Allt Goch Fawr
1266.0 m · gain 82.5 m · overall 6.5% · Nant Meigan · Sir Ynys Mon - Isle of Anglesey
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 — sign: 25% (2026). Signs get repainted and tags get retagged, so the date is part of the fact; on no cycling list we know of.
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, 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) | ≈37.7% | 612–614 m | under overhead cover terrain reads 57.9% at 700–702 m across terrain that holds no road surface (dense canopy, no bench) — excluded |
| 3 m (diagnostic) | ≈27.7% | 611–614 m | under overhead cover terrain reads 53.6% at 700–703 m across terrain that holds no road surface (dense canopy, no bench) — excluded |
| 5 m | ≈28.0% | 609–614 m | under overhead cover terrain reads 42.5% at 699–704 m across terrain that holds no road surface (dense canopy, no bench) — excluded |
| 10 m | ≈18.7% | 662–672 m | under overhead cover terrain reads 26.4% at 696–706 m across 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 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 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 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 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
© OpenStreetMap contributors · topo tiles © OpenTopoMap (CC-BY-SA) · vector tiles OpenFreeMap
Profile
Reported values vs. measurement
| Source | Claim | Evidence | Best-matching window |
|---|---|---|---|
| OSM incline tag | 25% | road_sign | 3 m (-2.7 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_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",
"signed_gradient_pct": 25,
"signed_source": "OSM incline tag (-25%)",
"notes": "Secondary road near Llanfairfechan; ways listed in verified chain order. EXTENDED by graph walk: +7 segment(s) at base, +1 at top (stop: base=no_descending_continuation, top=no_ascending_continuation; min_avg=2.0%). Review before adopting."
}