Definition of "geodetic_crs" and "geodetic_cs"

NOTE: Markdown does not support highlighting in color, so bold/italic has been used to highlight the material being discussed. This issue is extracted from the document submitted by Roger Lott at https://github.com/opengeospatial/CRS-JSON-Encoding/blob/main/ProjJson%20v0-7%20RL%202024-06-16.docx for discussion at the CRS SWG meeting OGC held during the Montreal June 2024 TC meeting.

"geodetic_crs": {
  "type": "object",
  "properties": {
    "type": { "type": "string", "enum": ["GeodeticCRS", "GeographicCRS"] },
    "name": { "type": "string" },
    "datum": {
        "oneOf": [
            { "$ref": "#/definitions/geodetic_reference_frame" },
            { "$ref": "#/definitions/dynamic_geodetic_reference_frame" }
            { "$ref": "#/definitions/datum_ensemble" }
        ]
    },

"datum_ensemble": { "$ref": "#/definitions/datum_ensemble" }, STRIKE THIS LINE
"coordinate_system": { "$ref": "#/definitions/coordinate_system" }, STRIKE THIS LINE

"coordinate_system": { "$ref": "#/definitions/geodetic_cs" },

    "deformation_models": {
      "type": "array",
      "items": { "$ref": "#/definitions/deformation_model" }
    },
    "defining_transformation": { "$ref": "#/definitions/defining_transformation" },
    "defining_transformations": {
      "type": "array",
      "items": { "$ref": "#/definitions/defining_transformation" }
    "$schema" : {},

"scope": {},
"area": {},
"bbox": {},
"vertical_extent": {},
"temporal_extent": {},
"usages": {},

    "remarks": {},
    "id": {}, "ids": {}
  },

"required" : [ "name", "datum", "coordinate_system" ],
"description": "One and only one of datum and datum_ensemble must be provided",

  "allOf": [
    { "$ref": "#/definitions/object_usage" },
    { "$ref": "#/definitions/one_and_only_one_of_datum_or_datum_ensemble" }
  ],
  "additionalProperties": false
},

"$comment": "[RL] why does this subtype of CRS not include "allOf": [{ "$ref": "#/definitions/object_usage" }],?"

"$comment": "[RL] Can the choice of datum v datum ensemble be simplified by including datum ensemble in the datum one of list? (Description would then not be necessary)."

"$comment": "[RL] A geodetic CRS must have a datum or a datum_ensemble. The schema would be clearer if this was included in the required list."

"$comment": "[RL] See comment regarding usage under compound_crs."

"$comment": "[RL] (i) A geodetic CRS should be constrained to having a geodetic_cs. (ii) A geographic CRS should be constrained to having an ellipsoidal_cs. To make these constraints clearer, should geographic_crs be defined separately?"

"$comment": "[RL] Defining transformation(s) attribute (0..*) missing. Equivalent of geoid_model(s) in vertical CRS. See comments below under vertical_crs."

"$comment": "[RL] Is the geoid model(s) attribute correctly modelled? A geoid model is usually described as a transformation. If this were done here then is the geoid_model member definition above needed in the schema? However, although 19111/Topic 2 has the association to transformation, implying that the full definition is required, for brevity WKT2 requires only the geoid model name. Should the CRS JSON schema give option for either full definition or just name (or ID)?"

"$comment": "[RL] Same question about full definition or name/ID for deformation model."

"geodetic_cs": {
  "type": "object",
  "properties": {
       "oneOf": [
            { "$ref": "#/definitions/Cartesian_cs" }
            { "$ref": "#/definitions/ellipsoidal_cs" }
            { "$ref": "#/definitions/spherical_cs" }
       ]
  "additionalProperties": false
},

Start with the linked ProjJson v0-7 document and the geodetic_crs and geodetic_cs schema definitions quoted in the issue. Review each comment with the CRS SWG discussion context, especially the datum, coordinate-system, usage, and transformation questions. Done means the design decisions are resolved and the corresponding schema definitions consistently reflect them.