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Q17: Does the ZIP Code file you have available for downloading contain a complete and
up-to-date list of ZIP Codes?

The ZIP file doesn't include all ZIPs because it was based only on those areas
for which we had city-style addresses (which leaves out a lot of rural
areas). The file was created as a byproduct of another operation (a data
product based on the ZIP codes we collected with the 1990 Census data). For
this Census data product, we took advantage of the fact that we had some
ZIP Code data as a consequence of trying to collect addresses for the Census
questionnaire mailout so we published data for those ZIP Codes. This ZIP
internal point file is basically the lat/long for the ZIPS in that product.
We put it on the Web in case anyone found it useful. It was not intended to
be an authoritative source on ZIP Codes.

Note that the task of creating a lat/long or polygon file of all ZIP Codes not
as easy as it seems since ZIPs are not designed to be polygons and can't
easily be forced into them - particularly in rural areas. To our knowledge
there are no official internal point or polygon files available.

For further information on ZIPs here are sites that may be of interest.

The first is at Lawrence Berkeley Labs:

The second is the Post Office:

Next: Q18: Does the Census Bureau produce ZIP Code maps?
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