How USPS Zip Code Polygons were calculated
My USPS Zip Code area maps are a combination of the Tiger mathematical representation of the US and my AIS USPS Zip+4 based Reverse Geocoder to produce up to date zip code areas. As there are no "official" zip code areas, each company that produces a map showing such a thing has calculated a close approximation of what a zip code area could look like. In my case, I believe my maps are pretty accurate.
To produce USPS Zip Code polygons, the reverse geocoding process was applied to the 16 million Tiger/Line type 'A' records to arrive at an approximation that would be independent of any errors and omissions the Tiger/Line database contains. The assumption was made that towns, cities and counties with a single zip code would be processed as that zip code.