[paulsmith]

This is a fact of ZIP Codes that a lot of people stumble one. I've worked on GIS/mapping projects in the past where stakeholders wanted or assumed ZIP Codes to be polygons.

Another complexity that surprises folks is you can't guarantee a one-to-many state-to-ZIP Code relationship. There are several (I forgot offhand how many, I used to have them memorized) that span across state boundaries.

[y-c-o-m-b]

Yep, this fact eluded me earlier in the year. I was supposed to map out all ZIP codes in the US and color their boundaries based on certain stats we had. We were surprised to find many areas in the US were empty because they didn't have ZIP codes. I did a quick search and found out ZIP codes are driven by mail routes, and that instantly made sense to me, but the product stakeholders were very surprised to learn it.

[paulsmith]

One thing I just recalled is that if you maintain a small exceptions lookup table (i.e. the ones that span state boundaries), you can use ZIP Codes as a way to uniquely look up a county name.

[graywh]

ZIP codes also span county boundaries that aren't state boundaries -- I know of several in my county alone

[mulmen]

Why would you do this?

[paulsmith]

For example, health care plans in the US are county-specific with regard to premiums, co-pays, etc. (based on demographics). Allowing someone to type in their ZIP Code to get started can be a better user experience than having them pick their county.

https://www.healthcare.gov/see-plans/

[mulmen]

Except when it isn’t. I’d be curious to know the population in areas where a zip code spans multiple counties.

[paulsmith]

In that case, each county that corresponds to that ZIP Code is shown and the user can disambiguate manually.