What is a zip code boundary?
A zip code boundary is an imaginary polygon drawn around United States Postal Service (“USPS”) delivery addresses that all share the same five-digit zip code. There are no official USPS zip code boundaries. The USPS only publishes delivery points along each delivery route. Other companies must us cartographic techniques to create the zip code boundaries. This is normally achieved using Voronoi diagrams and a coastline.
Since the zip code delivery routes change from time-to-time, the zip code boundaries also need to change.