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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: [gislist] Zip Code Boundaries |
| Date: |
10/20/2003 09:50:01 AM |
| From: |
Dimitri Rotow |
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> I am trying to align Zip Code Boundaries in North Carolina to actual
> streets, rivers, etc. Do zip codes follow the same rationale as census
> tracts? I know that the boundaries change frequently, but I cannot find a
> free source that shows the accurate physical locations. HELP!
>
A free source of zip codes shown as area features is the Census ZCTA
collection.
Zip codes are linear features aligned to postal carrier routes on existing
streets. An essay (with a very useful illustration) explaining why zip
codes are not polygonal area features is at (watch the line wrap!):
http://exchange.manifold.net/manifold/manuals/5_userman/mfd50Zip_Codes_are_N
ot_Areas.htm
Because zip codes are linear features that are usually intermingled with
each other, the creation of any particular representation of zip codes as
polygonal area features (which requires interpolation and grouping of linear
features into more-or-less arbitrary area constructions) is highly
subjective and unlikely to align to any other such representation.
ZCTA's are evolving as the standard simply because if one must choose from a
wide range of more or less arbitrary representations, one may as well choose
the version offered for free by the Census Bureau, especially since that
particular version will be coordinated with demographic data compiled by the
Census Bureau. However, I don't know if the construction rules for ZCTA's
included alignment of the synthesized boundaries to any pre-existing area
boundary features, such as census tracts.
It's true, by the way, that the Census ZCTAs include coverage over water as
well as land. However, this points out the inherent logical disconnect in
attempting to represent linear postal route features (zip codes) in the form
of areas. There are no postal routes that require postmen to walk on water,
so intrinsically such parts of the ZCTAs are make-believe, just like those
parts that purport to tile empty desert or inaccessible mountain terrain
with zip codes.
Cheers,
Dimitri
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