Postal code layer approach(es)

[ianthetechie]

I've been mulling over what a postal code layer could look like and wanted to loop in others to get some feedback.

I'm aware of at least 2 ways of representing postal codes in OSM:

• Boundary relations (or ways, maybe? multipolygon features in GDAL terms) - these are the easy ones... unfortunately, they are also the minority 😂 Most of the world does not define postal codes in terms of geographic boundaries, but rather assigns postal codes to buildings based on delivery route or some similar logic.
• On features with addresses, via the addr:postcode property.

The question is what / how do we slice this up in Layercake. My suggestion to start the discussion is:

1. Create a postal code boundaries layer that only includes valid boundary=postal_code (multi-)polygons.
2. Leave it to data consumers if they want to derive postal areas, Voroni polygons, or centroids (i.e. what Nominatim does) from point data can get this from the address layer (Address layer #10).

I suspect that doing something like computing postal code centroids will be enough work, and not general-use enough that the OSM US might not want to run such jobs on its servers, but maybe I'm wrong :)