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2135.9 m · net rise 142.2 m · total ascent 159.3 m · overall 6.7% · evidence quality U (needs confirmation) · Gwaelod-y-garth · South Glamorgan

Shape: stepped — 49% of the climbing sits in a quarter of the distance; longest continuous stretch at 10%+ is 110 m.

Documented on the ground — sign: 16% (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, texture unresolved — 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?

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: Trimmed 2026-10-02 (Peter, Welsh laser review page 2026-10-02): the leading turn is a stub, so the line now starts past it and was re-measured on the 1 m terrain model; the steepest 25 m now reads 15.8 % (was 21.9 %), at 391-416 m. The hold is lifted: the Welsh laser shows the road bed under the line. The figure is below the ranking door — kept as a listed page (never ranked below the doors, D8).

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

Load the interactive map

Route colour = gradient over a 25 m window: <10% 10–20% 20–25% ≥25%

Route line = the road surface as the LiDAR survey finds it. Basemap roads are for orientation and may sit a few metres off.

Topo (contours) Vector   © OpenStreetMap contributors · topo tiles © OpenTopoMap (CC-BY-SA) · vector tiles OpenFreeMap

Profile

The map, 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

Measurement card

Provenance

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    "code_git_dirty": "false",
    "analysis_version": "0.1.0",
    "analysis_config_digest": "sha256:8c94015b54267da4",
    "python": "3.14.4",
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    "rasterio": "1.5.1",
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    "pyosmium": "4.3.1",
    "libosmium": "2.23.1",
    "gdal": "3.12.4",
    "proj": "9.8.1",
    "run_id": "remeasure-261002-hooktrim",
    "road_pbf": "chunk-route-des-alpes.pbf",
    "region_key": "wales",
    "crs": "EPSG:27700",
    "elevation_provider_key": "wales",
    "authority": "os-open-roads",
    "registry_pbf": "wales-2026-08-11.osm.pbf"
  },
  "sample_interval_m": 1.0,
  "interpolation": "bilinear",
  "smoothing": "none",
  "gradient_windows_m": [
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  "diagnostic_windows_m": [
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  ],
  "elevation_crs": "EPSG:27700",
  "elevation_resolution_m": 1.0,
  "elevation_tiles": [
    "data/cache/wales/dtm_1m/wg_del_2_309183_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_2_309184_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_309182_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_310182_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_310183_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_310184_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_311182_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_311183_20201125dtm.tif",
    "data/cache/wales/dtm_1m/wg_del_3_311184_20201125dtm.tif"
  ],
  "calculated_at": "2026-10-02T17:29:32+00:00",
  "road_source": "OpenStreetMap (local PBF extract)",
  "road_attribution": "\u00a9 OpenStreetMap contributors, ODbL",
  "osm_pbf_path": "../analysis/hooktrim/chunk-route-des-alpes.pbf",
  "osm_snapshot": "",
  "run_id": "remeasure-261002-hooktrim",
  "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": [
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    "wg_del_3_311183_20201125dtm.tif",
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  ],
  "elevation_capture_dates": [
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  ],
  "climb_origin": "discovered",
  "seam_filled_samples": {
    "count": 0,
    "distances_m": []
  },
  "seam_filled_in_maximum": false,
  "structure_own_ways": 4,
  "structure_own_ways_source": "map-matched to the road network: the entry carried no way list, so the ways the route runs along were found by geometry (ukclimb.batch.own_ways_by_geometry)",
  "carriageway": {
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    "stations_resolved": 92,
    "width_median_m": 3.5,
    "width_p10_m": 1.52,
    "width_min_m": 1.0,
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    "bounded_share": 0.63,
    "unbounded_share": 0.37,
    "confidence_median": 0.579,
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    ],
    "width_at_max25_m": 4.5,
    "window_stations": 7,
    "single_track": true,
    "single_track_rule": "PROVISIONAL (2026-09-15): width_p10_m <= 3.0 m with bounded_share >= 0.6; recorded, not a rule",
    "status": "ok",
    "source": "dtm",
    "station_m": 25.0,
    "window_station_m": 5.0,
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    "stations_failed": 0,
    "hedge_to_hedge": {
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      "width_median_m": 2.25,
      "width_p10_m": 1.0,
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      "width_at_max25_m": 3.25,
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    "why": "leading stub trimmed off the line (Peter, Welsh laser review page)",
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    "max25_pct": 15.77
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  "license": "ODbL-1.0",
  "license_url": "https://howsteep.org/licensing"
}