- August 21, 2017Uniform Fire Code Proposed Amendments N. -2.6, 2.8, 3.4, 3.7, 4.4, 4.9, and 4.11 - Proposed new Rules: N. In areas without a regular postal route or no mail delivery, ZIP codes may not be defined or have unclear boundaries.
The additional 4 digits help USPS more precisely group mail for delivery.
The basic 5-digit format was first introduced in 1963 and later extended to add an additional 4 digits after a dash to form a ZIP 4 code.
The main issue is discussed above: there simply isn't always a clear geographic boundary for a ZIP code.
The Census Bureau and many other commercial services will try to interpolate the data to create polygons (shapes using straight lines) to represent the approximate area covered by a ZIP code, but none of these maps are official or entirely accurate.
On this site, all ZIP code maps use the ZIP Code Tabulation Areas (ZCTAs) as specified by the United States Census Bureau in 2010 (or newer) and discussed below.
They provide a very close approximation of the area covered by a ZIP code.
For instance, a single ZIP code is used for all US Navy mail.
When ZIP codes appear to be geographically grouped, a clear shape cannot always be drawn around the ZIP code because ZIP codes are only assigned to a point of delivery and not the spaces between delivery points.
In the map, 0 is closer to white and 9 is much more vivid.
It's easy to follow the gradient across each of the zones even though there are a few exceptions (such as the southwest tip of Georgia which uses 39XXX like central Mississippi).
The ZIP 4 code is not required, but it aids the post office in additional sorting of mail.