Given how postcodes are supposed to work, this is what I would expect. Curious if many of the polygons in the data set contain more than one polygon and or if there is any overlap
> how postcodes are supposed to work
Not sure under what basis you think they are "supposed" to be a certain way but postal codes originated as a signifiers of areas, even in the UK.
US postal codes absolutely have overlap in area, because they do not divide land, they direct delivery routing.
Different places have things like postal codes that mean different things, but in the US postal codes are tied to mail delivery routes.
https://web.archive.org/web/20030119031055/http://users.rcn....
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