Data Sources and Map Methodology
See exactly where our county boundaries, names, FIPS codes, and area figures come from, how maps are generated, and what the maps should not be used for.
Primary boundary source
All county shapes in the current atlas come from the U.S. Census Bureau Cartographic Boundary Files: 2025 counties, 1:20,000,000 scale. The downloaded archive is named cb_2025_us_county_20m.zip.
Cartographic files are generalized for mapping. They are smaller and clearer at state and national scale than full-resolution TIGER/Line data, but they are not appropriate for parcel, engineering, or legal boundary work.
Fields used
| Displayed item | Source field or method |
|---|---|
| County name | NAME and NAMELSAD |
| County identifier | GEOID (state and county FIPS) |
| Land area | ALAND, converted from square meters and rounded |
| Water area | AWATER, converted from square meters and rounded |
| Boundary geometry | Census shapefile geometry rendered to SVG |
Generation method
The source geometry is processed during development, filtered to the 50 states and District of Columbia, grouped by state FIPS code, and projected into a fixed SVG view box. The national map uses an Albers USA projection; individual state maps use a fitted projection to make efficient use of the page.
Map labels are placed at geometry centroids and shortened by removing a trailing area type such as “County” or “Parish.” The complete legal/statistical area name remains in the table below each map, so a crowded visual label is never the only source of a name.
Update policy
We review Census cartographic boundary releases annually. A new vintage is adopted when it materially improves current geography and passes build and visual checks. Map pages identify their current vintage and site-wide methodology changes are dated.
Limitations
- Boundaries are generalized and should not be used as a legal description.
- The atlas does not show parcel, ZIP Code, voting precinct, municipal, road, or school district boundaries.
- Centroid labels can be tight in states with many small counties; the adjacent table is authoritative for the page.
- Land and water figures are rounded for quick reference.
Reuse and public-domain status
Census Bureau boundary data is a work of the U.S. federal government and is generally public domain. The surrounding site design, writing, and presentation remain subject to this site’s terms.