Zip Code Boundary Maps

In 2005, I wrote one of my first custom mapping projects using the recently released Google Maps API. It was a fairly crude map that used the Census Tiger ZCTA's (Zip Code Tabulation Area) to display a representation of a Zip Code area. Matt Cutts, at the time, a major player at Google in search, wrote a blog post called "Fun with Zip Codes" that caused my site to get an average of 70,000 unique visitors a day for about a week.

The traffic that Matt's blog post caused made me consider that there could be a market for this type of site so I looked deeper into the issue and learned a number of interesting things.

The most important thing I learned was that Zip Codes are not areas. They are delivery routes. The USPS draws these delivery routes based on the efficient delivery of mail and nothing else. They can cross city, county and even state lines if it means the mail gets delivered efficiently.

Not every address in the US has a Zip Code. This is not obvious for people living in populated areas but for those folks living in the rural USA, they know all about this. Rural and low population areas have P.O. boxes and other ways of getting their mail instead of direct to home delivery that most of the population enjoys. So for these people, they don't technically live in ANY Zip code. An important factoid there.

The Census ZCTA is not a Zip Code area. It is an artificial representation of a Zip Code laid out for the sole purpose of tabulating Census data. Not all Zip Codes have corresponding ZCTA's. There are quite a few, thousands in fact, of Zip Codes that have no ZCTA.

Some Zip Codes represent a single point, like a building, block or an artificial construct such as the Zip Code used to mail tax returns. These Zip Codes cannot even be imagined as areas.

People and companies rely on the imaginary fact that Zip Codes are areas though. Virtually everyone imagines that they live in a particular Zip Code when in fact, this is absolutely untrue. Nobody lives IN a Zip Code. They live on a Zip Code route. A very important distinction.

So how is it that numerous websites, including our own show Zip Codes as areas?

The process of turning a delivery route into an area is more of a black art than science. Each company that builds these areal representations of delivery routes makes certain assumptions in their algorithms that are unique to their company. The resulting polygons are as different as fingerprints. No two would be the same, unless one company pilfered the data from another.

Our site uses a set of polygons we created using an algorithm I invented. The algorithm makes some assumptions about where the geographic boundaries for a particular Zip Code would be based on about a dozen factors, and then uses interpolation to get smooth polygons that border on roads or significant geologic and political boundaries like rivers, counties and states.

Users of our site might find differences between our depictions and other depictions and that is absolutely normal and correct. Who is right? The answer is simple. Nobody has the perfect or official Zip Code boundary map. Not even the United States Postal Service.

Here is a pretty good summary from Wikipedia:

"Despite the geographic derivation of most ZIP Codes, the codes themselves do not represent geographic regions; in general, they correspond to address groups or delivery routes. As a consequence, ZIP Code "areas" can overlap, be subsets of each other, or be artificial constructs with no geographic area (such as 095 for mail to the Navy, which is not geographically fixed). In similar fashion, in areas without regular postal routes (rural route areas) or no mail delivery (undeveloped areas), ZIP Codes are not assigned or are based on sparse delivery routes, and hence the boundary between ZIP Code areas is undefined. For example, some residents in or near Haubstadt, Indiana, which has the ZIP Code 47639, have mailing addresses with 47648, the ZIP Code for neighboring Fort Branch, Indiana, while others living in or near Fort Branch have addresses with 47639. Many rural counties have similar logistical inconsistencies caused by the aforementioned sparse delivery routes, often known as Rural Routes or by some other similar designation."


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