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Comment by macintux | Hacker News Reader

  - Zipcodes have defined areas
  - Zipcodes are areas
  - Zipcodes are stationary areas

Can you elaborate on these? I always assumed (thankfully not for any professional purposes) there was indeed an area assigned to each zip code.

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Military zipcodes often do not have a defined area - as military units are mobile. The zipcode may direct mail to varying actual physical places.

Certain extremely high-traffic entities (e.g., government agencies) may also receive their own zipcodes - and mail directed at them may go to different places. Private places (see: Empire State Building, 10118) may also receive their own zipcodes which, while representing a physical building, defies the usual "defined boundary" zipcode definition.

It's worth noting that the zip-for-building case is very common, most cities with skyscrapers have a few with their own zip.

It still definitely counts as an edge case though, in the sense that if the building were torn down or foreclosed I don't think its block would still warrant its own code

Indeed, and it's important to remember that skyscraper-zipcodes create holes in otherwise regular polygons, which is often not accounted for when storing shapes of zipcodes.

Lots of edge cases abound when using zipcodes, especially zipcodes in a spatial context.

> It still definitely counts as an edge case though, in the sense that if the building were torn down or foreclosed I don't think its block would still warrant its own code

For example, the World Trade Center's zip code was retired after the 9/11 attacks.

https://en.wikipedia.org/wiki/10048_(ZIP_code)

In Singapore, every building of a reasonable size has its own zip code

Zipcodes in the United States are postal routes, which aren't really polygons like cities. We can make an approximation, but that's not how they're defined. In some countries, point postcodes are common. We just spent a bunch of time on postcodes in Great Britain, which commonly only have a few addresses in each one. Even in the Unites States, some universities or other large institutions will have a "postcode" which is a single address. The US military also assigns postcodes to ships, which tend to move a bit more than your average building.

As others have mentioned, zip codes might not have any geometry at all (point zips), and they are frequently updated/changed. In addition, zips do not follow standard constraints that other defined region types do. For instance, counties are entirely contained by their parent state, but that is not the case for zip codes, as they frequently cross county and state borders.

My understanding: Zipcodes are postal routes. We just typically visualize them as the box containing those routes.

USPS maintains them as lists of addresses (with some sort of relationship to delivery routes). A new address will be assigned a zip code based on convenience.

So at a given moment you can generalize those lists into areas, but the way they are defined, there isn't a real boundary, just a line that happens to separate the addresses with different postcodes.

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