What should I know about using zip codes in geocoding?
While a lot of health and environmental data is geocoded to and presented at a zip code level, zip codes have a large number of limitations. First and foremost, zip codes are not areas and their exact spatial boundaries are not generally known. Most people think of zip codes as sub-divisions of their cities or counties, but in fact zip codes are simply mail delivery routes. When you see zip code boundaries on a map, those are merely approximations, usually based on a set of addresses where mail is being delivered. Data vendors have various techniques to estimate zip code boundaries. According to the USPS, “The ZIP Code system was created and designed to provide an efficient postal distribution and delivery network. ZIP Code assignments are, therefore, closely linked to factors such as mail volume, delivery area size, geographic location, and topography, but not necessarily to municipal or perceived community boundaries.” The first digit of a zip code divides the US into large regions.
