zlacker

>>voxada+(OP)
To put it in plain mathematical language, ZIP codes are not defined as polygons [0]. The consequence is that performing any analysis with an assumption that ZIP codes are polygons is bound to be error-prone.

0: https://manifold.net/doc/mfd8/zip_codes_are_not_areas.htm

>>jihadj+t7
Yeah. ZIP codes are sets in the abstract-dimensional space of carrier delivery points. I suppose you could think of them as lines, but definitely not polygons.

>>mholt+rf
Zip codes (in the US) are machine readable numbers a mail sorter can use to send a parcel to the right delivery truck for final delivery. In the US, they represent the hierarchy of postal centers with the most significant digit representing the primary hub for a region and the smallest number the actual post office that will be in charge of delivering the letter (or truck if you do the extended post code).

They don't represent geography at all, they represent the organizational structure of USPS.

They work by making the address on a letter almost meaningless. For some smaller population zip codes you can practically just put the name and zip code down and achieve delivery.

>>cogman+ci
I agree that they weren't explicitly meant to represent geography, but implicitly they do, right? Are there cases where this is violated?

In other words, is it safe to assume that for entity in a zip code is less than x distance away from the closest entity in the same zip code?

>>alsodu+us
> less than x distance away

zip codes don't even need to be contiguous. It's a mail delivery route, not a polygon.

There are 5 cases where the assumption is violated:

- Non-contiguous areas

- Zip codes that are a single point (some big companies get their own zip with a single mailbox, e.g. GE in Schenectady, NY is zip 12345)

- Zip codes that are a single line (highway-based delivery routes)

- Overlapping boundaries (since mail routes are linear, choosing a polygon representation is arbitrary and often not unique in space)

- Residents of some zip codes are not stationary (e.g. houseboats)

In short, asking questions about the area of a zip code is a category error - zip codes do not have a uniform representation in space. And we should be highly skeptical of any geospatial analysis that assumes polygons.