Definition of "datum_ensmble"

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.

"datum_ensemble": {
  "type": "object",
  "properties": {
    "$schema" : { "type": "string" },
    "type": { "type": "string", "enum": ["DatumEnsemble"] },
    "name": { "type": "string" },
    "members": {
        "type": "array",
        "items": {
            "type": "object",
            "properties": {
                "name": { "type": "string" },
                "id": { "$ref": "#/definitions/id" },
                "ids": { "$ref": "#/definitions/ids" }
            },
            "required" : [ "name" ],
            "allOf": [
                { "$ref": "#/definitions/id_ids_mutually_exclusive" }
            ],
            "additionalProperties": false
        }
    },

"ellipsoid": { "$ref": "#/definitions/ellipsoid" },

    "accuracy": { "type": "string" },
    "id": { "$ref": "#/definitions/id" },
    "ids": { "$ref": "#/definitions/ids" }
  },
  "required" : [ "name", "members", "accuracy" ],
  "allOf": [
    { "$ref": "#/definitions/id_ids_mutually_exclusive" }
  ],
  "additionalProperties": false
},

"$comment": "[RL] this does not seem to define that a member of datum ensemble must be a datum, nor that all members of a datum ensemble must have the same conventionalRS."

"$comment": "[RL] The ellipsoid attribute is not included in either 19111/Topic 2 or WKT2. However there are merits in including the attribute - saves having to mine for the information in a member. But the same argument applies to having Usage as an extra attribute - should that be added?"

Start with the datum_ensemble schema excerpt and the linked ProjJson v0-7 document, then review the questions about member types, shared conventionalRS, ellipsoid, and Usage. Done means the CRS SWG has decided the intended definition and the schema requirements and attributes are updated consistently.