Ffordd Pen Llech
236.9 m · net rise 40.8 m · total ascent 41.3 m · overall 17.2% · confidence: 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) — 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 |
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
This climb's data: OCL JSON — the full gradient ladder, quality flags and provenance in the OpenClimb Ladder format.
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.
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© 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",
"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.",
"license": "ODbL-1.0",
"license_url": "https://howsteep.org/licensing.html"
}