Ffordd Pen Llech
236.9 m · net rise 40.8 m · total ascent 41.3 m · overall 17.2% · evidence quality B (good) · Harlech · Gwynedd · share
Shape: sustained — 41% of the climbing sits in a quarter of the distance; longest continuous stretch at 10%+ is 173 m.
Matches: Guinness World Records, Wikipedia, 100 Greatest Cycling Climbs (Simon Warren), BikeRadar, "7 of the steepest UK climbs, ranked" — Steepest street in the world, 2019 (title returned to Baldwin St, NZ in 2020 after a measurement-method dispute) — Has its own article (public road, Harlech) — No. 92, as "Ffordd Penllech" (official app, 2026 numbering).
≈ 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 — the flag does not say whether the true value is higher or lower. Longer windows average over more clean ground and are steadier.
Gradient ladder what do windows mean?
| Window | Maximum | Located at | |
|---|---|---|---|
| 2 m (diagnostic) | ≈55% | 101–103 m | under overhead cover |
| 3 m (diagnostic) | ≈51% | 100–103 m | under overhead cover |
| 5 m | ≈44% | 98–103 m | under overhead cover |
| 10 m | ≈36% | 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 |
Measured on the mapped road line; moving the line 1 m either way reads 32.2–33.0% over 25 m.
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
No aerial layer here: Britain has no open or government source of aerial photography at road scale — the pan-government APGB imagery is licensed to the public sector only, unlike the LiDAR this survey is measured from. Street View, where it exists, is the visual check.
This climb's data: OCL JSON — the full gradient ladder, quality flags and provenance in the OpenClimb Ladder format.
Access: Measured along the mapped road line, which here runs on the inside of the hairpin — where the 2019 Guinness survey stood (37.45 % over 10 m). On this line the laser model reads 32.6 % over 25 m and 36.5 % over 10 m. Down the middle of the road the same model reads about 28 % over 25 m and about 32 % over 10 m — the direction of Guinness's 2020 centreline correction (28.6 %). Both surveys were right about the line each measured; the classified point cloud, when Wales releases it, will place the crown to the metre.
Measured, not vetted. This page records what the laser record shows about the road's shape — nothing more. Nobody has necessarily ridden or inspected this road for the survey, and a page here is not a recommendation and no guarantee that the road is open, surfaced, rideable or safe. Gates, private sections, missing rights of way and hazards do not show up in elevation data. Check access and conditions on the ground and make your own judgment.
Ridden it, or know better? Submit data for this climb no account needed · Share this climb
Route colour = gradient over a 25 m window: <10% 10–20% 20–25% ≥25%
© OpenStreetMap contributors · topo tiles © OpenTopoMap (CC-BY-SA) · vector tiles OpenFreeMap
Profile
The profile and gradient chart share one position — move your pointer over them and the marker follows. Colors are the gradient bands; the shaded span is the steepest 25 m. Any 25 m figure is good to about a point either way (why).
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) |
Measurement card
| Elevation | Welsh Government — Welsh Government LiDAR 1m DTM (2020-2023) |
|---|---|
| Resolution / CRS | 1.0 m grid, EPSG:27700; sampled every 1.0 m, bilinear interpolation |
| Tiles | wg_del_27_258331_20220111dtm.tif |
| Road centreline | OpenStreetMap (local PBF extract), snapshot 2026-08-10T20:21:40Z |
| Quality status | evidence quality B; caveated windows: canopy or structure |
| Analysis | OCL 0.1.0, analysis 0.1.0, run identifier not recorded (legacy measurement, before the campaign driver stamped runs), calculated 2026-08-12 |
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",
"license": "ODbL-1.0",
"license_url": "https://howsteep.org/licensing"
}