Data Sources and Map Methodology

Sources, scales and methods for our 2025 Census county boundaries, labels, map downloads, area statistics, neighboring counties and optional map backgrounds.

Boundary sources and scales

The atlas uses the U.S. Census Bureau’s 2025 Cartographic Boundary Files, filtered to the 50 states and District of Columbia.

The more detailed county source retains smaller bends and coastal features. All three sources are generalized cartographic files; none should be used to establish property lines, legal jurisdiction at a specific address, or engineering measurements.

Names, identifiers and areas

Names use the Census NAME and NAMELSAD fields. GEOID supplies the five-digit state-and-county FIPS code. Land and water area use ALAND and AWATER, converted from square meters using 2,589,988.110336 square meters per square mile and rounded to one decimal place. Rankings compare land area. Summed and displayed figures may differ slightly because of rounding.

The atlas follows current Census county-equivalent geography, including Connecticut planning regions, Alaska boroughs and census areas, Louisiana parishes, and independent cities. A historical county name may therefore differ from the current statistical geography.

Projection, labels and colors

The national map uses an Albers USA projection. Most state maps use fitted Mercator projections; Alaska uses a conic equal-area projection to keep the Aleutian chain together. Individual outlines use a fitted Mercator projection centered on the county. These are reference drawings without a fixed display scale.

State labels use an interior point in the largest suitable polygon, rather than a centroid that might lie outside a boundary. On screen, overlapping or very small labels are hidden and reappear as space permits when zooming. The searchable county list always contains every name. Downloaded state maps retain every county name, using numbered callouts and a key for crowded areas; numbered maps and their CSV answer keys are useful for dense states.

Colors distinguish neighboring county shapes. They do not represent population, political affiliation, or any measured value. County selection is also marked with a heavier boundary.

Neighboring counties

Neighbor links are derived from line segments shared by two county geometries in the 1:500,000 source. Point-only contacts are excluded. Generalized coastline geometry may omit water-separated relationships, so this is a reproducible map-based adjacency list rather than a legal determination of every neighbor.

Optional background maps

The interactive viewer loads backgrounds only after a visitor chooses a view. Streets come from OpenStreetMap contributors. Topography comes from USGS The National Map and its contributors. Aerial and satellite imagery comes from the USGS Imagery Only service, including USDA NAIP, NASA and SPOT sources.

Backgrounds have different collection dates and detail levels, and imagery is not live. Coverage and availability depend on the provider. Alaska imagery includes licensed material. Provider attribution remains visible in the viewer. These backgrounds are not included in our downloadable boundary maps.

Downloads and reuse

SVG files are generated from the same boundary data as the on-screen atlas. PNG files are rendered at 3,000 pixels wide in the browser. PDF files preserve vector paths on landscape Letter pages; crowded labelled maps place the name key on a second page. Exports show the complete map regardless of screen zoom. State CSV files provide the alphabetical county list, numbered map key, FIPS identifiers, and land and water areas.

Census source data is generally public domain. Our downloadable map artwork can be reused under the map reuse terms. Site writing, branding, and external background layers have separate terms.

Updates and corrections

Map generation and methodology were updated September 27, 2026. We review Census cartographic releases annually and check names, links, downloads, and map behavior before adopting a new vintage. Report a correction with the page URL and supporting source.