Andy Berkvam - 2008-09-19

  I did some research to figure out discrepancies between this list and an existing list of Zip codes that we had.  The discrepancies mostly boil down to the fact that this is not a Zip code database, it's a ZCTA database.  There is an FAQ about ZCTAs at http://www.census.gov/geo/ZCTA/zctafaq.html.  The important bit is:

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Is there a corresponding ZCTA for every for every United States Postal Service ZIP Code?

Based on the January 2000 list of ZIP Codes from the USPS's Delivery Type File, the ZCTA delineation process excluded 10,068 ZIP Codes in the United States and Puerto Rico (not counting overseas military ZIP Codes). These included 2,523 ZIP Codes that served specific companies or organizations with high volumes of mail and 6,419 ZIP Codes dedicated to Post Office (PO) Box and/or general delivery addresses primarily located in areas otherwise served by rural route or city style mail delivery. The remainder represents ZIP Codes that were either inactive or insufficiently represented in the MAF and therefore did not become ZCTAs.

New ZIP Codes created by the USPS since January 2000 will not have a corresponding Census 2000 ZCTA. The 2003 TIGER/Line® Files will contain updated ZCTAs which (while maintaining the restrictions mentioned above) will correspond to the October, 2002 list of ZIP Codes from the USPS's Delivery Type File.

  Also of note is the question about the XX and HH suffixes:

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What do the ZCTAs with an "XX" or "HH"suffix represent?

The three-digit ZCTA code and an "XX" suffix are applied to a large land area (generally larger than 25 square miles) where we had insufficient information to determine the five-digit codes. The USPS may not provide five-digit ZIP Code delivery service in this area. These are generally rural areas with little settlement; for example, parks, forest lands, and desert and mountainous areas. Smaller areas were generally absorbed into adjacent five-digit ZCTAs, but for larger areas, we did not want to overextend the surrounding five-digit ZCTAs.

The requirements for ZCTA delineation state that all census blocks receive a ZCTA code. Because large water features may border many five-digit ZCTAs, it would be difficult to divide up the water features and assign the parts to the five-digit ZCTAs on the land. Furthermore, any attempt to add these water features to land-based ZCTAs, would greatly distort the size and shape of the land ZCTAs, particularly in coastal areas. To avoid these problems, such water features receive ZCTA codes ending in "HH" (special code for hydrographic features).

  For many people, the information in this project will be adequate.  However it would be nice to add a note about the difference between Zip codes and ZCTAs to the documentation.

Thank you