What is the difference between ZIP Code data provided by the USPS for free and that provided by a mapping service?
The USPS does not maintain or release ZIP Code maps; it only releases ZIP Code correspondence information in database format. Commercial vendors then take each ZIP Codes information and attach it to a polygon that represents the dimensional area covered by that ZIP Code. Each polygon is then placed at the correct longitude and latitude within a map framework thereby creating ZIP Code maps. The resulting ZIP Code map data is then integrated with other map software in a GIS system to be used in creating detailed multi-layered ZIP Code-related maps.
