Anthony wrote:
>
> The actual areas are basically only useful for reverse geocoding
> (click a spot on the map and get the postal code). But whether or not
> that's even possible is highly dependent on whether or not the post
> office provides such information. For some post offices, such
> information is not meaningful. What is the postal code for the middle
> of a highway? Maybe there is one defined, which represents what the
> postal code would be if there were a post box there. But maybe there
> isn't. It depends on the post office.
>
In the US, ZIP codes are really sets of points (addresses that get
delivered to), not geographies.
There are commercially available ZIP code boundaries that are made
by a process of assigning plausible shapes that fit the points known to
be in the ZIP codes. I suppose these are done with alpha shapes or
maximum-margin or some similar algorithm.
There's a lot of demand for ZIP code-indexed data in the US because
everybody is familiar with them, and because they're roughly on the
scale of marketing activities: you're likely to drive to stores that
are in your ZIP code or nearby ZIP code. On the other hand, the more I
learn about ZIP codes the more I learn how bad they are: for instance,
ZIP codes are insufficient to determine a person's congressional
district, commonly cross counties, and I'm aware of at least one ZIP
code that spans two US states. ZIPs are also too big to do effective
geodemographics: for instance, the ZIP code 14850 (most of "Ithaca,
NY") contains some very rich neighborhoods, some very white
neighborhoods, and also some neighborhoods that are poor and minority.
There are good commercial databases, however, that give geodemographic
profiles at the census block or individual household level: enough that
you could find out that the most of the 'rich' people in 14850 are
college professors who don't spend ostentatiously so they wouldn't
support a Nordstrom's or a Jaguar dealership.
Personally I like the TIGER county shapes for spatial control in the
US. These are accurate and tile nicely and, I find I that the union of
several counties is generally a good proxy for the kind of 'semantic
regions' that I work with... For instance, even if I'm targeting a
city, the noosphere density falls off so much in the suburbs that the
county is an effective boundary: and if something out in the 'burbs has
that much 'interestingness' it's probably semantically associated with
the city anyway.
I'm currently establishing a spatial control system for the world
and it's probably going to be based on second-level administrative
divisions, though I've got good third-levels for a lot of interesting
places.
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