landcheck — offline land/sea lookup

This is created as Trifold application (also test and demonstration)

Is this lat/long point on land or in the sea? gets answered anywhere on Earth in ~1–13 µs, fully offline, from a small 182 KB bundled dataset — with a confidence value for every answer. Works with Python and JavaScript.

Live in-browser demo — classify sample or your own points (CSV / GeoJSON) on a map and watch the measured lookup rate; the page embeds the real JS library and dataset.

import sys; sys.path.insert(0, "landcheck/python")
from landcheck import LandCheck

lc = LandCheck()
lc.is_land(24.7536, 59.4370)          # True   (lon, lat — Tallinn)
lc.check(-0.1276, 51.5072)
# LandResult(land=True, kind='land', confidence=1.0, land_fraction=1.0,
#            cell='TFA95BM', refined=False)

import { LandCheck } from "./landcheck/js/landcheck.mjs";
const lc = await LandCheck.fromFile();          // NodeJs takes bundled data directly
// OR:
const lc = await LandCheck.fromUrl("/data/landsea_L10.tfls"); // browser needs to load data file online
lc.isLand(24.7536, 59.437);           // true
lc.check(-0.1276, 51.5072);
// { land: true, kind: 'land', confidence: 1, landFraction: 1,
//   cell: 'TFA95BM', refined: false }

How it works

The Trifold level-10 grid (~7 km triangles, 21M cells globally) is classified against Natural Earth 1:50m land: 6.15M cells touch land, of which 168,833 are coastal (mixed land/sea) and the rest are wholly interior. Because Trifold addresses sort hierarchically, every cell at any level maps to a contiguous range in the canonical level-10 index space (face·4¹⁰ + path), so the entire classification collapses to 153,884 run-length intervals — 182 KB compressed, including a 4-bit land-area fraction for every coastal cell.

A lookup is: locate the point's level-10 triangle (pure float math, no dependencies), then binary-search the runs.

Measured accuracy (30,000 uniform random points vs. exact polygon containment in the source dataset): 99.82% agreement overall; land and sea answers 100% correct; all residual error lives in coast answers, which self-report their lower confidence (mean calibration credit 0.997).

Caveats inherited from the source data: Natural Earth 1:50m treats lakes as land and omits islets below its resolution; coast cells flag exactly where such risk is concentrated.

Optional coastal refinement (OSM)

For applications that need near-exact coastlines, a second dataset (coastal_osm_L10.tflr, 12.7 MB) stores OSM simplified land polygons (osmdata.openstreetmap.de) clipped to every triangle crossed by either the Natural Earth or OSM coastline, quantized to a cell-local 16-bit grid (~0.1 m) with delta-varint rings. When loaded, covered answers switch from the Natural Earth base or bulk land-fraction guess to an exact point-in-polygon test. This has 99.95% agreement with full polygon containment on 4,000 random points inside the original coastal-cell sample (~23 µs per refined lookup, Python):

lc = LandCheck(refine_path="landcheck/data/coastal_osm_L10.tflr")

await lc.loadRefinement("landcheck/data/coastal_osm_L10.tflr");

Only covered coastline cells pay the polygon-test cost. OSM can override a base land or sea answer when its coastline crosses a triangle that Natural Earth classified differently.

Note on dataset semantics: the base layer keeps Natural Earth's view of the world for land/sea kinds. NE and OSM systematically disagree about Antarctica (OSM land polygons are clipped near the pole and draw ice-shelf edges differently). With refinement enabled, OSM is authoritative in every covered coastline cell.

Command-line one-off checks (uses refinement automatically when the file is present):

$ python landcheck/python/landcheck.py 24.7536 59.4370
LAND  kind=land  confidence=1.000  land_fraction=1.0  cell=TFAVKGR  refined=False

Performance

The Python library is dependency-free (stdlib only); is_land_batch optionally uses numpy + the trifold SDK. The JS library is a single ES module, works with browser + Node.

Files

build.py          TFLS builder: compacted L10 grid GeoJSON -> landsea_L10.tfls
refine_build.py   TFLR builder: land polygons + TFLS -> coastal_osm_L10.tflr
python/           landcheck.py (public API) · _fastloc.py (point location)
js/landcheck.mjs  the JS library (same data, same answers)
data/             landsea_L10.tfls (bundled) · coastal_osm_L10.tflr (optional)
tests/            pytest + node:test suites, shared fixture (points.json)

Rebuild from a Trifold grid product:

python landcheck/build.py                  # needs data/global_tri_L10_compacted.geojson
python landcheck/refine_build.py --land osm_simplified_land_polygons.geojson
python landcheck/tests/make_fixture.py     # refresh cross-language fixture
pytest landcheck/tests/ && node --test landcheck/tests/test_landcheck.mjs

Format notes

Custom data format is used to ensure compactness.

TFLS (land/sea runs): 16-byte header + zlib stream of varint(gap), varint(length<<1 | coastal) per run, then 4-bit land fractions for coastal cells. TFLR (refinement): 12-byte header + zlib stream of varint(Δindex), varint(code) per covered cell, where code 0/1 = all sea/land and code n≥2 introduces n−1 quantized zigzag-delta rings combined by the even-odd rule. Both formats are level-agnostic (the level lives in the header), so the same tooling can serve an L8 (~30 KB) or L12 (~3 MB) variant.

Roadmap

• Country detection: the run-length layer maps each cell to a country id instead of a land bit (runs split at borders; border cells carry clipped boundary polygons like TFLR does for coastlines). Probably can use same formats, same lookup path, same confidence model.
• Level-12 (~1.8 km trifolds) variant for higher-precision use.
• Published packages (pip install trifold-landcheck, npm equivalent).