{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Vertical Datum Reference",
  "description": "Schema component for encoding the vertical datum metadata required for unambiguous height referencing across land and marine domains. Required by FMSDI Best Practice 1 (Unified Geospatial Reference). Always record datum, epoch, transformation method, and vertical orientation convention in metadata (FMSDI BP1 mandate).\n",
  "type": "object",
  "required": [
    "datumName",
    "verticalOrientation"
  ],
  "properties": {
    "datumName": {
      "type": "string",
      "description": "Human-readable name of the vertical datum or reference surface. Examples include \"Chart Datum\" (CD), \"Mean Sea Level\" (MSL), \"Ordnance Datum Newlyn\" (ODN), \"WGS84 Ellipsoid\", \"Lowest Astronomical Tide\" (LAT), \"Mean High Water Springs\" (MHWS), \"Southampton Chart Datum\".\n",
      "examples": [
        "Chart Datum",
        "Mean Sea Level",
        "Ordnance Datum Newlyn",
        "WGS84 Ellipsoid",
        "Lowest Astronomical Tide",
        "Southampton Chart Datum"
      ],
      "x-jsonld-id": "http://www.w3.org/2000/01/rdf-schema#label",
      "x-jsonld-type": "http://www.w3.org/2001/XMLSchema#string"
    },
    "datumEpoch": {
      "type": "string",
      "description": "Reference epoch for the datum, required when using a dynamic reference frame subject to crustal motion or sea-level change. Format follows ISO 8601 decimal year notation.\n",
      "examples": [
        "2020.0",
        "2015.5"
      ],
      "x-jsonld-id": "https://w3id.org/iliad/fmsdi/datumEpoch"
    },
    "crsIdentifier": {
      "type": "string",
      "format": "uri",
      "description": "URI identifying the coordinate reference system in an authoritative registry. Prefer EPSG URIs from the OGC CRS resolver. Examples: EPSG:5714 (MSL height), EPSG:5861 (LAT depth), EPSG:4979 (WGS84 3D), EPSG:7405 (ODN height).\n",
      "examples": [
        "http://www.opengis.net/def/crs/EPSG/0/5714",
        "http://www.opengis.net/def/crs/EPSG/0/5861",
        "http://www.opengis.net/def/crs/EPSG/0/4979",
        "http://www.opengis.net/def/crs/EPSG/0/7405"
      ],
      "x-jsonld-id": "http://purl.org/dc/terms/conformsTo",
      "x-jsonld-type": "@id"
    },
    "separationSurface": {
      "type": "object",
      "description": "Describes the separation model used to transform between this datum and another reference surface (ellipsoid, geoid, or tidal datum). FMSDI BP1 recommends using national models such as NOAA VDatum, UKHO VORF, or CHS HyVSEPs.\n",
      "required": [
        "type"
      ],
      "properties": {
        "type": {
          "type": "string",
          "enum": [
            "geoid",
            "tidal",
            "ellipsoidal",
            "chart"
          ],
          "description": "Category of separation surface: geoid (gravity-based), tidal (derived from tidal observations), ellipsoidal (geometric ellipsoid surface), or chart (hydrographic chart datum surface).\n",
          "x-jsonld-id": "https://w3id.org/iliad/fmsdi/separationSurfaceType"
        },
        "modelReference": {
          "type": "string",
          "description": "Name and version of the separation model. Examples: \"NOAA VDatum 4.3\", \"UKHO VORF 2022\", \"CHS HyVSEPs\", \"EGM2008\", \"OSGM15 (UK Geoid)\".\n",
          "examples": [
            "NOAA VDatum 4.3",
            "UKHO VORF 2022",
            "CHS HyVSEPs",
            "EGM2008",
            "OSGM15"
          ],
          "x-jsonld-id": "https://w3id.org/iliad/fmsdi/modelReference"
        }
      },
      "x-jsonld-id": "https://w3id.org/iliad/fmsdi/separationSurface",
      "x-jsonld-type": "@id"
    },
    "transformationMethod": {
      "type": "string",
      "description": "Tool, model, or procedure used to transform observations to this vertical datum. This field is required when datum conversion has been applied; its absence implies no conversion was performed or the source datum is unknown. Record this to support data lineage under FMSDI BP2 (FAIR Data Principles).\n",
      "examples": [
        "NOAA VDatum 4.3 web service",
        "UKHO VORF vertical offset raster",
        "CHS HyVSEPs interpolation",
        "Manual offset -2.74m (Southampton Chart Datum)"
      ],
      "x-jsonld-id": "http://www.w3.org/ns/prov#wasGeneratedBy"
    },
    "verticalOrientation": {
      "type": "string",
      "enum": [
        "up",
        "down"
      ],
      "description": "Defines the positive direction of the vertical axis. \"up\" means positive values indicate higher elevation (land/topographic convention, e.g. MSL height in metres). \"down\" means positive values indicate greater depth below the datum surface (hydrographic convention, e.g. Chart Datum depth in metres). This is the single most critical field for preventing land-sea data integration errors: the Compusult FMSDI demonstrator encountered exactly this problem when ODN (up) was assumed where Southampton Chart Datum (down, offset -2.74m) was the correct reference.\n",
      "x-jsonld-id": "https://w3id.org/iliad/fmsdi/verticalOrientation"
    },
    "verticalAccuracy": {
      "type": "object",
      "description": "Quantifies the vertical accuracy of observations referenced to this datum. Include confidence level to support ISO 19115 quality reporting and FMSDI BP2.\n",
      "required": [
        "value",
        "unit"
      ],
      "properties": {
        "value": {
          "type": "number",
          "description": "Accuracy magnitude (1-sigma or as specified by confidenceLevel).",
          "minimum": 0,
          "x-jsonld-id": "http://qudt.org/schema/qudt/value"
        },
        "unit": {
          "type": "string",
          "description": "Unit of measure. Use \"m\" for metres.",
          "examples": [
            "m",
            "cm"
          ],
          "x-jsonld-id": "http://qudt.org/schema/qudt/unit"
        },
        "confidenceLevel": {
          "type": "string",
          "description": "Statistical confidence level for the accuracy value. IHO S-44 uses 95% confidence (2-sigma) for hydrographic surveys. ISO 19115 typically uses 1-sigma or 95%.\n",
          "examples": [
            "1-sigma (68%)",
            "95% (2-sigma)",
            "IHO S-44 Order 1a"
          ],
          "x-jsonld-id": "https://w3id.org/iliad/fmsdi/confidenceLevel"
        }
      },
      "x-jsonld-id": "http://vocab.nerc.ac.uk/collection/P01/current/UNCEJJ01/"
    }
  },
  "x-jsonld-prefixes": {
    "rdfs": "http://www.w3.org/2000/01/rdf-schema#",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "fmsdi": "https://w3id.org/iliad/fmsdi/",
    "dct": "http://purl.org/dc/terms/",
    "prov": "http://www.w3.org/ns/prov#",
    "qudt": "http://qudt.org/schema/qudt/",
    "skos": "http://www.w3.org/2004/02/skos/core#",
    "sdo": "https://schema.org/",
    "geosparql": "http://www.opengis.net/ont/geosparql#",
    "nerc": "http://vocab.nerc.ac.uk/collection/"
  }
}