Allt Goch Fawr
1266.0 m · net rise 82.5 m · total ascent 86.5 m · overall 6.5% · evidence quality U (needs confirmation) · Beaumaris · Gwynedd · share
Shape: stepped — 46% of the climbing sits in a quarter of the distance; longest continuous stretch at 10%+ is 107 m.
Documented on the ground — tag: 25% (2026). Signs get repainted and tags get retagged, so the date is part of the fact; not yet matched against any cycling list.
needs review short window divergence — treat short-window maxima here as provisional.
≈ Overhead cover (tree canopy or structures) spans 55% 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) | ≈38% | 612–614 m | under overhead cover terrain reads 57.9% at 700–702 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 3 m (diagnostic) | ≈28% | 611–614 m | under overhead cover terrain reads 53.6% at 700–703 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 5 m | ≈28% | 609–614 m | under overhead cover terrain reads 42.5% at 699–704 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 10 m | ≈19% | 662–672 m | under overhead cover terrain reads 26.4% at 696–706 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 20 m | ≈15.3% | 651–671 m | under overhead cover terrain reads 22.1% at 698–718 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 25 m | ≈15.0% | 783–808 m | under overhead cover terrain reads 21.7% at 713–738 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 50 m | ≈14.5% | 782–832 m | under overhead cover terrain reads 20.7% at 696–746 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
| 100 m | ≈14.1% | 779–879 m | under overhead cover terrain reads 15.6% at 646–746 m across a structure and terrain that holds no road surface (dense canopy, no bench) — excluded |
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.
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 |
|---|---|---|---|
| OSM incline tag | 25% | mapper_estimate | 3 m (-2.7 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_19_259376_20211221dtm.tif, wg_del_29_260376_20220319dtm.tif |
| Road centreline | OpenStreetMap (local PBF extract), snapshot 2026-08-10T20:21:40Z |
| Quality status | evidence quality U; review flags: short window divergence |
| 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_19_259376_20211221dtm.tif",
"data/cache/wales/dtm_1m/wg_del_29_260376_20220319dtm.tif"
],
"calculated_at": "2026-08-12T09:11:53+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_19_259376_20211221dtm.tif",
"wg_del_29_260376_20220319dtm.tif"
],
"elevation_capture_dates": [
"19-03-2022",
"22-12-2021"
],
"climb_origin": "osm_incline",
"license": "ODbL-1.0",
"license_url": "https://howsteep.org/licensing"
}