ZIP Codes versus ZCTAs

A short history of ZCTAs

Beginning with the 2000 Census, the US Census Bureau compiled and released a new set of geographic areas called ZIP Code Tabulation Areas (ZCTAs) intended to align census data tabulation to ZIP Code areas.  The 1990 Census reported data directly by ZIP Codes but because ZIP Codes are determined by the US Postal Service (USPS) and are not constructed from the same building blocks as the US Census, ZCTAs were created in an attempt to reflect ZIP Codes as much as possible using Census geography.

ZIP Codes are determined by the USPS, not the US Census.  ZIP Codes are defined by sets of postal addresses (Maponics uses this data to draw accurate polygons around ZIP Codes to form geographic boundaries) whereas the fundamental component of ZCTA geography is the Census block level.  Census blocks are formed by streets, roads, railroads, streams and other bodies of water, other visible physical and cultural features, and the legal boundaries shown on Census Bureau maps.  

Important Differences between ZCTAs and ZIP Codes

In the 2000 Census, more than 10,000 ZIP Codes were excluded from the process used to create ZCTAs.  The excluded ZIP Codes represent a number of different scenarios, including ZIP Codes for companies or dedicated high mail volume sites, post office boxes, and inactive ZIP Codes.

ZCTA follows the same 5-digit ZIP Code format but for ZCTAs where there is insufficient information to determine the corresponding five-digit code, the US Census uses a 3-digit format and appends a generic code to the end.  There are more than 1,000 ZCTA that do not include the full 5-digits corresponding to an actual ZIP Code.   

While the US Census attempts to align ZCTAs to ZIP Codes as much as possible, they also clearly state that addresses with one ZIP Code may be assigned to another ZCTA.  In fact, on their ZCTA FAQ page, the Census includes questions such as: “Why does the census block where I live have a different ZCTA than my ZIP Code?”

Why Choose ZIP Codes

There are several key reasons to choose an accurate and consistent source of ZIP Code Boundaries instead of ZCTAs.   Here are a few:

1. ZCTAs are only updated as part of the decennial Census and even the 2010 ZCTAs will not be availability until the middle of 2011 whereas Maponics ZIP Code Boundaries are updated and released every quarter and reflect the latest changes made by the USPS.
   
   
2. Because ZCTAs are only updated with the decennial Census, they do not reflect any changes that the USPS makes to ZIP Codes related to the optimization of mail delivery and for things like post office moves or consolidations.
   
   
3. According to US Census website, it is possible for addresses of the same ZIP Code to be assigned to different ZCTAs.  This can happen for several reasons, but one important one is that ZCTAs are based on a set of complete Census blocks and ZIP Codes do not always align to Census blocks.  Since the US Census does not release a cross-reference file that correlates ZCTAs to ZIP Codes, it is not possible to tell exactly how many households are misrepresented.
   
   
4. Because ZCTAs and ZIP Code boundaries do not fully align, direct mail and marketing campaigns that are built around Census demographics cannot be fully reconciled to ZIP Code-based data.