Zip Code Tabulation Area

Terms: 
zip code tabulation area
zcta

Generalized area representations of U.S. Postal Service (USPS) ZIP Code service areas. Simply put, each one is built by aggregating the Census 2000 blocks, whose addresses use a given ZIP Code, into a ZCTA which gets that ZIP Code assigned as its ZCTA code. They represent the majority USPS five-digit ZIP Code found in a given area. For those areas where it is difficult to determine the prevailing five-digit ZIP Code, the higher-level three-digit ZIP Code is used for the ZCTA code. ZCTAs do not precisely depict the area within which mail deliveries associated with that ZIP Code occur. A five-digit ZCTA ending in "HH" (e.g., "006HH") represents the water area within a three-digit ZCTA that is not associated with any mail delivery route. A five-digit ZCTA ending in "XX" (e.g., "006XX") represents the land area within a three-digit ZCTA that is not associated with any mail delivery route.

Source and for further information:  U.S. Census Bureau http://www.census.gov/geo/ZCTA/zcta.html