Ffordd Pen Llech
236.9 m · gain 40.8 m · overall 17.2% · Harlech · Gwynedd
Shape: sustained — 41% of the climbing sits in a quarter of the distance; longest continuous stretch at 10%+ is 173 m.
Known from: Guinness World Records — Steepest street in the world, 2019 (title returned to Baldwin St, NZ in 2020 after a measurement-method dispute).
≈ Overhead cover (tree canopy or structures) spans 15% of this route — detected by comparing the surface model (DSM, treetops and rooftops) against the terrain model (DTM, bare ground). Under cover the bare-ground model is built from fewer laser returns, so metre-scale readings can wobble; maxima whose window sits in a covered zone are marked ≈ below — less certain, not wrong. Longer windows average over more clean ground and are steadier.
Gradient ladder what do windows mean?
| Window | Maximum | Located at | |
|---|---|---|---|
| 2 m (diagnostic) | ≈55.0% | 101–103 m | under overhead cover |
| 3 m (diagnostic) | ≈51.3% | 100–103 m | under overhead cover |
| 5 m | ≈43.7% | 98–103 m | under overhead cover |
| 10 m | ≈36.3% | 100–110 m | under overhead cover |
| 20 m | ≈34.2% | 97–117 m | under overhead cover |
| 25 m | ≈32.6% | 89–114 m | under overhead cover |
| 50 m | ≈29.8% | 73–123 m | under overhead cover |
| 100 m | ≈26.5% | 64–164 m | under overhead cover |
may_continue_beyond_end Extent detection suggests reviewing where this climb starts/ends.
Map
Check this route yourself: Google Maps route via their router · Street View at the base where coverage exists · geojson.io our exact measured line · OpenTopoMap
© OpenStreetMap contributors · topo tiles © OpenTopoMap (CC-BY-SA) · vector tiles OpenFreeMap
Profile
Reported values vs. measurement
| Source | Claim | Evidence | Best-matching window |
|---|---|---|---|
| Road sign / OSM incline tag | 40% | road_sign | 5 m (-3.7 pp) |
| Guinness World Records (2019) | 37.45% | survey | 10 m (+1.2 pp) |
Provenance
{
"analysis_version": "0.1.0",
"sample_interval_m": 1.0,
"interpolation": "bilinear",
"smoothing": "none",
"gradient_windows_m": [
5.0,
10.0,
20.0,
25.0,
50.0,
100.0
],
"diagnostic_windows_m": [
2.0,
3.0
],
"elevation_crs": "EPSG:27700",
"elevation_resolution_m": 1.0,
"elevation_tiles": [
"data/cache/wales/dtm_1m/wg_del_27_258331_20220111dtm.tif"
],
"calculated_at": "2026-08-12T09:11:48+00:00",
"road_source": "OpenStreetMap (local PBF extract)",
"road_attribution": "\u00a9 OpenStreetMap contributors, ODbL",
"osm_pbf_path": "data/cache/pbf/wales-2026-08-11.osm.pbf",
"osm_snapshot": "2026-08-10T20:21:40Z",
"elevation_provider": "Welsh Government",
"elevation_dataset": "Welsh Government LiDAR 1m DTM (2020-2023)",
"elevation_licence": "Open Government Licence v3 \u2014 \u00a9 Crown copyright Welsh Government",
"elevation_catalogue_layer": "geonode:welsh_government_lidar_tile_catalogue_2020_2023",
"elevation_tile_ids": [
"wg_del_27_258331_20220111dtm.tif"
],
"elevation_capture_dates": [
"11-01-2022"
],
"climb_origin": "named",
"signed_gradient_pct": 40,
"signed_source": "OSM incline tag (-40%); Harlech; former Guinness steepest-street record holder",
"notes": "One-way spiral street, Harlech. Guinness measured 37.45% at centreline-adjacent line in 2019; record returned to Baldwin St NZ after measurement-method dispute \u2014 exactly this project's subject. EXTENDED by graph walk: +1 segment(s) at base, +1 at top (stop: base=no_descending_continuation, top=no_ascending_continuation; min_avg=2.0%). Review before adopting."
}