Where can I obtain a function that takes a geographic location
expressed in GPS coordinates (Latitude/Longitude in decimal degrees)
as input, and returns the FIPS code for the US county(s) that contains
the point location?

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Request for Question Clarification by tar_heel_v-ga on 11 Dec 2002 12:37 PST

Are you looking for a commercial application that will do this for
you?  If so, do you have a price point that you are looking at?

-THV

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Clarification of Question by kenjin-ga on 11 Dec 2002 13:16 PST

I was hoping for a computer program (or source code) in the public domain.

I've done a project like this. Here's how I did it: 

- Purchase a zipcode database like http://www.zipinfo.com
- look up the nearest zipcode centroid (in longitude/latitude provided
by zipinfo db) thus yielding the associated FIPS

If you need high performance/real-time look-up, I have developed an
efficient method to store this data in-memory.

The ZIP code products I've looked at (including zipinfo) map a 5 digit
zip code to one FIPS code. So, when a ZIP spans more than one county,
errors will occur.

Hmmm, are you sure this is common? Zipcodes are fairly grainual. How
else would Zipinfo list FIPS if a given zip crosses county line? Maybe
this is a sales question for Zipinfo.

Otherwise you need to find some way to look up your coordinates with
nearest "county centroid". Not sure if there are such database
available. Even so I think this is less accurate than zipcode centroid
(zipcodes cover much smaller area than counties.)

USPS zones definitly cross political boundaries.

ZipInfo probably maps to the dominant(weighted by population or
area)FIPS code. I assume that they document their standard.

This isn't an issue of 'accuracy', so much as correctness. An
algorithm is defective if it fails to produce correct results for all
(even infrequently encountered) inputs.

I found what I was looking for at a local university's geography department.