What are ZCTAs?
ZCTA is an acronym for ZIP Code Tabulation Areas. ZCTAs were created by the United States Census Bureau to provide a ZIP Code like unit of aggregation for tabulating summary statistics for Census 2000. They are comprised of Census geographies in an effort to approximate the shapes of actual ZIP Codes. Take a look at the video below and watch Melissa explain ZCTAs to Carl...
Is there that much difference between ZCTA and ZIP Codes?
Yes, there is. Click here for some examples. The ZCTA data contains approximately 32,200 ZIP Codes. By way of comparison, the ZIPboundary.com data contains approximately 42,000 ZIP Codes. Our data has more than 30% more ZIP Codes! There are also significant differences in rosters (ZIP Codes are deleted as well as added) and in the boundaries themselves. Keep in mind that the ZCTAs are based on the 2000 Census which, at this point in time is over 10 years old!
Should I consider using ZCTAs for my project?
That depends on how you intend to use the data. If your application does not require complete or accurate ZIP Code data then ZCTAs may be of value to you. We typically recommend them for prototyping or for non-mission critical maps where the ZIP Code layer is not used with any attribute data.
ZCTAs are similar to but not equivalent to United States Postal Service (USPS) ZIP code service areas. Due to the fact that individual USPS ZIP codes can cross state, county, census tract and other census area boundaries, the Census Bureau tells us that "there is no correlation between ZIP codes and Census Bureau geography." In addition, since USPS ZIP Codes are active service areas they change frequently. In any given month the USPS may report dozens if not hundreds of changes including realignments, merges, or splits, new and deleted ZIP codes to meet ongoing service demands. These changes are usually not reflected in updated ZCTA releases. ZCTAs are constructed by aggregating the Census 2000 blocks whose addresses use a given ZIP code. When creating ZCTAs, the Census Bureau used the ZIP code used by the majority of addresses in each census unit at the time the data was compiled. As a result, some addresses end up with a ZCTA code that is different from their ZIP code. ZCTAs are not developed for ZIP codes that comprise only a small number of addresses nor do they include US PO Box ZIP Codes or unique ZIP Codes. In addition, there are ZCTAs that represent ZIPs that no longer exist due to their discontinuation by the USPS.
A good description of ZCTAs can be found at the US Census ZCTA FAQ. Specifically, from the FAQ:
Is there an equivalency or comparability data product that shows the relationship between Census 2000 ZCTAs™ (ZIP Code Tabulation Areas) and USPS 2000 ZIP Codes?
The Census Bureau is not planning to 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 will continue to 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.