The US Census Bureau provides Keyhole Markup Language (KML) files for every state in the US, containing the boundaries of the counties in that state as of the 2010 census. Files are available as detailed (the exact boundaries) and generalized (boundaries are smoothed and do not represent the exact boundaries). Files are downloadable in the KML format. KML boundary files are also available for Census tracts at this site.
Naming convention for these files:
Type=gz (generalized) or dt (detailed)
Layer= county or tract
FIPS 2-digit state FIPS code
Note that a listing of State FIPS codes can be found at the Environmental Protection Agency website at epa.gov/envirofw/html/codes/state.html.
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