BETA — measurements and names under active refinement. Seen a sign, a gate, or a better name? Report it (photos welcome).

Twtil

352.2 m · gain 49.5 m · overall 14.1% · Harlech · Gwynedd

Shape: sustained — 32% of the climbing sits in a quarter of the distance; longest continuous stretch at 10%+ is 294 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.

≈ Overhead cover (tree canopy or structures) spans 9% 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)	≈29.2%	261–263 m	under overhead cover terrain reads 87.2% at 347–349 m across a structure — excluded
3 m (diagnostic)	≈29.1%	261–264 m	under overhead cover terrain reads 74.8% at 347–350 m across a structure — excluded
5 m	≈25.0%	259–264 m	under overhead cover terrain reads 46.8% at 346–351 m across a structure — excluded
10 m	≈23.0%	124–134 m	under overhead cover terrain reads 24.7% at 340–350 m across a structure — excluded
20 m	21.9%	115–135 m
25 m	21.5%	111–136 m
50 m	19.7%	97–147 m
100 m	17.2%	202–302 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

Profile

Reported values vs. measurement

Source	Claim	Evidence	Best-matching window
OSM incline tag	25%	road_sign	5 m (-0.0 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",
    "data/cache/wales/dtm_1m/wg_del_30_257331_20220320dtm.tif"
  ],
  "calculated_at": "2026-08-12T09:11:50+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",
    "wg_del_30_257331_20220320dtm.tif"
  ],
  "elevation_capture_dates": [
    "11-01-2022",
    "20-03-2022"
  ],
  "climb_origin": "osm_incline",
  "signed_gradient_pct": 25,
  "signed_source": "OSM incline tag (25%)",
  "notes": "Harlech, neighbour of Ffordd Pen Llech. EXTENDED by graph walk: +0 segment(s) at base, +6 at top (stop: base=no_descending_continuation, top=no_ascending_continuation; min_avg=2.0%). Review before adopting."
}