Why do some ZCTAs form discontiguous areas?
Discontiguous parts of ZCTAs may represent offshore islands. In other situations, they may result from using census blocks to build ZCTAs. United States Postal Service (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 a 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.
