todo and/or todiscuss

"version"?
other geometry object for single ones? like "Point" or "LineString"? Why having "Solid" then?
abolish "CompositeX" and just "MultiX"?

the tu3djson formal specification

What is tu3djson?

It stands for TU Delft 3D JSON specification, named after the TU Delft 3D geoinformation research group. It is meant as a simple format to store/exchange 3D features since GeoJSON is restricted to 2D primitives stored in WGS84 (even if the elevation with respect to the WGS84 ellipsoid can be stored, it is only 2.5D and not 3D).

An example

{
  "type": "tu3djson",
  "crs": "https://www.opengis.net/def/crs/EPSG/0/7415",
  "features": [
    {
      "type": "Building",
      "geometry": {
        "type": "MultiSurface",
        "boundaries": [
          [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
        ],
        "vertices": [
          [0.0, 0.0, 0.0],
          [1.0, 0.0, 0.0],
          [0.0, 0.0, 0.0]
        ]
      },
      "properties": {
        "name": "big rock"
      }
    },
    {
      "type": "Tree",
      "geometry": {
        "type": "Solid",
        "boundaries": [
          [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]] ]
        ],
        "vertices": [
          [0.0, 0.0, 0.0],
          [1.0, 0.0, 0.0],
          [0.0, 0.0, 0.0]
        ]
      },
      "properties": {
        "name": "another rock",
        "colour": "bright red"
      }
    }
  ]
}

tu3djson object

A tu3djson object is a single JSON object containing zero or more Feature Objects. It:

must have one member with the name "type", whose value must be "tu3djson".
must have one member with the name "features", whose value is an array containing 0 or more Feature Objects
may have a member with the name "crs" (coordinate reference system), whose value is a URL formatted according to the OGC Name Type Specification. All Features in the tu3djson object must be geo-referenced according to this CRS.

Feature object

must have one member with the name "type", whose value must be a string. The values of the type is not prescribed and should be used to describe the Feature Object. "type": "Feature is an acceptable property/value.
must have one member with the name "geometry", whose value must be one Geometry Object.
may have one member with the name "properties", whose value is a JSON object.

Geometry object

must have one member with the name "type", whose value must be either:
- "MultiPoint"
- "MultiLineString"
- "MultiSurface"
- "CompositeSurface"
- "Solid"
- "MultiSolid"
- "CompositeSolid"
must have one member with the name "boundaries", whose value is a hierarchy of arrays with integers (the depth depends on the Geometry object, see this guide). An integer refers to the index in the "vertices" array of the Geometry object, and it is 0-based (ie the first element in the array has the index "0", the second one "1", etc.).
must have one member with the name "vertices", whose value is an array of coordinates of each vertex of the current Geometry object. Their position in this array (0-based) is used to represent the Geometry Object.
must have one member with the name "vertices", whose value is an array of coordinates of each vertex of the current Geometry Object, stored with integers. Their position in this array (0-based) is used as an index to be referenced by "boundaries" of the Geometry Object.

Schemas

In the schemas/ folder.

tudelft3d / tu3djson