rather than binary numeric fields. In reports they will display with leading zeroes (01 as the code for Alabama, for example, rather than just 1), and these leading zeroes also are written to output CSV files by the Geocorr application. When the latter get imported into Excel, however, the import routine (by default) turns them into numerics and the leading zeroes disappear.
The MABLE database is really a collection of 51 state-level databases. The state geocode is almost always added to output files even if it is not explicitly selected. The District of Columbia is considered a state for the purposes of this application. Other statistically equivalent areas, such as territories and outlying areas (including Puerto Rico) are not states and are thus not part of MABLE. This is a two-digit FIPS (Federal Information Processing Standard) code with leading zeroes.
The FIPS county codes are three-digit numbers assigned within states. They generally are odd numbers assigned in alphabetical order. Exceptions are independent cities (i.e., cities like Baltimore and St. Louis that are not in any county and serve as county equivalents), which are usually assigned codes over 500 (such as 510). On output files and listings we usually combine the FIPS state and county codes. Thus, the value of the County variable for Autauga County, Alabama is 01001 and for Baltimore City, Maryland is 24510. In some states (such as Louisiana and Alaska), the primary substate legal entities are not called "counties;" for the sake of this application they are county equivalents and act exactly the same as counties. Counties appearing here are those defined at the time of the 2000 census. See the Census Bureau web page describing any changes that may occur. There are approximately 3,141 counties in the U.S.
The value of this county code is the same value as its 2000 version, with only a few exceptions in the states of Alaska, Colorado, and Virginia. The change in Alaska was to the Skagway-Hoonah-Angoon Census Area (Alaska) with FIPS code 02-232. It was split into two new county equivalents, apparently following the boundary of the city of Skagway. The new entities are 02-230, called Skagway Municipality, and 02-105, called Hoonah-Angoon Census Area. This change apparently was made some time in 2007 or early 2008.
These are the primary geographic units recognized by the Census Bureau which are just below the county level. Most states have Minor Civil Divisions (MCDs), which are legally recognized governmental or administrative units. MCDs are defined in 28 states and in D.C. In the remaining states, the Census Bureau has defined Census County Divisions (CCDs). Most states have either all MCDs or all CCDs (with Missouri being an example of a state that has both). "MCD" is a generic category; the specific types of MCDs vary by state. The most common type of MCD is the township. Other types of areas that can be MCDs include towns or incorporated places, election districts, plantations, magisterial districts, etc. In the geographic hierarchy, these divisions provide a complete coverage of all counties in the county. There were 35,414 such geographic areas in the U.S. at the time of the 2000 census.
This excerpt from the technical documentation released with the 2010 PL94 files summarizes some of the odd and inconsistent (across states, at least) behavior of these entities:
In some states, all or some incorporated places are not part of any MCD; these places are termed independent places. Independent places also serve as primary legal subdivisions and have a Federal Information Processing Series (FIPS) county subdivision code and National Standard (ANSI) code that is the same as the FIPS and ANSI place code. In nine states — Maine, Massachusetts, New Hampshire, New Jersey, North Dakota, Pennsylvania, Rhode Island, South Dakota, and Wisconsin — all incorporated places are independent places. In other states, incorporated places are part of, or dependent within, the MCDs in which they are located, or the pattern is mixed — some incorporated places are independent of MCDs and others are included within one or more MCDs.
Smcdcnvt format table shows the relationship between the Census Bureau codes used for these entities and the FIPS codes (as used in MABLE). The FIPS codes are five digits and are unique within state, whereas the Census Bureau codes are only three digits and are unique within county. The Census Bureau codes are no longer used for current data, and we are not even sure if such codes are assigned for new county subdivisions. They are of interest only when needing to link to earlier data that used these codes.
On output files and listings generated by Geocorr, this variable goes by the name
cousubfp ("COUnty SUBdivision FiPs").
Lots of variety for this geographic level. Places have different names in different states (e.g., "cities", "towns", "boroughs", "villages", etc.) There are also approximately 4,000 entities called Census Designated Places (CDPs) which have no formal, legally recognized boundaries but which the Census Bureau has designated as areas which are generally recognized by the local population as worthy of having data tabulated for them. Places are within states, but otherwise they can cross just about any other boundary. A place can be in multiple counties, in multiple county subdivisions, ZCTAs, etc. Places are mutually exclusive but are not exhaustive — there are areas that are not contained in any place. The Census Bureau (and MABLE) assign a code of all 9s to areas that are not within any place. These "pseudo-places" are simply referred to as Unincorporated Remainders; they are not simply "unincorporated portions" (in general), because in many cases part of the unincorporated area of a county or MCD is in a CDP.
The relationship of MCD-CCDs (i.e., county subdivisions) and places varies from state to state, but in general places may cross MCD-CCD boundaries. There are some places which are also MCDs (common in New England). In these cases, the FIPS MCD and place codes are the same (but not the census codes). There were approximately 25,000 places recognized for the 2000 census.
Note: Places are among the most unstable of geographic entities over time, and are perhaps the most difficult to identify accurately since their boundaries are often "invisible" — i.e., do not follow physical features that are easily identifiable. Because of this, the Census Bureau has a real challenge trying to keep up with accurate place definitions. The codes used in MABLE come from the Bureau's official SF1 geographic headers files, which define the geography of the United States as of 1-1-2000. The place codes that appear in MABLE reflect what the Bureau recognized for city boundaries when it tabulated the 2000 census. It is an accepted fact of the census-taking business that there will always be mistakes regarding these boundaries. The Bureau has published a special file called the CQR (for "Census Quality Review") that is the official list of known geographic coding problems.
On output files and listings generated by Geocorr this variable goes by the name
placefp. The Splccnvt format module shows the relationship between the Census Bureau codes used for these entities and the FIPS codes (as used in MABLE). The FIPS codes are five digits and are unique within state, while the Census Bureau codes are four digits and are also unqiue within state.
The census tract is part of the very useful four-level hierarchy of census data, in which each lower level is completely contained within its parent level. The four levels are county, tract, block group, and block.
The Census Bureau has complete control over these "small-area" geographic units. The Bureau defines them solely for the purpose of collecting and tabulating the results of the census. In most metropolitan areas, local census tract committees are appointed which are responsible for drawing up suggested boundaries for the census tracts in their areas. In most rural areas, there can be (but usually are not) such committees, and the Census Bureau defines the tracts. In the 1990 census, areas that were defined without the input of a local tract committee were called Block Numbering Areas (BNAs), but this terminology was dropped in 2000 and everything since then is called a census tract.
Among the criteria that the Census Bureau has established for defining tracts is that they should be compact contiguous areas with populations of about 4,000 persons and that the area should, if possible, be homogeneous. The ideal urban census tract would be a locally recognized "neighborhood" within a city. Prior to 1990, census tracts were strictly an urban concept; they were only defined within metropolitan areas.
Census tracts are assigned four-digit numeric codes, unique within counties. Tracts can also have a two-digit suffix code, usually indicating that this is a "split" of a tract from an earlier census year. Thus if 1234.00 was a tract in 1900 with 5,000 persons, and that area grew to a population of 12,000 by 2000, you might see three tracts in 2000 with codes 1234.01, 1234.02, and 1234.03. Suffix codes of .97 and .98 are special and have to do with details most people would rather not be bothered with. The short explanation is that it represents where there was a "temporary problem" with a tract assignment that was "fixed," but this suffix code had to be attached. Suffix codes of .99 are used for pseudo-tracts used to tabulate "crews of vessels" residing in nearby rivers and lakes.
Census tract codes on all output files and reports from Geocorr are named tract and are always represented in a full seven-character xxxx.xx format with leading and trailing zeroes. There were about 65,400 of these entities defined for the 2000 census.
Whenever you select this geocode (using Geocorr) from either the source or target geocode select lists, the county code is also automatically selected for you — you should never process tract data without carrying along the county code, unless, of course, your entire analysis is taking place within a single county.
Tracts do not have names associated with them, in general, and there are no format codes available for them.
If you understand census tracts, then all you need to know is that block groups are the next level down in the hierarchy. A typical census tract will comprise about five or six block groups. The name comes from the fact that each block group (bg) is composed of census blocks, grouped together within a tract. The first digit of the four-digit block number is the code for the area. Thus the block group geocode is only one character long, which, of course, is meaningless outside of the context of the tract and county. Block groups with a code of 0 indicate a coastal area entirely comprised of water. There were approximately 209,000 block groups defined for the 2000 census, with an average population of 1,348.
From a data perspective, block groups have the distinction of being the smallest geographic unit (well, almost) for which the Census Bureau tabulates detailed demographic data. This means that if you are looking for data regarding income, occupation, or education (to name three popular subjects only available in the sample data) then the smallest geographic unit for which you'll be able to get that data is the block group.
On all Geocorr output files, this geocode will be called bg and will be a single character (digit) long.
Block groups do not have names associated with them, in general, and there are no format codes available for them.
This is the atom in the MABLE view of the matter. It is the smallest geographic entity recognized by the Census Bureau. It is generally the smallest area that can be formed by intersecting visible features. The classic census block is the rectangular city block bounded by four streets. With the extending of block assignment to rural areas for the 1990 census, we also now have the 100-square-miles-of-open-desert blocks and the classic single farm or portion of farm block. Each of the over 10 million observations on the MABLE database describes one census block. All other geographies are defined in terms of which of these blocks can be added together to form it. This is a fudge in some cases — notably with ZIP (ZCTA) codes, but makes perfect sense for most geographies. This is because the Census Bureau made a decision when it redesigned census block geography for the 1990 census (and beyond) that it would not have any blocks that crossed county subdivision or place boundaries. The Geocorr concept is based on the idea that geography can be reduced to a set of block-level "pixels" that can be used to examine how other geographies are related using simple algebra instead of complex geometry.
Census blocks are the last level of the county-tract-bg-block hierarchy. The block code itself is four digits (the first of which, as mentioned above, indicates the block group.) If the first digit is 0 it indicates a water block, which as the name implies is located under water and rarely has anyone living in (or "on") it. These water blocks were not included in the 1990 version of MABLE but are now. They tend not to contribute much, however, since they have almost all 0 values for the weight variables (population, households, and land area).
On all output files and listing produced by Geocorr, the census block geocode is called blk and is four characters long. Census blocks do not have names associated with them.
For the 2000 Census, the Census Bureau decided to change the name of the geographic entity that they had previously referred to as ZIP codes. They would now be called ZCTAs (ZIP Census Tabulation Areas). There were actually rather minor changes in the way these entities were defined as compared to 1990, but the Bureau decided that it would be helpful to alert the users to the fact that these entities were not exactly ZIP codes. But, for most purposes, pretty close. To see the definitive word on the what, why, and how of ZCTAs see the Bureau's ZCTA web page or the Geographic Terms and Concepts — ZCTAs web page.
Only "residential" ZIP codes — those containing household addresses — have corresponding ZCTA codes (and hence will appear in a MABLE database). There are no business or Post Office Box-only ZCTAs, etc. The latter account for about a fourth of all ZIP codes in the U.S.
Another problem is that ZIP codes are not really spatial entities — they are simply lists of addresses, organized to facilitate mail delivery. While they often do form areas that can be viewed as geographic areas, that is not what they really are. This can create problems when you try to relate them to a spatial entity such as a census block. Think of a classic census block formed by the intersection of 1st St., Elm Ave, 2nd St., and Pine Ave. If 1st St. is the northern border of the block, then folks living on the south side of 1st St. between Elm and Pine are in our block (let's call it 101). But people living across the street — on the north side of 1st St. — are living in a different block, say 102. But the U.S. Postal Service would never have a ZIP boundary go down the middle of a street. If this were an area where the ZIP changed, it would almost certainly divide along (vague and invisible) "back-lot lines." For example, the folks living on both sides of 1st St. in our example might live in ZIP 12345, while the folks living on 2nd St. might live in 12346. Thus you have households in the same census block, but in different ZIP codes. Hence, the fundamental concept of census block as the atomic unit is violated. Of course, this only happens in a certain percentage of blocks, and in many cases the ZIP boundaries are on commercial streets where not many people live and you can assign most of the population in the boundary blocks to the right ZIP. These issues are dealt with in the Bureau's definitive web page (see link above) that describes how these issues were dealt with when defining ZCTAs.
On output files this geocode is stored as ZCTA5. The variable Zipname is also included (unless otherwise requested). This name is based on an old file we obtained from the USPS back in the 1990s with post office names, slightly supplemented. In a few cases there will not be a name available, in which case the code value also appears as the ZIPName value.
Pseudo-ZCTA codes are those that end with XX or HH (e.g. 594XX and 594HH in Montana). The 594XX ZCTA is the set of blocks within the 594 three-digit ZIP area that the USPS had not assigned to any ZIP code. (Not every location in the country has a ZIP code.) The HH pseudo-ZIP areas are the combination of all water blocks within the three-digit ZIP area. These usually (but not always) have zero population.
To get information regarding any ZCTA (or even non-ZCTA ZIP), the MCDC offers the ZIP Code Lookup web application.
For more information about ZIP codes, see the MCDC's own ZIP Code resources page.
This format code was derived from a file from the U.S. Postal Service. It's a combination of post office and local geographic names. It is the source for the ZIPNAME fields that will be added to your Geocorr outputs if you specify that you want names to got with your geocodes and you also select ZIP as one of your geocodes.
This is not exactly a geocode in the same sense as the other ones used here, but it is far too useful to discard on a technicality. All census blocks are assigned this characteristic by the Census Bureau based on their standard definition of the concept of "urban." That definition can get rather complicated when you follow it down to the source, and it has changed over time. See our Ten Things to Know About Urban vs. Rural page for almost all you need to know about this topic.
This geocode has two values: U means urban and R means rural. On Geocorr output files, this field is called ur and is one character long. There is no name field associated with it.
We referred to these entities above in our discussion of Urban/Rural. To be urban starting with the 2000 census is be part of an Urbanized Area or an Urban Cluster. These two entities share a variable on the MABLE database and a set of five-digit codes. The only difference between them is the size of its densely settled core that is the primary requirement that must be met before an area can be considered urban. That densely settled core area must have a population of at least 1,000 persons per square mile and must have a total size of 10,000 persons. If it is over 50,000, then you have an Urbanized Area; if it is between 10,000 and 49,999, then you have an Urban Cluster. The area comprises this central core plus the surrounding contiguous area with a density of at least 500 persons per square mile. There are some important exceptions to these general rules (to allow for special situations where there may be "hops" across certain bodies of water or "green belts".) See the Census Bureau's FAQ page on the subject.
On all Geocorr output files this field is called urbarea and is five characters wide with leading zeros.
UA and UC codes are included in the MCDC's geographic codes lookup web app.
The smallest geographic area identified on the 2000 Public Use Micro Sample files (File A — 5% sample), as well as on the ACS PUMS files. Boundaries of PUMA areas had to be defined in terms of counties, places, county subdivisions or census tracts. In a very large majority of cases PUMAs consist of one or more counties. In larger metro counties, they are frequently broken down along the smaller geographic area lines (places and/or census tracts). A strict guideline for defining PUMAs is that they must have a minimum population of 100,000 persons (as of the 2000 census.) For a more detailed description of these entities with links to related resources, see the MCDC's PUMAs page.
PUMA codes are five digits (characters) long. Most end with 00. Generally when the last two digits are not zeroes, it represents a county that has been split into subareas. Thus, for example, the PUMA codes for the St. Charles county (MO) are 01801, 01802, and 01803.
On all Geocorr output files these fields will be called puma5 and will be five characters wide with leading and trailing zeroes. There will be no names associated with them.
There are also codes called Super PUMAs (aka "1% PUMAs") that were used on the 2000 PUMS 1% sample files. These have never been kept on the MABLE database and we have no plans to add them.
Maybe the most important thing to know about this geocode is that it does not represent the current metropolitan areas. There is some confusion on this point because the Census Bureau uses the same term — Metropolitan Statistical Area — to refer to two similar but distinct statistical entities. For more on this, see the entry for CBSAs.
The Office of Management and Budget defined these metropolitan area entities based on decennial census data and official intercensal estimates. The code used here was the definition that was in effect at the time of the 2000 census. They are the last set of such definitions that will ever be assigned. In 2002 they were replaced by the new Core Based Statistical Areas.
MSA stands for Metropolitan Statistical Area. Most metropolitan areas (like St. Louis, Pittsburgh, Des Moines and Louisville) are simple MSAs. Consolidated MSAs (CMSAs) occur when two or more contiguous MSAs are joined, or when there is a large urban area with more than one central city. Examples of CMSAs are Chicago-Gary-SE Wisconsin, Washington,DC-Baltimore, Dallas-Ft. Worth, and the San Francisco Bay Area. The metro areas that comprise CMSAs are then called PMSAs (primary MSAs), which are also available as a separate layer. This more complex system of classifying metro areas replaced the simpler SMSA concept that was used in the 1980 and earlier censuses. Note that codes for metro areas are unique without any qualifiers and that metro areas can span states. Note also that MSA and CMSA codes share this field/variable, while PMSAs have their own, separate field (layer). If an area is not contained within a metro area, then the MSA/CMSA code will be 9999 and the PMSA codes will be blank. If any area is within an MSA but is not part of a CMSA, then the PMSA code will be blank.
Except in New England, metro areas are made up of complete counties. In New England they are made up of complete towns (MCDs) and can cross county boundaries.
The Census Bureau has created an excellent web page describing Metro Area concepts with links to current codes and geographic components.
On Geocorr output files, this field will be called msacmsa and will be four characters wide. A value of 9999 is used to indicate an area that is not within a metro area.
Most of what was said for the previous geocode — MSA/CMSA applies here as well. Except that PMSA will have a value of 9999 for any area that is not inside a PMSA. Even if it is metropolitan, if it is not in a CMSA (and hence a PMSA) then it has the all nines code. (See also the Bureau's metro area web page.)
Format table: Smetro.sas
This format code handles MSA, CMSA, and PMSA codes and returns the names of the areas. Note that these three kinds of codes do not overlap (i.e., if there is an MSA with code 1234, then there will never be a PMSA or CMSA with that code). The format code will return a (P) at the end of the metro name to indicate a Primary MSA.
On Geocorr output files, this field will be called pmsa and will be four characters wide. It will have a value of 9999 to indicate not applicable.
This is the current geocode to use when you want to have data for a Metropolitan Area or a Micropolitan Area. We mentioned in the section above on Metro Area (MSA or CMSA 2000) that there was some confusion over the term "metropolitan area". Here is where we will follow up on that issue.
Note that this field has a year specified (which will vary over time as we update MABLE to have the latest metro area codes). We currently (late October, 2009) have the 2009 vintage (last modified by OMB as of November, 2008) CBSA and related codes (CBSA type, metropolitan division, and combined statistical area). These are county-based areas and new ones can be added, old ones modified, deleted, renamed, etc. each year. There are two major changes involved in going from the previous msa/cmsa/pmsa metro area concepts and this new "core-based" system:
CBSAs have a five-digit code. Metropolitan and Micropolitan areas share the same pool of codes; you cannot tell from the CBSA code which kind it is. (That is why we have the CBSA Type as a separate geocode.)
Note the source of confusion here. We have the old St. Louis MSA (FIPS code 7040) that was defined in 2000 and used as a summary unit in the 2000 census products. And we have the current (core-based) St. Louis MSA (FIPS code 41180) that includes Washington County in Missouri and Bond, Calhoun and Macoupin counties in Illinois (which 7040 does not). Both are referred to simply as "the St. Louis Metropolitan Statistical Area", but they are really different entities, as reflected in the different FIPS codes. Adding to the confusion is the Census Bureau's decision to discourage use of the term "core based" statistical areas, apparently because they thought it confused or scared users. Better to just use the more friendly and familiar term "Metropolitan Statistical Area" and not confuse people with the fact that things have actually changed.
For the official definition (generic as well as specific) of these "Metropolitan and Micropolitan Statistical Areas," see the Census Bureau's metrodef web page, maintained in the estimates section. Remember, these things can change from year to year: counties can be added or deleted from existing CBSAs, new CBSAs can be created, and old ones can be deleted or renamed (in which case it will get a new code, so it is very much like having a new entity).
CBSAs comprise complete counties or county equivalents (parishes, independent cities, etc). There is only one exception to this general rule: The portion of the city of Sullivan in Crawford County, MO was legislated to be part of the St. Louis metropolitan area. It has to do with a veteran's hospital being located there and something about a formula for federal subsidies in metro vs. non-metro areas. As of July, 2009 there were approximately 1,500 people living in this area.
Hardly needs any additional explanation. The value is stored in mixed case, with the M in upper case and the rest in lower case. When CBSA is blank then so is this field. Having this field is handy for letting you generate reports that show the latest population estimates by state for the metropolitan, micropolitan, and "neither" portions of a state. You do not really need to keep it for ID when you select CBSA and keep name fields since the CBSAName field includes the type.
Some core-based MSAs are large enough that they have significant subareas comprised of related counties. These subareas are called Metropolitan Divisions and they have their own five-digit FIPS codes. An example is the Chicago-Naperville-Joliet, IL Metropolitan Division, which is the Illinois portion of the tri-state Chigago core-based MSA. This would be comparable to a PMSA under the previous MSA-CMSA-PMSA scheme. There are not very many of these beasts.
Same concept as the Metropolitan Divsision except that here we are talking about a combining of CBSAs into a larger geographic area. Note that all thse CBSA related geocodes get updated in sych so they are all the same vintage.
They always have to be different in New England, where counties are disdained and towns are what count. So while they passed a law saying that all CBSA's would be county-based they could not force the good people of New England to actually use those entities. Instead, the powers that be agreed to create alternative metro area definitions using the core-based technology (i.e. the central densely setttled area not constricted by any municipal boundaries) but with towns ("MCD"s) as the component units rather than counties. These were added to the MABLE database in 2007.
See Metropolitan Divsion, above. NECTA Divsions are to NECTAs as Metropolitan Divisions are to CBSAs. They are town-based subareas of NECTAs.
Anyone involved in redistricting will recognize these units, and anyone not involved in redistricting is unlikely to be very interested. These were geographic entities that the states identified and labeled back in 1999 so that in 2001 the Census Bureau would deliver basic population data for these geogaphic units, which could then be used as the building blocks for the state's congressional and state legislative districts. These can be anything the state (or the local entity within the state — things can vary widely within a state) wanted them to be but typically they correspond to precincts or other voting units. The codes assigned can be mnemonic (i.e., related to the codes for the precints or wards to which they correspond) or arbitrary (as in most of Missouri). The real ID in many cases is in the corresponding VTDName field. The Bureau wrote guidlelines for how the states were to assign the codes but they were largely ignored in Missouri. The codes in that state — or at least for many counties within it — were just sequentially assigned and stored as left-justified character strings (in direct violation of the guidelines). So, for example, the VTD code for Boone County, Precinct 34 is 39, while the correponding VTDName field has the more useful value "Voting Dist 34." On the other hand, in Cook County, IL, if the code is 03039, it turns out the area is Ward 3, Pct 39.
We currently have these codes for three congresses. The most recent is the 109th congress, elected in the fall of 2004. The current congress (in 2010) is the 111th (elected in 2008). There were changes to the CDs in Texas and Georgia for the 110th; in all other states the 109th and 110th district codes are identical. The Geography Division of the Census Bureau maintains a web site where you can view 109th Congressional District wall maps (very large PDF files), including maps for individual districts. They also provide a 110th Congressional Districts Geographic Relationships Files with links to files that define relationships of multiple geographies to these latest CDs. Unfortunately, these files to not provide sufficient detail to allow us to add the revised codes to the MABLE database. These codes should be available in 2011 when we go to the new MABLE database for the new decade.
These are the districts which were redrawn circa 2011 using results (block level pop counts) from the 2000 census. They were released as a data product by the Census Bureau early in 2007. They should remain in effect until around 2012 when new districts will be drawn using the results of the 2010 census. The "Upper Chamber" is referred to as the "Senate" in most states, while the "Lower Chamber" is usually called the "House" (in California it is called the "Assembly.") Nebraska has a unicameral legislature (meaning they have only one state legislative body instead of the usual two). The codes for the Lower Chamber districts are blank for Nebraska. These are three-digit codes with leading zeroes, even though in most states just one or two digits would suffice. There are no names associated with the codes.
These codes were added to the Census Bureau's TIGER system in the 1990s. These values are as of 2000 (i.e., as used in the geographic headers of the 2000 census summary files.) They are "LEA" codes as assigned by the National Center for Educational Statistics. There are three levels of districts, but not all three will be defined for a census block. In many/most states, not all three levels are used. In Missouri, for example, only Unified and Elementary are used, and they are mutually exclusive (i.e., they do not overlap). These are five-digit codes as established by the National Census for Education Statistics. They are unique within state.
Counties are categorized by nine population interval categories. Even though we use the original name from the geographic headers portions of the census summary files where it is stored as a code, the values stored in MABLE (cntysc) are just character strings indicating the interval. The values are:
Same idea as above, but this is for place size (placesc) rather than county.
Based on census 2000 place boundary definitions and populations.
This is a category variable that allows you to look at a breakout of an area based on how much of it is inside or outside of a place, with detail as to whether the place is incorporated or a Census Designated Place (CDP). The one-character code values are:
Based on census 2010 place boundary definitions.
This was an experiment that probably has failed, partly due to lack of documentation. We never told people what it was and only one person ever asked since we put it out there over five years ago. We were trying to get a handle on the concept of just how densely populated it was just in this little area. But that becomes very tricky, because all we can do is measure the persons per square mile for the census block. But that can be misleading, since you really need to look at the density of surrounding or nearby blocks as well. We plan to delete this in the next release unless we hear from users saying they want us to keep it.
The Census Bureau has their own set of four-digit place codes that were used before they switched to five-digit FIPS codes some time between the 1980 and 1990 censuses. This field is blank for places that did not exist in 1980. Only useful for capturing codes that you plan to use in conjunction with earlier census data (1980 or earlier). Normally you will use the regular Place geocode, which is the FIPS version.
This does for county subdivisions what the previous entry does for places. These are three-digit codes that the Census Bureau assigned and used in their products before switching to the more standard FIPS codes. These codes are only unique within county. They have leading zeroes and usually appear in increments of five. So most counties have CenCouSub codes of 005, 010, 015, etc.
These areas are used by the transportation planning community to do things like setting up work trip matrices that count commuting flows between these zones. These are important geographic units on special products like the various Census Transportation Planning Packages (CTTP) that the Census Bureau creates based on specs from the U.S. Department of Transportation. Coding schemes vary from state to state and metro area to metro area. They can be up to 6 characters long. A value of 999999 indicates an area where no TAZ has been defined. They appear to be unique within county (but you have to select County explicitly when selecting TAZ as a Source or Target code.) There are no names associated with these codes.
These county typology codes are assigned by the Economic Research Service at the U.S. Dept of Agriculture in an attempt to measure the "degree of ruralness" (or "urban-ness") for a county. (The codes are named after Calvin Beale, the USDA researcher who came up with the scheme.) The codes are scaled from 1 to 9, with 1 being the least rural and 9 being the most. The 2003 codes are based on the status of the county as measured in the 2000 census. We also have a 1993 version of the code, which classifies an area based on the results of the 1990 census. There is also a slight modification of the coding scheme. In 1993 the code 0 was used to indicate central counties of metro areas of a million or more population, with code 1 used for all other (non-central) counties in those metro areas. For 2003, the distinction between central county and other was dropped and so there is no 0 code for 2003.
An alternate set of codes for classifiying counties according to their geographic context. There are 12 categories used here. The explanation at the USDA web site states:
An area's geographic context has a significant effect on its development. Economic opportunities accrue to a place by virtue of both its size and its access to larger economies. And, access to larger economies — centers of information, communication, trade, and finance — enables a smaller economy to connect to national and international marketplaces. These relationships among economies are basic concepts of the central place theory commonly studied in regional economics. Population size, urbanization, and access to larger communities are often crucial elements in research dependent on county-level data sets. To further such research, ERS developed a set of county-level urban influence categories that captures some differences in economic opportunities.
These categories make use of the new distinction being made between metropolitan and micropolitan statistical areas (not available until 2002). See the entry for CBSAs, above.
For additional background information on any of the geographic units maintained or utilized by the Census Bureau you can view the Geographic Areas Reference Manual which is now available on the Geography Divsion's home page.
Other useful links: