# Data Sources and Licensing

The data bundled with `uszipinfo` is derived entirely from US government
public-domain sources. These sources are freely usable, modifiable, and
redistributable without restriction.

## Sources

### US Census Bureau — American Community Survey (ACS) 5-Year Estimates

Provides demographic, economic, housing, and education data at the ZIP Code
Tabulation Area (ZCTA) level.

- Source: https://www.census.gov/programs-surveys/acs
- License: Public domain (US government work)

### US Census Bureau — Gazetteer Files

Provides geographic identifiers including latitude, longitude, land area,
and water area for each ZCTA.

- Source: https://www.census.gov/geographies/reference-files/time-series/geo/gazetteer-files.html
- License: Public domain

### US Department of Housing and Urban Development (HUD) — USPS ZIP Code Crosswalk Files

Provides ZIP-to-county and ZIP-to-CBSA mappings with population-weighted
ratios for ZIPs spanning multiple administrative areas.

- Source: https://www.huduser.gov/portal/datasets/usps_crosswalk.html
- License: Public domain

### Office of Management and Budget (OMB) — CBSA Delineations

Provides metropolitan and micropolitan statistical area definitions and
their hierarchical relationships (Metro/Micro classification, CSA membership).

- Source: https://www.census.gov/geographies/reference-files/time-series/demo/metro-micro/delineation-files.html
- License: Public domain

## Notes

- USPS authoritative ZIP type classifications are NOT redistributable.
  This package derives ZIP types heuristically from population, area, and
  other public signals. The `zip_type` field is documented as approximate.

- All data is refreshed annually following the December ACS release cycle.

- Build pipeline source code is included in this repository for full
  transparency of data lineage.
