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The acronym ZCTA should be pronounced zik'tah.
Census 2000 and 2010 Census ZCTAs were created for all 50 states, the District of Columbia, and Puerto Rico. Later releases of 2010 Island Areas products will include 2010 ZCTAs for American Samoa, Guam, the Commonwealth of the Northern Marianas Islands, and the United States Virgin Islands. Large water bodies and unpopulated land area such as national parks were excluded from the 2010 delineations, and for this reason the Census 2010 product does not have complete national coverage.
For the Census 2000 product, there were 33,178 ZCTAs nationwide (all 50 states, the District of Columbia, and Puerto Rico), including generic three-digit ZCTAs. Of that total, 32,038 ZCTAs carried the full five-digit numeric ZIP Code. The remainder had a generic code ending in either “HH” or “XX” suffix.
For the 2010 Census, there are 33,120 ZCTAs. All 2010 Census ZCTAs are full five-digit ZCTAs. No three-digit ZCTAs will be published, nor will any ZCTAs be published with an “HH” or “XX” suffix. Large water bodies and unpopulated land area such as national parks were excluded from the 2010 delineations, and for this reason the Census 2010 product does not have complete national coverage.
For Census 2000 counts, refer to the Census 2000 Tabulation Entity Counts site. 2010 Census counts will be released at a later date.
U.S. Postal Service ZIP Codes are designed to meet the day-to-day operational needs of the U.S. Postal Service and tend to change more frequently than every ten years. They are networks of streets served by mail carriers or just individual post offices and are a tool for mail delivery. They change periodically as required to meet U.S. Postal Service operational needs. In creating ZCTAs, the Census Bureau took the most frequently occurring ZIP Code used by addresses in an area for the ZCTA code. Therefore, some addresses will end up with a ZCTA code different from their ZIP Code. Some ZIP Codes represent very few addresses (sometimes only one). If a ZIP Code was never the most frequently occurring ZIP Code in an area during the ZCTA delineation process, the ZIP Code will not appear in the ZCTA universe.
The Census Bureau did not produce a 2000 ZIP Code to 2000 ZCTA relationship file. We created the ZCTAs specifically to address the inadequacies of ZIP Codes for census data tabulation.
For those who may want to do this, the TIGER/Line® files show address ranges with mailing ZIP Codes. These files can be processed using a GIS to compare the ZCTA code for a block to the mailing ZIP Code associated with the address ranges on each block side. Such a comparison can provide a general idea of how the two relate.
The relationship between ZIP Code and ZCTA can be determined fully only by comparing individual block-geocoded addresses to the ZCTAs. This process is quite involved. Some examples of why the process can become quite involved are as follows: ZCTAs follow census block boundaries. In contrast, USPS ZIP Codes serve addresses with no correlation to census block boundaries; therefore, the area covered by a ZCTA may include mailing addresses associated with ZIP Codes that are not the same as the ZCTA.
A ZCTA may include a mailing address with a unique or PO Box ZIP Code that is ineligible to become a ZCTA. Addresses with PO Box ZIP Codes generally cluster around a post office, but they may be widely scattered across several ZCTAs. Consequently, the relationships that exist between ZCTAs and ZIP Codes can become quite complicated, so that within the boundaries of a single ZCTA there may exist several ZIP Codes; likewise, within the boundaries of a single ZIP Code, there may exist more than one ZCTA.
Some addresses included in the census and used to define ZCTAs (typically in rural areas) have incomplete or, in some cases, no mailing ZIP Code, thus making it difficult to determine the full extent of the relationships between ZCTAs and ZIP Codes.
Dedicated PO Box ZIP Codes present a special challenge for ZCTA delineation. While addresses with these ZIP Codes generally cluster around a post office, they may also be widely scattered. Additionally, there may be the option of regular/street delivery present in the areas around these post offices, resulting in a mix of valid USPS ZIP Code options for residents.
For ZCTA delineation, the Census Bureau attempted to isolate those PO Box Only ZIP Codes for which there was a relatively compact, well-defined cluster of census blocks where the majority of residents chose or were required to receive their mail via PO Box delivery. For areas where the households receiving their mail via PO Box delivery are more widely scattered and intermixed with households receiving mail via regular/street delivery, the delineation process gives preference to the street delivery ZIP Code.
The ZCTA delineation process included both residential and commercial addresses. A ZIP Code representing a commercial district may appear as a ZCTA if the ZIP Code serves the majority of the addresses for the census blocks in the area. For the 2000 Census, individual firms or organizations with their own ZIP Codes were automatically excluded and did not become ZCTAs. 2,523 of these unique ZIP Codes were excluded from Census 2000 ZCTA delineation. However, for the 2010 ZCTA delineation these addresses were included in the universe, to include many group quarters, and other businesses and institutions with such large mail volumes that they have been assigned a unique ZIP Code by the US Postal Service.
This was a convention used in the Census 2000 ZCTAs, but was not used for the 2010 ZCTAs. The three-digit ZCTA code and an "XX" suffix are applied to a large land area (generally larger than 25 square miles) where we had insufficient information to determine the five-digit codes. The USPS may not provide five-digit ZIP Code delivery service in this area. These are generally rural areas with little settlement; for example, parks, forest lands, and desert and mountainous areas. Smaller areas were generally absorbed into adjacent five-digit ZCTAs, but for larger areas, we did not want to overextend the surrounding five-digit ZCTAs.
For Census 2000, the requirements for ZCTA delineation state that all census blocks receive a ZCTA code. Because large water features may border many five-digit ZCTAs, it would be difficult to divide up the water features and assign the parts to the five-digit ZCTAs on the land. Furthermore, any attempt to add these water features to land-based ZCTAs would greatly distort the size and shape of the land ZCTAs, particularly in coastal areas. To avoid these problems, such water features receive ZCTA codes ending in "HH" (special code for hydrographic features).
For the 2010 Census, there were no “XX” or “HH” ZCTAs assigned. Only five-digit ZCTAs were produced, and large unpopulated areas were excluded from the delineations. For the 2010 Census, there is no wall-to-wall national coverage for ZCTAs.
The requirements for the Census 2000 ZCTA delienation state that all census blocks receive a ZCTA Code. Because large water features may border many five-digit 2000 ZCTAs, it would be difficult to divide up the water features and assign the parts to the five-digit 2000 ZCTAs on the land. Furthermore, any attempt to add these water features to land-based ZCTAs would greatly distort the size and shape of the land ZCTAs, particularly in coastal areas. To avoid these problems for the Census 2000 delineation, such water features receive ZCTA codes ending in "HH" (special code for hydrographic features). Note that a single code could not be used for all water areas for data processing reasons.
This classification scheme was not implemented for the 2010 ZCTA delineation. Large water bodies were excluded from the delineation in order to avoid distortion of the 2010 Census ZCTAs.
After Census 2000, efforts to improve the spatial accuracy of the TIGER database have and continue to add land area such as small islands or sections of shoreline to census blocks that were entirely water in Census 2000. Census 2000 tabulation census blocks once consisting entirely of water features may now contain a mix of water and small land features. As a result, "HH" ZCTA codes may no longer represent purely water areas.
This classification scheme was not implemented for the 2010 ZCTA delineation. Large water bodies were excluded from the delineation in order to avoid distortion for the 2010 Census ZCTAs.
ZCTAs are approximations of mailing ZIP Codes and should not be confused with them. There are some common reasons why someone's ZIP Code may not match the ZCTA code:
Based on the January 2000 list of ZIP Codes from the USPS's Delivery Type File, the Census 2000 ZCTA delineation process excluded 10,068 ZIP Codes in the United States and Puerto Rico (not counting overseas military ZIP Codes). These included 2,523 ZIP Codes that served specific companies or organizations with high volumes of mail and 6,419 ZIP Codes dedicated to PO Box and/or general delivery addresses primarily located in areas otherwise served by rural route or city style mail delivery. The remainder represents ZIP Codes that were either nonactive or insufficiently represented in the Master Address File (MAF) and therefore did not become ZCTAs.
ZCTAs almost always exist within a single state but are independent of all other statistical and governmental entities except census tabulation blocks. While data from the USPS may indicate that a given governmental entity is serviced by a single ZIP Code, that should not be taken to indicate that every household in that entity is serviced by that particular ZIP Code, nor that the indicated ZIP Code services only those addresses located within the boundaries of that governmental entity.
Final Census 2000 ZCTA statistics show that 27 percent of Census 2000 ZCTAs cross county or county equivalent boundaries nationwide. There are 47,010 unique Census 2000 ZCTA-county combinations (32,038 are five-digit Census 2000 ZCTAs). Of these five-digit Census 2000 ZCTAs, 42 cross state boundaries.
The TIGER/Line Shapefiles provide geographic descriptions of the legal and statistical entities included in census tabulations. By overlaying different layers from the TIGER/Line Shapefiles, data users can generate a wide range of ZCTA-to-block or ZCTA-to-entity relationship data. In addition, the Census Bureau provides a Census 2000 ZCTA-to-Metropolitan Statistical Area (MSA) relationship file for download, and several 2010 ZCTA-to-2010 Entity files.
Discontiguous parts of ZCTAs may represent offshore islands. In other situations, they may result from using census blocks to build ZCTAs. USPS carrier routes tend to extend out along the local street network. Addresses along a street may have one ZIP Code while addresses along the cross streets have another. If a block is characterized by addresses with the ZIP Code of the cross streets it may isolate another block characterized by addresses of the first ZIP Code, and it becomes a discontiguous area. Joining the fragments by changing the ZCTA code for intermediate blocks could degrade the match between ZIP Code and ZCTA. Rather than degrade this match, the ZCTA coverage retains some discontiguous fragments.
The USPS may assign several three-digit ZIP Codes to the areas around some cities. The five-digit ZIP Codes within these areas are distributed across the area and do not form specific three-digit clusters. To identify these areas see the USPS maps of three-digit ZIP Codes, located in the 2000 National Five-Digit ZIP Code and Post Office Directory USPS Publication 65. The maps show certain areas with multiple three-digit ZIP Codes. For an example, see the area around Cedar Rapids, Iowa with the three-digit ZIP Codes 522 and 523.
If the fragmented areas are problematic for your application, you might try to do what the USPS does in the Publication 65 maps and aggregate the fragmented three-digit areas into a single combined area.
Currently, there are no restrictions limiting how large or small a ZCTA can be in terms of either a minimum/maximum number of housing units or geographic area. However, as ZCTAs are comprised of census tabulation blocks, a ZCTA must be composed of at least one census tabulation block. Therefore, ZCTAs reflect the size of the census tabulation blocks in areas where we find addresses with the specified ZIP Code. As a result, ZCTAs may be as small as a few city blocks or cover many square miles.
The downloadable Census 2000 and 2010 Census ZCTA gazetteer files contain the total land and water area (in square meters and in square miles) for each Census 2000 and 2010 Census ZCTA.
The TIGER/Line Shapefiles also include the land and water area for ZCTAs.
You can also get this information from American Factfinder. Use the Geographies tab to select the ZCTAs you are interested in. Then look for table G001, Geographic Identifiers for the appropriate year (2000 or 2010). This table contains both land and water area measurements.
The downloadable Census 2000 and 2010 Census ZCTA gazetteer files contain the latitude/longitude coordinates for each Census 2000 and 2010 Census ZCTA. Coordinates for ZCTA internal points (calculated by considering the latitude/longitude of all bounding features and then creating an area-weighted average point inside the polygon) are also available in the TIGER/Line® Shapefiles and in American Factfinder (use the same instructions as those for finding area measurements, as described in the question above, "Does the Census Bureau have any data available on land per ZCTA?").
Even though the codes may appear the same, the addresses and areas covered by these areas may not be the same. We strongly advise data users who wish to compare 1990 and 2000 data to determine and evaluate any coverage differences that exist before making any comparisons. There are several reasons for this caution:
For the 2003 TIGER/Line® files, the U.S. Census Bureau produced updated national current ZCTAs reflecting USPS ZIP Code changes through October, 2002. The U.S. Census Bureau also adjusted Census ZCTA boundaries to account for new growth and more precise ZIP Code information, and modified some of the generic “XX” and “HH” ZCTAs to reflect expansion of five-digit ZCTAs and to achieve more consistent “XX” and “HH” coverage within and across county boundaries.
“XX” and “HH” ZCTAs were not produced for the five-digit Census 2010 ZCTA product, nor were three-digit ZCTAs produced for the 2010 Census. Five-digit ZCTAs will be released for all Island Areas (U.S. Virgin Islands, Guam, American Samoa, and the Commonwealth of the Northern Marianas Islands) at a later date. The U.S. Census Bureau is not planning another release of ZCTAs until after the 2020 Census.
ZCTAs are generalized approximations of USPS five-digit ZIP Codes. There are no plans to create subdivisions of five-digit ZCTAs that approximate the USPS ZIP+4 sector-segment codes.