Hi HN, I built this. Listings that say "borders state land" can't easily be checked: you open the county's GIS viewer and line up parcel lines against public-land maps by hand. I did that a lot while looking for land of my own, then automated it.

How it works, once a week:

1. About 700,000 active U.S. listings come in from a licensed listings API.

2. Each listing near public land gets its county parcel boundary, from state and county GIS services where they publish them and from a licensed parcel provider elsewhere. With no boundary, the page says so and measures from the listing point, but never claims a shared edge.

3. Public land is USGS PAD-US 4.1, narrowed to land an agency actually owns (Category = Fee, so no conservation easements), that's open to the public, and that's at least 50 acres.

4. Parcel and public land are compared in a local metric projection (shapely/geopandas): within (an inholding), bordering (the boundaries touch, and the shared edge is measured in feet), or adjacent (within 75 m, about a road's width).

Each property also gets the size of the connected public land behind it, its elevation (USGS 3DEP), road access (Census TIGER roads), FEMA flood zone and Forest Service wildfire hazard. The output is about 16,000 static pages with Leaflet maps on Cloudflare Pages, rebuilt every Monday on a Mac mini. ZIP search looks the ZIP up in your browser from Census centroids, so nothing you type is sent anywhere.

Things that were harder than expected:

- Lot sizes sometimes arrive in square feet in the acres field. A Birmingham, AL listing came through at 7,000 acres. Sizes are now checked against price, property type and the parcel itself, and corrected or withheld with a note.

- Condo and subdivision listings often resolve to the association's common tract, so a single town lot can appear to share miles of boundary. Those are flagged as shared parcels.

- PAD-US stores some units as one multipart polygon. My first "connected public land" figure for one Montana property was 69.7 million acres, about three-quarters of the state. Units are now split into pieces and joined only where they actually touch.

- Road centerlines and lot lines come from different agencies and are often tens of meters apart, so "does a road touch this parcel" needs a tolerance, and frontage only counts where the boundary runs along the road.

It's free, with no account and no cookies. There are free weekly email alerts for any search, starting Monday. I'd especially like to hear from anyone who has bought rural land: what should it measure that it doesn't?

Show HN: Unfenced – Land for sale that borders public land