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I have been searching for the most recent list of Postal Codes in US but I most of them are two or three years old. I did cross check them with the Google Maps, though most of them them were correct but I can say 5% of them were not. Then it made me thinking from where does Google update it's data about Postal Codes? Any leads?

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Zipcodes ftp2.census.gov/geo/tiger/TIGER2010/ZCTA5/2010 (dated 5th November 2011) - a blog showing you which ones have been updated - maps.huge.info/blog/zip_codes –  Mapperz Oct 1 '12 at 15:30
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Recognize that ZCTA's are not actual USPS zip codes. "ZIP Code Tabulation Areas (ZCTAs™) are a statistical geographic entity produced by the U.S. Census Bureau for tabulating summary statistics from the 2010 Census, first developed for Census 2000...ZCTAs are generalized area representations of U.S. Postal Service (USPS) ZIP Code service areas. They represent the most frequently occurring five-digit ZIP Code found in a given area. Simply put, each ZCTA is built by aggregating 2010 Census blocks, whose addresses use a given ZIP Code." –  RyanDalton Oct 1 '12 at 16:01
    
Further: "The Census Bureau does not have U.S. Postal Service ZIP Code boundary files, nor do we have information or possible sources of such files" –  RyanDalton Oct 1 '12 at 16:05
    
The USPS ZIP FAQ explains how ZIPs may not be areas at all: ZIP Code Type Definitions Standard: It is the established and usual ZIP Code used in a specified area. PO Box™ Only: A ZIP Code that is assigned only to a set of PO Boxes at a particular facility. Unique: Assigned to a company, government agency, or entity with sufficient mail volume, based on average daily volume of letter size mail received, availability of ZIP Code numbers in the postal area, and USPS cost-benefit analysis. Military: A ZIP Code assigned to a US Military installation APO/FPO/DPO. –  neuhausr Oct 1 '12 at 16:53
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1 Answer

One of the biggest misconceptions GIS users have about ZIP codes is that they are a set of polygons that cover the United States--they are not. ZIP codes are a system used by the Postal Service for sorting mail before delivery, and nothing more. If an address receives enough mail, the USPS will just assign them a ZIP code to improve sorting efficiency. Many post offices also have a separate ZIP code just for their P.O. Boxes.

The ZIP Code Tabulation Areas (ZCTAs) are a best-estimate made by the Census Bureau about the spatial delineation of ZIP codes, reverse-engineered from responses to the Bureau's surveys. But it is not exact. I've found significant differences between ZIP codes (from geocoded addresses) and ZCTAs, especially in areas with uneven distribution of postal mail, like dense urban centers, commercial and industrial parks, and very rural areas. Depending on your workflow, this may make ZCTAs inappropriate for further use.

The USPS does make some of their own ZIP code data available, as described in this USPS FAQ. But certainly nothing like a shapefile with national coverage.

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