The United States Postal Service has done a great deal for geographers in the United States by maintaining a very large set of data that can be used to estimate specific geographic locations. However figuring out the location of an address, zip code, zip+4, or mailing city is far from as simple as it seems like it should be, as none of these data points can inherently be used to determine exact location from afar without the help of supplementary data.
A zip code does represent geographic location to a degree, the first digit tells us it is in one of 9 regions of the US, the next 2 digits indicate the facility the mail is routed through, and the last 2 indicate the specific post office the mail will go to for delivery. So knowing a zip code can give us a very general idea of location. But zip codes themselves do not have defined boundaries. The closest we can get to a zip code’s shape is drawing a border around all of the addresses that share a zip code. But this is a limited exercise because the boundaries would be fuzzy, overlap each other, and leave gaps in many areas where zip codes are not assigned (or where no addresses even exist).
It may be more correct to think of a zip code as a label that is attached to each of a group of addresses rather than as a geographic area with a specific boundary. It is not so much that addresses are within a zip code, but rather that addresses have a zip code.
Enjoy this lovely USPS video from the 1960s, explaining the new zip code system: Zip Code PSA
A zip+4 is a five-digit zip code plus four additional digits that were tacked onto the zip code system in the 1980s to allow for even more granular organization of addresses by the USPS. Usually, a relatively small group of addresses will share a zip+4, for example, all of the addresses on the one side of one block of one street. This data can get us much closer to an approximation of specific location. But because several addresses share the same zip+4, in may be correct that a single zip+4 is used by addresses both inside and outside of a city, or in different cities. And while it is likely that all of the addresses that share the same zip+4 will be located right next to one another, it is not always true. The reasoning behind assignment of zip+4s has to do with mail delivery systems, not necessarily location.
As we all know, every letter we ever send is required to have a city included in the address on the envelope. But, as we also know, not every inch of the nation is included within a city. There are large areas of land that are within “unincorporated” portions of counties, and therefore not geographically within any city at all. The city names that are used on envelopes are known as “mailing cities” and do not share a one-for-one relationship with incorporated municipalities and all that they represent. Mailing cities are used by the USPS to aid in the delivery of mail. Just because you are sending a letter to “Widefield, CO” you cannot assume that the address on your letter is included inside the city limits of Widefield or even that Widefield has an incorporated city government and defined city limits!
Curious about specific addresses and how they can be used to determine location? Well, that’s for another time, keep your eye out for my discussion of geocoding!