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Q18: Why am I coming up with gaps in coverage when I try to use TIGER/Line files
to create ZIP Code polygon coverage for an area?

There are a variety of reasons why you're having this problem. First, we
don't have complete ZIP Code coverage in TIGER because we don't have complete
address coverage for the entire country. We only have city-style addresses in
our data base. P.O. boxes and rural route addresses aren't particularly
useful to us for taking the census. We have to use the more traditional
"knocking on the door" approach to insure that we have complete coverage.
This means our address, and therefore ZIP Code, coverage is not good in the
rural areas.

A more basic problem is that ZIP Codes were not developed as polygons by the
Postal Service, but as a way to manage mail carrier routes. This means that
they are really a collection of linear features (carrier routes) at best.
Many, particularly in urban areas, can be encompassed neatly by an imaginary
polygon. That can't be done easily (or at least accurately) in many other
areas, particularly rural. In rural areas there is not necessarily a complete
assignment of all land area to one or another post office. By that I mean, a
ZIP Code is really a point feature (assigned to a post office, really a
building). It may provide some, or no, house delivery service. People can
more or less go the one that is closest or most convenient for other reasons
to get their mail.

The ZIP Code maps produced by mapping companies aren't being made using an
authoritative cartographic boundary file coming from the USPS (in our dealings
with them we haven't found one). They are the result of these companies
making their own judgments and using a variety of sources to come up with an
unofficial map.

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