Census offers county adjacency files for identifying which counties are neighbors.
The fipscounty variable in these files can be linked to Census SF1 2010 880 5-Digit ZIP Code Tabulation Area-State-County sf12010880us1 SAS, Stata, or CSV files by creating a fipscounty variable from the state and county variables in the SF1 files.
ZIP Codes are created by the U.S. Post Office for mail delivery and a small percentage may cross state lines. ZIP Code Tabulation Areas (ZCTAs) are representations of ZIP Codes made by Census.
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