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      "use": "Static physical mapped non-fictitious road-network length, including restricted/private mapped roads",
      "notMileage": true,
      "licence": "Contains OS data \u00a9 Crown copyright and database right 2026; Open Government Licence"
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      "licence": "ONS and Ordnance Survey open geography data; Open Government Licence"
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    "numpy": "2.3.5",
    "pandas": "2.2.3",
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      "sourceUrl": "https://api.os.uk/downloads/v1/products/Terrain50/downloads?area=GB&format=ASCII+Grid+and+GML+%28Grid%29&redirect",
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      "observationPeriod": "OS Terrain50 July2026; OS Open Roads April2026; retrospective geographic context",
      "sourceUrl": "https://api.os.uk/downloads/v1/products/Terrain50/downloads?area=GB&format=ASCII+Grid+and+GML+%28Grid%29&redirect",
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      "unit": "percent of terrain-covered road metres",
      "definition": "100 * terrain-covered road metres with sampled gradient strictly above8% / terrain-covered road metres."
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    "decision": "Numeric terrain diagnostics only; national road-gradient bands withheld",
    "reasons": [
      "No independent road-surface elevations or representative bridge/tunnel validation were acquired.",
      "OS Terrain50 is a 50m terrain grid; 4m publisher height RMSE is not evidence of road-grade accuracy.",
      "OS Open Roads structure field flags some tunnels; it does not establish complete bridge coverage or independent carriageway elevations."
    ],
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      "Validate a prespecified stratified sample against independent road-surface elevations, including bridges, tunnels, urban cuttings and upland minor roads.",
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    "secondarySpacingMetres": 100,
    "steepOperator": ">",
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    "sensitivityThresholdsPct": [
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    "status": "estimated_terrain_proxy",
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    "Roundabout": 51794,
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    "Shared Use Carriageway": 2992,
    "Guided Busway": 153
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    "Road In Tunnel": 492
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  "extremeIntervalInterpretation": "Largest computed intervals flag follow-up locations, not confirmed actual steep roads. No clipping or silent removal of extreme gradients.",
  "outputs": [
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