US Census Bureau ZCTA-to-County Relationship File
Postal code -> state (and county), the resolution key every other source in this list is joined through.
- License class
- P — public domain / open US Government work, public domain.
- Attribution
- Not legally required — every figure derived from it still carries a source and date stamp.
- Fetch cadence
- Annual (Census releases a new vintage roughly once a year)
- Last fetched
- pipeline/normalize/data/zip-state.csv (retrieved_at stamp)
Worth knowing
- This project resolves the question from a static relationship file rather than a live geocoding API call, because it only needs a postal code's state, not a full street-address lookup.
- A postal code spanning a state line is reported, not silently resolved -- the runner-up county is kept so a coverage report can say how often the choice was close.
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