geoprimitives US Geography FIPS Lookup

API ↗

geoprimitives US Geography FIPS Lookup is a paid API for AI agents from api.geoprimitives.dev, paid per call via x402, $0.01/call, status unknown (last checked 2026-10-10).

Returns Census FIPS codes (state, county GEOID, and congressional district GEOID) for any latitude/longitude point within the United States.

How this service works

Give it a longitude and latitude inside the United States and it answers with the Census FIPS codes for that point: the 2-digit state code, the 5-digit county GEOID, and the GEOID of the congressional district. Parameters go in the query string: this is a GET, there is no body. The codes are the product and the names are a courtesy. A county GEOID is the join key to essentially every geography-keyed federal dataset -- the American Community Survey, BLS Local Area Unemployment Statistics, BEA regional accounts, CDC WONDER, HUD, USDA, FEMA -- and the state code is its first two digits, so one call gives you both keys already in the form those tables expect. County GEOID is also the first five digits of a tract GEOID (11 digits) and of a block group GEOID (12), so it is the prefix you would filter a tract-level extract down with. COVERAGE, stated up front because the shortfall is the point: this endpoint answers state, county and congressional district, and it does NOT answer tract, block group, place or ZCTA. That is a size limit, not an oversight, and it was measured rather than guessed. A national compacted H3 index of tracts is 3196 KiB gzipped and of block groups 5335 KiB, at a resolution already too coarse for either; the whole Worker has 3 MiB compressed to spend and most of that was gone before this route existed. Those four tiers need an object store behind the Worker and will arrive with one. Every response repeats this under `coverage`, so an agent never has to infer it from a missing field. In the meantime geocoding.geo.census.gov answers all seven tiers, free, authoritative, at the cost of a round trip and a dependency. VINTAGE: TIGER/Line 2025, congressional districts for the 119th Congress. It is on every response under `vintage` and it matters more here than almost anywhere else in this catalog, because TIGER is re-cut annually and districts move with redistricting, so a stale district code is a wrong answer a caller cannot detect from the outside. Nothing here self-updates; a new vintage means a rebuild. The data is a work of the United States government and is in the public domain (17 U.S.C. 105) -- no attribution clause and no redistribution terms. ACCURACY, and this is the part to read before you trust a code. The boundaries themselves are full-resolution TIGER, the same geometry the Census Geocoder answers from, not a simplified copy. What is approximate is the lookup: the polygons are indexed as compacted H3 cells at resolution 7, about 1.22 km across, so a point within roughly 610 m of a boundary may be attributed to the neighbour. County equivalents and districts too small to describe at that resolution -- Falls Church city, the other Virginia independent cities, the congressional districts inside New York City -- carry a second index at resolution 9, about 87 m, and `boundaryAccuracyMetres` says which one answered you. Because a 610 m index is only useful if you know when not to trust it, every response carries `boundaryUncertain.county` and `boundaryUncertain.congressionalDistrict`. They are true when a neighbouring cell resolves to a different code, which means the boundary is close enough that the index cannot settle which side you are on. Measured against geocoding.geo.census.gov on 650 coordinates: on 400 population-weighted points the county code was right 389 times and the district code 383, and every one of the 11 and 17 disagreements was flagged; on 250 points drawn deliberately 100 m to 1500 m from a county line the county code was right 183 times, and every one of the 67 wrong county codes and 25 wrong district codes was flagged. Across all 650 it never returned a wrong code with the flag clear. It is generous in the other direction -- it flagged 74 of the 400 correct-heavy sample -- so treat `true` as "verify this one if it matters" rather than as "this is wrong". Outside the United States the answer is `found: false` with null codes and a paid 200, because "this coordinate is not in the US" is the answer you asked for. County polygons include their legal water area out to the three-mile limit and across the Great Lakes, so harbours, piers and lake coordinates resolve normally and only genuinely foreign or high-seas points fall out. Coverage is the 50 states, DC, Puerto Rico, American Samoa, Guam, the Northern Mariana Islands and the US Virgin Islands. A small amount of water is inside a county but inside no congressional district, which TIGER publishes as a real polygon; that answers with a district whose number is null rather than with a guess. Longitude and latitude are checked against EPSG:4326 bounds before anything else touches them, because H3 silently normalises an out-of-range latitude into a real but wrong cell rather than refusing it. Reproject with POST /v1/transform first if you are not already in WGS 84. TIGER publishes in NAD83, which this reads as WGS 84; the two differ by under two metres, which is a fortieth of the index's own resolution.

Output

A JSON object containing the 2-digit state FIPS code, the 5-digit county GEOID (the standard join key for ACS, BLS, BEA, CDC WONDER, HUD, USDA, FEMA, and other federal datasets), and the GEOID of the congressional district. Human-readable names are included as a courtesy. County GEOID also serves as the prefix for 11-digit tract GEOIDs and 12-digit block group GEOIDs.