Agent Guidance, United States Counties (2023, 1:500k)

County polygons whose value is the GEOID column, the five-digit FIPS key to US county statistics. Join ACS and decennial downloads on GEOIDFQ instead, it matches their GEO_ID column verbatim. COUNTYNS is the identifier that survives FIPS changes across years.

Query the GeoParquet data asset in place with DuckDB spatial, read_parquet('us-counties.parquet'), or load it with GeoPandas. It streams over HTTP range requests, so query the published URL directly rather than downloading first. For rendering use the visual PMTiles asset with its MapLibre styles.

Quirks

• GEOID is a zero-padded string. Cast it to integer and Alabama's 01001 becomes 1001, silently unjoining every state below FIPS 10. CSV round-trips are the usual culprit.
• NAME is wildly ambiguous, 31 Washingtons, and independent cities collide with the counties that share their name. Baltimore city is 24510, Baltimore County is 24005. The lowercase word city in NAMELSAD is what marks an independent city.
• Connecticut has nine planning regions, GEOIDs 09110 to 09190, LSAD PL, since the 2022 change. Datasets keyed to the old 09001 to 09015 match nothing here.
• Aleutians West Census Area crosses the antimeridian, so its stored bbox spans nearly all longitudes and it passes almost every naive bbox filter. Handle it before bbox-windowing the world.
• Geometry is clipped to shoreline but AWATER is not, so San Francisco's polygon covers 122 km2 while ALAND plus AWATER is 601 km2. Offshore points in legal county water fall in no polygon.

Coordinate Reference System

EPSG:4269, NAD83, a geographic coordinate reference system whose coordinates are in degrees. Planar distance and area functions return degrees and square degrees, which are not ground units and vary with latitude. For real distances and areas use a sphere or spheroid function, or transform to a projected CRS first. The data asset carries the same code as proj:code.

The spheroid function works here once geometry_always_xy is set. Without it 1,646 of the 3,235 counties return NaN, which reads like a geometry problem and is not. Transforming to an equal-area CRS is still the better habit, since it leaves you somewhere you can measure, and the query below does that.

For web mapping, treating the NAD83 coordinates as WGS84 shifts nothing visible at this generalization level.

Tested Queries

Counties per state.

Which county contains a point, longitude first.

Cheap spatial windows from the bbox covering column.

Related Collections

No attribute join inside this catalog, the FIPS ecosystem lives outside it. Spatial joins against boundaries/boston-open-space or the Natural Earth layers work at matching scales. Converted from the upstream zipped Shapefile with DuckDB spatial into web-optimized GeoParquet 2.0.