Represents US Postal Service (USPS) Zip Codes.

The USPS doesn't produce ZIP Code maps. A common way to establish zip codes is by geocoding a file that contains ZIPS and to subsequently interpret where a boundary might exist. One thing to keep in mind is the fact that ZIP codes are postal codes in the United States created by the US Postal Service. The most common misconception is that Zip codes are polygonal regions or areas. People often think of mapping in the US as a hierarchy of ever-subdivided polygonal areas: states, counties, cities, zip codes. If they need higher resolution than a county, they next leap to zip codes because they think of zip codes as polygons. This is not true. "ZIP Codes" are simply collections of addresses that have the same 5-digit code. They usually DO NOT follow legal town/city boundaries and usually DO NOT follow the set of geography defined by the census bureau. They are established for mail carriers that follow routes. For these reasons, any map that purports to show "Zip Code Areas" or "Zip Code Polygons" should not be taken as a precise map showing Zip code locations. It is at best some sort of approximation and most likely is wildly inaccurate in certain regions. The approximations can be useful, but they should not be confused with the real thing. The US Postal Service, of course, doesn't make it any easier to deal with such issues by making it easy to get Zip code information. Zip code information is not available for download via Internet from the US Postal Service. It is best obtained from (of all agencies!) the US Bureau of the Census. For statistical tabulation purposes the Census Bureau has long found it convenient to work with Zip code groupings of population. Zip codes have been so useful that the Bureau embarked on a project to create a standardized map of the US showing the approximate region of coverage of various Zip codes as areas. These areas are known as ZIP Code Tabulation Areas (ZCTAs). The zipcode boundaries inside the City limits were defined by symbolizing properties by their address zip code values and digitizing the boundaries. The zip codes' lines extending outside of the city were not altered by City GIS staff and were provided to ESRI Desktop customers.