Why are there instances of fragmented or discontiguous three-digit ZCTAs?
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.
