8.1.  Attributes for all City Objects

The attributes for a given City Object must be stored in the "attributes" member and, unlike CityGML, they are not prescribed. Note that any valid JSON value (including an array and/or object) is a valid attribute value.

"CityObjects": {
  "id-1": {
    "type": "LandUse",
    "attributes": {
      "function": "Industry and Business",
      "area-parcel": {
        "value": 437,
        "uom": "m2"
      },
    },
    "geometry": [{...}]
  },
  "id-2": {
    "type": "WaterBody",
    "attributes": {
      "name": "Lake Black",
      "some-list": ["a", "b", "c"]
    },
    "geometry": [{...}]
  }
}

Figure 7

8.2.  Bridge

See the CityGML v3.0.0 Bridge module for more details.

Six City Objects are related to bridges:

• "Bridge"

• "BridgePart"

• "BridgeInstallation"

• "BridgeConstructiveElement"

• "BridgeRoom"

• "BridgeFurniture"

The geometry of both "Bridge" and "BridgePart" can only be represented with these Geometry Objects: (1) "Solid", (2) "CompositeSolid", (3) "MultiSurface", (4) "CompositeSurface". The geometry of the four other objects can be represented with any Geometry Object.

All the above City Objects, except "Bridge", must have a "parents" member. The installations and furniture can have as parent a "Bridge", a "BridgePart", or a "BridgeRoom".

A City Object of type "Bridge" or "BridgePart" may have a member with the name "address", whose value is an array of JSON objects listing one or more addresses of that bridge. The members of an address JSON object are not prescribed, to accommodate the different ways addresses are structured in different countries. If a location is necessary then a member with the name "location" can be added to the "address" member, and it should contain a "MultiPoint" geometry.

"CityObjects": {
  "LondonTower": {
    "type": "Bridge",
    "address": [
      {
        "city": "London",
        "country": "UK"
      }
    ],
    "children": ["Bext1", "Bext2", "Inst-2017-11-14"],
    "geometry": [{
      "type": "MultiSurface",
      "lod": "2",
      "boundaries": [
        [[0, 3, 2, 1]],
        [[4, 5, 6, 7]],
        [[0, 1, 5, 4]],
        [[1, 2, 6, 5]],
        [[2, 3, 7, 6]],
        [[3, 0, 4, 7]]
      ]
    }]
  }
}

Figure 8

8.3.  Building

See the CityGML v3.0.0 Building module for more details.

Eight City Objects are related to buildings:

• "Building"

• "BuildingPart"

• "BuildingInstallation"

• "BuildingConstructiveElement"

• "BuildingFurniture"

• "BuildingStorey"

• "BuildingRoom"

• "BuildingUnit"

The geometry of "Building", "BuildingPart", "BuildingStorey", "BuildingRoom", and "BuildingUnit" can only be represented with these Geometry Objects: (1) "Solid", (2) "CompositeSolid", (3) "MultiSurface", (4) "CompositeSurface". The geometry of "BuildingInstallation", "BuildingConstructiveElement", or "BuildingFurniture" objects can be represented with any Geometry Object.

All the above City Objects, except "Building", must have a "parents" member. The "BuildingInstallation", "BuildingConstructiveElement", "BuildingFurniture", "BuildingStorey" can have as parent a "Building", a "BuildingPart", or a "BuildingRoom".

A City Object of type "Building", "BuildingPart" or "BuildingUnit" may have a member with the name "address", whose value is an array of JSON objects listing one or more addresses of that building (an apartment building for instance). The members of an address JSON object are not prescribed, to accommodate the different ways addresses are structured in different countries. If a location is necessary (eg to locate the position of the front door) then a member with the name "location" can be added to the "address" member, and it should contain a "MultiPoint" geometry.

"CityObjects": {
  "id-1": {
    "type": "Building",
    "attributes": {
      "roofType": "gabled roof"
    },
    "geographicalExtent": [ 84710.1, 446846.0, -5.3, 84757.1, 446944.0, 40.9 ],
    "children": ["id-56", "id-832", "mybalcony"]
  },
  "id-56": {
    "type": "BuildingPart",
    "parents": ["id-1"],
    ...
  },
  "mybalcony": {
    "type": "BuildingInstallation",
    "parents": ["id-1"],
    ...
  }
  ...
}

Figure 9

"myroom": {
  "type": "BuildingRoom",
  "attributes": {
    "usage": "living room"
  },
  "parents": ["id-1"],
  "geometry": [{
    "type": "Solid",
    "lod": "2",
    "boundaries": [
      [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], ... ]
    ]
  }]
}

Figure 10

{
  "type": "Building",
  "address": [
    {
      "country": "Canada",
      "locality": "Chibougamau",
      "thoroughfareNumber": "1",
      "thoroughfareName": "rue de la Patate",
      "postcode": "H0H 0H0",
      "location": {
        "type": "MultiPoint",
        "lod": "1",
        "boundaries": [231]
      }
    }
  ]
}

Figure 11

8.4.  CityFurniture

See the CityGML v3.0.0 CityFurniture module for more details.

The geometry of a City Object of type "CityFurniture" can be represented with any Geometry Object.

"mystopsign": {
  "type": "CityFurniture",
  "attributes": {
    "function": "bus stop"
  },
  "geometry": [{
    "type": "MultiSurface",
    "lod": "2",
    "boundaries": [
      [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
    ]
  }]
}

Figure 12

8.5.  CityObjectGroup

See the CityGML v3.0.0 CityObjectGroup module for more details.

The CityGML concept of groups, which is used to aggregate City Objects based on certain criteria (think of a neighbourhood in a city for instance), is also adopted in CityJSON. The group is a City Object, and it can contain, if needed, a geometry (the polygon representing the neighbourhood for instance).

Since a "CityObjectGroup" is also a City Object, it can be part of another group.

A City Object of type "CityObjectGroup":

• must have one member with the name "children". The value is an array of the IDs (of type string) of the City Objects that the group contains. As for other City Objects, the City Objects must have the ID of the group in their "parents" member.

• may have one member with the name "children_roles". The value is an array of strings describing the role of each City Object in the group. This member must be of the same length as that of "children".

• may have one member with the name "attributes". The value is an object where the attributes of the City Object are listed.

• may have one member with the name "geometry". The value is an array containing 0 or more Geometry Objects. Notice that since the "CityObjectGroup" is a container of different City Objects, the concept of Level of Detail does not apply to it. Nevertheless, the "lod" member is still used for enforcing uniformity with all the other geometries.

"CityObjects": {
  "my-neighbourhood": {
    "type": "CityObjectGroup",
    "children": ["building1", "building2", "building3"]
  }
}

Figure 13

"CityObjects": {
  "my-neighbourhood": {
    "type": "CityObjectGroup",
    "attributes": {
      "location": "Chibougamau Sud"
    },
    "children": ["building1", "building3"],
    "children_roles": ["residential building", "voting location"],
    "geometry": [{
      "type": "MultiSurface",
      "lod": "2",
      "boundaries": [ [[2, 41, 5, 77]] ]
    }]
  }
}

Figure 14

8.6.  GenericCityObject

While the CityGML v3.0.0 Generics module has different classes for each space type, CityJSON has only one class to cover all 3 space types.

This object should be used for objects that are not covered explicitly by any of the CityGML classes.

The geometry of a City Object of type "GenericCityObject" can only be represented with these Geometry Objects: (1) "MultiPoint", (2) "MultiLineString", (3) "MultiSurface", (4) "CompositeSurface", (5) "Solid", or (6) "CompositeSolid".

"whatisthat": {
  "type": "GenericCityObject",
  "attributes": {
    "class": "a big bucket of water",
    "usage": "it's not clear"
  },
  "geometry": [{
    "type": "CompositeSurface",
    "lod": "1",
    "boundaries": [
      [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
    ]
  }]
}

Figure 15

8.7.  LandUse

See the CityGML v3.0.0 LandUse module for more details.

The geometry of a City Object of type "LandUse" can only be represented with these Geometry Objects: (1) "MultiSurface" or (2) "CompositeSurface".

"oneparcel": {
  "type": "LandUse",
  "geometry": [{
    "type": "MultiSurface",
    "lod": "1",
    "boundaries": [
      [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
    ]
  }]
}

Figure 16

8.8.  OtherConstruction

See the CityGML v3.0.0 Construction module for more details (OtherConstruction is one class).

This is used for constructions that are not buildings, bridges, or tunnels. Examples are:

• electricity pylon

• fence

• permanent water tank

• pontoon

• railway platform

• shed

• windmill

The geometry of a City Object of type "OtherConstruction" can be represented with any Geometry Object.

"mypylon": {
  "type": "OtherConstruction",
  "attributes": {
    "class": "windmill",
    "conditionOfConstruction": "underConstruction"
  },
  "geometry": [{
    "type": "MultiSurface",
    "lod": "2",
    "boundaries": [
       [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], ...
    ]
  }]
}

Figure 17

8.9.  PlantCover

See the CityGML v3.0.0 Vegetation module for more details (PlantCover is one class).

The geometry of a City Object of type "PlantCover" can only be represented with these Geometry Objects: (1) "Solid", (2) "CompositeSolid", (3) "MultiSolid", (4) "MultiSurface", (5) "CompositeSurface".

"myplants": {
  "type": "PlantCover",
  "attributes": {
    "averageHeight": 11.05
  },
  "geometry": [{
    "type": "MultiSolid",
    "lod": "2",
    "boundaries": [
      [
        [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[10, 13, 22, 31]] ]
      ],
      [
        [ [[5, 34, 31, 12]], [[44, 54, 62, 74]], [[111, 123, 922, 66]] ]
      ]
    ]
  }]
}

Figure 18

8.10.  SolitaryVegetationObject

See the CityGML v3.0.0 Vegetation module for more details (SolitaryVegetationObject is one class).

The geometry of a City Object of type "SolitaryVegetationObject" can be represented with any Geometry Object.

"onebigtree": {
  "type": "SolitaryVegetationObject",
  "attributes": {
    "trunkDiameter": 5.3,
    "crownDiameter": 11.0
  },
  "geometry": [{
    "type": "MultiPoint",
    "lod": "1",
    "boundaries": [1]
  }]
}

Figure 19

8.11.  TINRelief

See the CityGML v3.0.0 Relief module for more details (TINRelief is one class).

The geometry of a City Object of type "TINRelief" can only be represented with the Geometry Object "CompositeSurface".

CityJSON does not define a specific Geometry Object for a TIN (triangulated irregular network). It is simply a CompositeSurface for which every surface is a triangle (thus a polygon having 3 vertices, and no interior ring).

Notice that in practice any "CompositeSurface" is allowed for encoding a terrain, and that arbitrary polygons could also be used (not just triangles).

"myterrain01": {
  "type": "TINRelief",
  "geographicalExtent": [ 84710.1, 446846.0, -5.3, 84757.1, 446944.0, 40.9 ],
  "geometry": [{
    "type": "CompositeSurface",
    "lod": "1",
    "boundaries": [
       [[0, 3, 2]], [[4, 5, 6]], [[1, 2, 6]], [[2, 3, 7]], [[3, 0, 4]]
    ]
  }]
}

Figure 20

8.12.  Transportation

See the CityGML v3.0.0 Transportation module for more details.

Four City Objects are related to transportation:

• "Road"

• "Railway"

• "Waterway"

• "TransportSquare" (to model for instance parking lots and squares)

Observe that the Section, Intersection, and Track classes from CityGML are omitted because they can be easily specified using specific attributes.

"ma_rue": {
  "type": "Road",
  "attributes": {
    "class": "backwards",
    "clearanceSpace": 2.23,
    "clearanceSpaceUnit": "meter"
  },
  "children": ["sect1", "sect2"],
  "geometry": [...]
}
"sect1": {
  "type": "Road",
  "attributes": {
    "class": "section"
  },
  "parents": ["ma_rue"],
  "geometry": [...],
}

Figure 21

Similarly, the CityGML classes TrafficArea, AuxiliaryTrafficArea, Marking, and Hole are implemented as semantic surfaces (see Clause 9.3). That is, the surface representing a road should be split into sub-surfaces (therefore forming a "MultiSurface" or a "CompositeSurface") in which each of the sub-surfaces has semantics.

"ma_rue": {
  "type": "Road",
  "geometry": [{
    "type": "MultiSurface",
    "lod": "2",
    "boundaries": [
       [[0, 3, 2, 1, 4]], [[4, 5, 6, 9, 12]], [[0, 1, 5]], [[20, 21, 75]]
    ]
  }],
  "semantics": {
    "surfaces": [
      {
        "type": "TrafficArea",
        "surfaceMaterial": ["asphalt"],
        "function": "road"
      },
      {
        "type": "AuxiliaryTrafficArea",
        "function": "green areas"
      },
      {
        "type": "TrafficArea",
        "surfaceMaterial": ["dirt"],
        "function": "road"
      }
    ],
    "values": [0, 1, null, 2]
  }
}

Figure 22

8.13.  Tunnel

See the CityGML v3.0.0 Tunnel module for more details.

Six City Objects are related to tunnels:

• "Tunnel"

• "TunnelPart"

• "TunnelInstallation"

• "TunnelConstructiveElement"

• "TunnelHollowSpace"

• "TunnelFurniture"

The geometry of both "Tunnel" and "TunnelPart" can only be represented with these Geometry Objects: (1) "Solid", (2) "CompositeSolid", (3) "MultiSurface", (4) "CompositeSurface". The geometry of the other four objects can be represented with any Geometry Object.

All the above City Objects, except "Tunnel", must have a "parents" member. "TunnelInstallation", "TunnelConstructiveElement", "TunnelHollowSpace", and "TunnelFurniture" can have as parents a "Tunnel" or a "TunnelPart".

"CityObjects": {
  "Lærdalstunnelen": {
    "type": "Tunnel",
    "attributes": {
      "yearOfConstruction": 2000,
      "length": "24.5km"
    },
    "children": ["stoparea1"],
    "geometry": [{
      "type": "Solid",
      "lod": "2",
      "boundaries": [
        [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]] ]
      ]
    }]
  }
}

Figure 23

8.14.  WaterBody

See the CityGML v3.0.0 WaterBody module for more details.

The geometry of a City Object of type "WaterBody" can only be represented with these Geometry Objects: (1) "MultiLineString", (2) "MultiSurface", (3) "CompositeSurface", (4) "Solid", or (5) "CompositeSolid".

"mygreatlake": {
  "type": "WaterBody",
  "attributes": {
    "usage": "leisure",
  },
  "geometry": [{
    "type": "Solid",
    "lod": "2",
    "boundaries": [
      [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]] ]
    ]
  }]
}

Figure 24

9.1.  Coordinates of the vertices

A CityJSON Object must have one member named "vertices". The value is an array of arrays of 3 integers representing the coordinates of each vertex of the city model. The position of a vertex in this array (0-based) is used to represent the "boundaries" of Geometry Objects.

• one vertex must be an array with exactly 3 integers, representing the (x,y,z) location of the vertex before it is transformed to its real-world coordinates (with the Clause 10).

• the array of vertices may be empty.

• vertices may be repeated.

"vertices": [
  [102, 103, 1],
  [11, 910, 43],
  [25, 744, 22],
  ...
  [23, 88, 5],
  [8523, 487, 22]
]

Figure 25

9.2.  Arrays to represent boundaries

The depth of the hierarchy of arrays depends on the Geometry object, and is as follows.

• A "MultiPoint" has an array with the indices of the vertices; this array can be empty.

• A "MultiLineString" has an array of arrays, each containing the indices of a LineString.

• A "MultiSurface", or a "CompositeSurface", has an array containing surfaces, each surface is modelled by an array of arrays, the first array being the exterior boundary of the surface, and the others the interior boundaries.

• A "Solid" has an array of shells, the first shell being the exterior shell of the solid, and the others the interior shells. Each shell has an array of surfaces, modelled in the exact same way as a MultiSurface/CompositeSurface.

• A "MultiSolid", or a "CompositeSolid", has an array containing solids. Each solid is modelled as above.

  NOTE 1:    JSON does not allow comments, the comments in the example below (C++ style: //-- my comments) are only to explain the cases, and should be removed.

{
  "type": "MultiPoint",
  "lod": "1",
  "boundaries": [2, 44, 0, 7]
}

Figure 26

{
  "type": "MultiLineString",
  "lod": "1",
  "boundaries": [
    [2, 3, 5], [77, 55, 212]
  ]
}

Figure 27

{
  "type": "MultiSurface",
  "lod": "2",
  "boundaries": [
    [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
  ]
}

Figure 28

{
  "type": "Solid",
  "lod": "2",
  "boundaries": [
    //-- exterior shell
    [ [[0, 3, 2, 1, 22]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[1, 2, 6, 5]] ],
    //-- interior shell
    [ [[240, 243, 124]], [[244, 246, 724]], [[34, 414, 45]], [[111, 246, 5]] ]
  ]
}

Figure 29

{
  "type": "CompositeSolid",
  "lod": "3",
  "boundaries": [
    [ //-- 1st Solid
      [ [[0, 3, 2, 1, 22]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[1, 2, 6, 5]] ],
      [ [[240, 243, 124]], [[244, 246, 724]], [[34, 414, 45]], [[111, 246, 5]] ]
    ],
    [ //-- 2nd Solid
      [ [[666, 667, 668]], [[74, 75, 76]], [[880, 881, 885]], [[111, 122, 226]] ]
    ]
  ]
}

Figure 30

9.3.  Semantics of geometric primitives

A Semantic Object is a JSON object representing the semantics of a primitive of a geometry (e.g. a surface of a building). A Semantic Object may also represent other attributes of the primitive (e.g. the slope of the roof, or the solar potential). For surface and volumetric geometries (e.g. MultiSurface, Solid and MultiSolid), a primitive is a surface. If a geometry is a MultiPoint or a MultiLineString, then the primitives are its respective sub-parts: points and linestrings.

A Semantic Object:

• must have one member with the name "type". The value is one of the allowed values. These depend on the City Object (see below).

• may have one member with the name "parent". The value is an integer pointing to another Semantic Object of the same geometry (index of it, 0-based). This is used to explicitly represent to which wall or roof a window or door belongs to; there can be only one parent.

• may have one member with the name "children". The value is an array of integers pointing to other Semantic Objects of the same geometry (index of it, 0-based). This is used to explicitly represent the openings (windows and doors) of walls and roofs.

• may have other members in the form of a JSON key-value pair, where the value must not be a JSON object (but a string/number/integer/boolean).

{
  "type": "RoofSurface",
  "slope": 16.4,
  "children": [2, 37],
  "solar-potential": 5
}

{
  "type": "Window",
  "parent": 2,
  "type-glass": "HR++"
}

Figure 31

"Building", "BuildingPart", "BuildingRoom", "BuildingStorey", "BuildingUnit", and "BuildingInstallation" can have the following semantics:

• "RoofSurface"

• "GroundSurface"

• "WallSurface"

• "ClosureSurface"

• "OuterCeilingSurface"

• "OuterFloorSurface"

• "Window"

• "Door"

• "InteriorWallSurface"

• "CeilingSurface"

• "FloorSurface"

For "WaterBody":

• "WaterSurface"

• "WaterGroundSurface"

• "WaterClosureSurface"

For Transportation ("Road", "Railway", "TransportSquare"):

• "TrafficArea"

• "AuxiliaryTrafficArea"

• "TransportationMarking"

• "TransportationHole"

It is possible to define and use other semantics, but these have to start with a "+", inline with the rules defined in the section about Extensions (Clause 14).

{
  "type": "+SupportingWall"
}

Figure 32

Because in a given City Object (say a "Building") several primitives can have the same semantics (think of a complex building that has been triangulated, there can be dozens of triangles used to represent one planar surface), a Semantic Object can be declared once, and each of the primitives that are represented by it should point to it. This is achieved by first declaring all the Semantic Objects in an array, and then having an array where each primitive links to a Semantic Object (position in the array).

If a Geometry object has semantics, then the Geometry object:

• must have one member with the name "semantics", whose values are two properties: "surfaces" and "values". Both must be present.

Also:

• the value of "surfaces" is an array of Semantic Objects.

• the value of "values" is a hierarchy of arrays with integers. The depth depends on the Geometry object: for MultiPoint and MultiLineString this is a simple array of integers; for any other geometry type it is two less than the array "boundaries". An integer refers to the index in the "surfaces" array of the same geometry, and it is 0-based. If one surface has no semantics, a value of null must be used.

  NOTE:    For legacy reasons, we use "surfaces" to name the array of Semantic Objects. Nevertheless, this member is used for points and linestrings of MultiPoints and MultiLineStrings, as well.

{
  "type": "MultiSurface",
  "lod": "2",
  "boundaries": [
    [[0, 3, 2, 1]],
    [[4, 5, 6, 7]],
    [[0, 1, 5, 4]],
    [[0, 2, 3, 8]],
    [[10, 12, 23, 48]]
  ],
  "semantics": {
    "surfaces" : [
      {
        "type": "WallSurface",
        "slope": 33.4,
        "children": [2]
      },
      {
        "type": "RoofSurface",
        "slope": 66.6
      },
      {
        "type": "+PatioDoor",
        "parent": 0,
        "colour": "blue"
      }
    ],
    "values": [0, 0, null, 1, 2]
  }
}

Figure 33

{
   "type": "CompositeSolid",
   "lod": "2.2",
   "boundaries": [
     [ //-- 1st Solid
       [ [[0, 3, 2, 1, 22]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[1, 2, 6, 5]] ]
     ],
     [ //-- 2nd Solid
       [ [[666, 667, 668]], [[74, 75, 76]], [[880, 881, 885]] ]
     ]
   ],
   "semantics": {
     "surfaces" : [
       {
         "type": "RoofSurface"
       },
       {
         "type": "WallSurface"
       }
     ],
     "values": [
       [ //-- 1st Solid
         [0, 1, 1, null]
       ],
       [ //-- 2nd Solid get all null values
         [null, null, null]
       ]
     ]
   }
 }

Figure 34

9.4.  Geometry templates

CityGML’s ImplicitGeometries, better known in computer graphics as templates, are one method of compressing files since the geometries (such as benches, lamp posts, and trees) need to be defined only once. In CityJSON, they are implemented differently from what is specified in CityGML: they are defined separately in the file, and each template can be reused. By contrast, in CityGML, the geometry used for a given City Object is reused by other City Objects, there is thus no central location where all templates are stored.

The Geometry Templates are defined as a JSON object that:

• must have one member with the name "templates". The value is an array of Geometry Objects.

• must have one member with the name "vertices-templates". The value is an array of coordinates of each vertex of the templates (0-based indexing). The reason the vertices’ indices are not global is to ensure that operations on the vertices (eg for CRS transformation, for Clause 10, or calculating the bounding box of a dataset) will not be affected by the templates (since they will often be defined locally, and translated/rotated/scaled to their final position).

Observe that the geometry of a template can have semantic surfaces, and that appearances can be assigned to it.

"geometry-templates": {
  "templates": [
    {
      "type": "MultiSurface",
      "lod": "2.1",
      "boundaries": [
         [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]]
      ],
      "semantics": {
        "surfaces" : [
          {
            "type": "+Skylight",
          },
          {
            "type": "+PatioDoor",
          }
        ],
        "values": [0, 0, 1]
      }
    },
    {
      "type": "MultiSurface",
      "lod": "1.3",
      "boundaries": [
         [[1, 2, 6, 5]], [[2, 3, 7, 6]], [[3, 0, 4, 7]]
      ],
      "material": {...}
    }
  ],
  "vertices-templates": [
    [0.0, 0.5, 0.0],
    ...
    [1.0, 1.0, 0.0],
    [0.0, 1.0, 0.0]
  ]
}

Figure 35

A given template can be used as the geometry (or as one of the geometries) of a City Object. A new JSON object of type "GeometryInstance" is defined, and it:

• must have one member with the name "template", whose value is the position of the template in the "geometry-templates" (0-indexing).

• must have one member with the name "boundaries", whose value is an array containing only one vertex index, which refers to one vertex in the "vertices" member of a CityJSON file. (This is the reference point from which the transformations are applied, it is the `referencePoint in CityGML.)

• must have one member with the name "transformationMatrix", whose value is a 4×4 matrix (thus 16 values in an array) defining the rotation/translation/scaling of the template. Note that these 16 values are ordered row-by-row, as the example below shows.

{
  "type": "SolitaryVegetationObject",
  "geometry": [
    {
      "type": "GeometryInstance",
      "template": 0,
      "boundaries": [372],
      "transformationMatrix": [
        2.0, 0.0, 0.0, 0.0,
        0.0, 2.0, 0.0, 0.0,
        0.0, 0.0, 2.0, 0.0,
        0.0, 0.0, 0.0, 1.0
      ]
    }
  ]
}

Figure 36

11.1.  geographicalExtent (bbox)

While the geographical extent can be computed from the dataset itself, it is often useful to store it. It may be stored as an array with 6 values: [minx, miny, minz, maxx, maxy, maxz]. Notice that these values are in the real-world coordinate system of the dataset (based on Clause 11.5]) and have not been compressed with the "transform" member (Clause 10) as the "vertices" have been.

"metadata": {
  "geographicalExtent": [ 84710.1, 446846.0, -5.3, 84757.1, 446944.0, 40.9 ]
}

Figure 40

11.2.  identifier

A unique identifier for the dataset. It is recommended to use a universally unique identifier, but it is not obligatory.

"metadata": {
  "identifier": "44574905-d2d2-4f40-8e96-d39e1ae45f70"
}

Figure 41

11.3.  pointOfContact

The point of contact for the dataset. This is a JSON object that

• must have one member with the name "contactName". The value is the name of the contact.

• must have one member with the name "emailAddress". The value is a string with the email.

• may have one member with the name "role". The value describes the role that contact person/organisation has, it is one of the following: "resourceProvider", "custodian", "owner", "user", "distributor", "originator", "pointOfContact", "principalInvestigator", "processor", "publisher", "author", "sponsor", "co-author", "collaborator", "editor", "mediator", "rightsHolder", "contributor", "funder", "stakeholder".

• may have one member with the name "website". The value is the URL of point of contact.

• may have one member with the name "contactType". The value is a string which is either "individual" or "organization". For an "organization", the "website" can also be given.

• may have one member with the name "address". The value is a JSON object and any properties can be used, to accommodate the different ways addresses are structured in different countries.

• may have one member with the name "phone". The value is a string with the phone number.

• may have one member with the name "organization". The value is the name of the organisation, to be used if the "contactName" is the name of a person.

"pointOfContact": {
  "contactName": "Justin Trudeau",
  "emailAddress": "justin.trudeau@parl.gc.ca",
  "phone": "+1-613-992-4211",
  "address": {
    "thoroughfareNumber": "24",
    "thoroughfareName": "Sussez Drive",
    "postcode": "H0H 0H0",
    "locality": "Ottawa",
    "country": "Canada"
  },
  "contactType": "individual",
  "role": "pointOfContact"
}

Figure 42

11.4.  referenceDate

The date when the dataset was compiled, without the time of the day, only a "full-date" as defined in RFC 3339; Section 5.6 should be used.

"metadata": {
  "referenceDate": "1977-02-28"
}

Figure 43

11.5.  referenceSystem (CRS)

The coordinate reference system (CRS) is given as a URL formatted according to the OGC Name Type Specification:

http://www.opengis.net/def/crs/{authority}/{version}/{code}

Figure 44

where {authority} designates the authority responsible for the definition of this CRS (usually EPSG or OGC), and where {version} designates the specific version of the CRS (0 (zero) is used if there is no version).

For instance, the Dutch national CRS in 3D:

"metadata": {
  "referenceSystem": "https://www.opengis.net/def/crs/EPSG/0/7415"
}

Figure 45

Be aware that the CRS should be a three-dimensional one, ie the elevation/height values should be with respect to a specific datum.

11.6.  title

A string describing the dataset.

"metadata": {
  "title": "3D city model of Chibougamau, Canada"
}

Figure 46

12.1.  Geometry Object having material(s)

Each surface in a Geometry Object can have one or more materials assigned to it. To store the material of a surface, a Geometry Object may have a member "material". The value of this member is a collection of key-value pairs, where the key is the theme of the material, and the value is one JSON object that must contain either:

• one member "values". The value is a hierarchy of arrays with integers. Each integer refers to the position (0-based) in the "materials" member of the "appearance" member of the CityJSON object. If a surface has no material, then null should be used in the array. The depth of the array depends on the Geometry object, and is equal to the depth of the "boundary" array minus 2, because each surface ([[]]) gets one material.

• one member "value". The value is one integer referring to the position (0-based) in the "materials" member of the "appearance" member of the CityJSON object. This is used because often the materials are used to colour full objects, and repetition of materials is not necessary.

In the following example, the Solid has 4 surfaces, and there are 2 themes (irradiation and irradiation-2). These could represent, for instance, the different colours based on different scenarios of an solar irradiation analysis. Notice that the last surface gets no material (for both themes), thus null is used.

{
  "type": "Solid",
  "lod": "2.1",
  "boundaries": [
    [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[1, 2, 6, 5]] ]
  ],
  "material": {
    "irradiation": {
      "values": [[0, 0, 1, null]]
    },
    "irradiation-2": {
      "values": [[2, 2, 1, null]]
    }
  }
}

Figure 48

12.2.  Geometry Object having texture(s)

To store the texture(s) of a surface, a Geometry Object may have a member with the name "texture". Its value is a collection of key-value pairs, where the key is the theme of the textures, and the value is one JSON object that must contain one member "values", which is a hierarchy of arrays with integers. For each ring of each surface, the first value refers to the position (0-based) in the "textures" member of the "appearance" member of the CityJSON object. The other indices refer to the UV positions of the corresponding vertices (as listed in the "boundaries" member of the geometry). Therefore, each array representing a ring has one more value than the number of vertices in the ring.

The depth of the array depends on the Geometry object, and is equal to the depth of the "boundary" array.

In the following example, the Solid has 4 surfaces, and there are 2 themes: winter-textures and summer-textures could for instance represent the textures during winter and summer.. Notice that the last 2 surfaces of the first theme gets no material, thus the value null is used.

{
  "type": "Solid",
  "lod": "2.2",
  "boundaries": [
    [ [[0, 3, 2, 1]], [[4, 5, 6, 7]], [[0, 1, 5, 4]], [[1, 2, 6, 5]] ]
  ],
  "texture": {
    "winter-textures": {
      "values": [
        [ [[0, 10, 23, 22, 21]], [[0, 1, 2, 6, 5]], [[null]], [[null]] ]
      ]
    },
    "summer-textures": {
      "values": [
        [
          [[1, 10, 23, 22, 21]],
          [[1, 1, 2, 6, 5]],
          [[1, 66, 12, 64, 5]],
          [[2, 99, 21, 16, 25]]
        ]
      ]
    }
  }
}

Figure 49

12.3.  Material Object

A Material Object:

• must have one member with the name "name", whose value is a string identifying the material.

• may have the following members (their meaning is explained there):

  1. "ambientIntensity". The value is a number between 0.0 and 1.0.

  2. "diffuseColor". The value is an array with 3 numbers between 0.0 and 1.0 (RGB colour).

  3. "emissiveColor". The value is an array with 3 numbers between 0.0 and 1.0 (RGB colour).

  4. "specularColor". The value is an array with 3 numbers between 0.0 and 1.0 (RGB colour).

  5. "shininess". The whose value is a number between 0.0 and 1.0.

  6. "transparency". The value is a number between 0.0 and 1.0 (1.0 being completely transparent).

  7. "isSmooth". The value is a Boolean value, is defined in CityGML as a hint for normal interpolation. If this boolean flag is set to true, vertex normals should be used for shading (Gouraud shading). Otherwise, normals should be constant for a surface patch (flat shading).

  NOTE:    If only "name" is defined for the Material Object, then it is up to the application that reads the CityJSON file to attach a material definition to the "name". This might not always be possible. Therefore, it is advised to define as many from the optional members as needed for fully displaying the material.

"materials": [
  {
    "name": "roofandground",
    "ambientIntensity":  0.2000,
    "diffuseColor":  [0.9000, 0.1000, 0.7500],
    "emissiveColor": [0.9000, 0.1000, 0.7500],
    "specularColor": [0.9000, 0.1000, 0.7500],
    "shininess": 0.2,
    "transparency": 0.5,
    "isSmooth": false
  },
  {
    "name": "wall",
    "ambientIntensity":  0.4000,
    "diffuseColor":  [0.1000, 0.1000, 0.9000],
    "emissiveColor": [0.1000, 0.1000, 0.9000],
    "specularColor": [0.9000, 0.1000, 0.7500],
    "shininess": 0.0,
    "transparency": 0.5,
    "isSmooth": true
  }
]

Figure 50

12.4.  Texture Object

A Texture Object:

• must have one member with the name "type". The value is a string with either PNG or JPG as value.

• must have one member with the name "image". The value is a string with the name of the file. This file can be a URL (eg "http://www.someurl.org/filename.jpg"), a relative path (eg "appearances/myroof.jpg"), or an absolute path (eg "/home/elvis/mycityjson/appearances/myroof.jpg").

• may have one member with the name "wrapMode". The value can be any of the following: "none", "wrap", "mirror", "clamp", or "border".

• may have one member with the name "textureType". The value can be any of the following: "unknown", "specific", or "typical".

• may have one member with the name "borderColor". The value is an array with 4 numbers between 0.0 and 1.0 (RGBA colour).

"textures": [
  {
    "type": "PNG",
    "image": "http://www.someurl.org/filename.jpg"
  },
  {
    "type": "JPG",
    "image": "appearances/myroof.jpg",
    "wrapMode": "wrap",
    "textureType": "unknown",
    "borderColor": [0.0, 0.1, 0.2, 1.0]
  }
]

Figure 51

12.5.  Vertices-texture Object

An Appearance Object may have one member with the name "vertices-texture". Its value is an array of the (u,v) coordinates of the vertices used for texturing surfaces. Their position in this array (0-based) is used by the "texture" member of the Geometry Objects.

• the array of vertices may be empty.

• one vertex must be an array with exactly 2 values, representing the (u,v) coordinates.

• vertices may be repeated

"vertices-texture": [
  [0.0, 0.5],
  [1.0, 0.0],
  [1.0, 1.0],
  [0.0, 1.0]
]

Figure 52

13.1.  Decomposing an area into parts/tiles

One solution to handle a large dataset is to subdivide it into tiles or regions, and ensure that each part has a reasonable size. Each part becomes a CityJSON file.

13.2.  Text sequences and streaming with CityJSONFeature

Another solution is to decompose a CityJSON object into its features (the City Objects), create several JSON objects, and store them in a JSON Text Sequences (one example being JSON Lines). This is a format to store several JSON objects in a single file, and allows the processing of each object one at a time.

A CityJSON Feature Object allows the storage of a single feature, for instance a "Building" together with its children (of type "BuildingPart" and/or "BuildingInstallation"). Unlike a CityJSON Object, all the vertices and appearances of the object are local.

A CityJSON Feature Object:

• is a JSON object.

• must have one member with the name "type". The value must be "CityJSONFeature".

• must have one member with the name "id". The value must be a string representing the identifier of the City Object Feature. This is used to clearly identify which of the CityObjects is the parent.

• must have one member with the name "CityObjects". The value is a collection of key-value pairs, where the key is the ID of the object, and the value is one City Object. The ID of a City Object should be unique (within one "CityJSONFeature"), and all the children of the "CityJSONFeature" must be included (and the children of the children (recursively), if there are any).

• must have one member with the name "vertices". The value is an array of coordinates of each vertex of the current City Object Feature (stored with integers). Their position in this array (0-based) is used as an index to be referenced by the Geometry Objects for the JSON object (warning: the vertices are local to the JSON object).

• may have one member with the name "appearance". The value may contain JSON objects representing the textures and/or materials of surfaces. See [Appearance_object] for details.

• must not have other members.

{
  "type": "CityJSONFeature",
  "id": "myid",
  "CityObjects": {},
  "vertices": [],
  "appearance": {}
}

Figure 53

{
  "type": "CityJSONFeature",
  "id": "id-1",
  "CityObjects": {
    "id-1": {
      "type": "Building",
      "attributes": {
        "roofType": "gabled roof"
      },
      "children": ["mypart"],
      "geometry": [...]
    },
    "mypart": {
      "type": "BuildingPart",
      "parents": ["id-1"],
      "children": ["mybalcony"],
      "geometry": [...]
    },
    "mybalcony": {
      "type": "BuildingInstallation",
      "parents": ["mypart"],
      "geometry": [...]
    }
  },
  "vertices": [...]
}

Figure 54

The following root members of a CityJSON Object are not allowed in a CityJSONFeature Object:

• "transform"

• "version"

• "metadata"

• "geometry-templates": these should either be resolved/dereferenced, or they should be placed in the metadata or collection

• "extensions": these should be placed in the metadata or collection

Note that a CityJSON Feature Object does not contain all the information that is required for parsing the feature. Most commonly, the transformation properties (the Transform Object) and CRS must be known by the client in order to correctly georeference the City Objects. These properties may be known by the client upfront, or they may be accessible in a CityJSON Object, which is sent as the first object in a JSON Lines text stream, or in other ways not described here (for instance RESTful APIs often have a mechanism to retrieve metadata).

In case the properties are stored in a CityJSON Object, this object needs to be a valid CityJSON Object. This implies that the CityJSON object must contain all the required properties, including "CityObjects" and "vertices", even though they are empty, because this information is stored in the subsequent CityJSON Features.

Below is an example of a CityJSONFeature stream (or a JSON Lines text file), with a CityJSON Object storing the metadata and transformation properties, as well as geometry templates:

{"type":"CityJSON","version":"2.0","transform":{...},"CityObjects":{},"metadata":{...},"vertices":[], "geometry-templates":{...}}
{"type":"CityJSONFeature","id":"a","CityObjects":{...},"vertices":[...]}
{"type":"CityJSONFeature","id":"b","CityObjects":{...},"vertices":[...]}
{"type":"CityJSONFeature","id":"c","CityObjects":{...},"vertices":[...]}

Figure 55

14.1.  Using an Extension in a CityJSON file

An Extension should be given a name (eg Noise) and the URL of the Extension file should be defined, including the version of the Extension that is used for this file. It is expected that the Extension is publicly available at the URL, and can be downloaded.

Several Extensions can be used in a single CityJSON Object, each one is indexed by its name in the "extensions" JSON object. In the example below we have two Extensions: one named Noise and one named Solar_Potential.

{
  "type": "CityJSON",
  "version": "2.0",
  "extensions": {
    "Noise": {
      "url" : "https://someurl.org/noise.json",
      "version": "2.0"
    },
    "Solar_Potential": {
      "url" : "https://someurl.org/solar.json",
      "version": "0.8"
    }
  },
  "CityObjects": {},
  "vertices": []
}

Figure 57

14.2.  The Extension file

A CityJSON Extension is a JSON object, and it must have the following 8 members:

1. one member with the name "type". The value must be "CityJSONExtension".

2. one member with the name "name". The value must be a string identifying the extension.

3. one member with the name "url". The value must be a string with the HTTP URL of the location of the schema where the JSON object is located.

4. one member with the name "version". The value must be a string identifying the version of the Extension.

5. one member with the name "versionCityJSON". The value must be a string (X.Y) identifying the version of CityJSON that uses the Extension.

6. one member with the name "extraAttributes". The value must be a JSON object. Its content is part of a JSON schema (explained below), or an empty object.

7. one member with the name "extraCityObjects". The value must be a JSON object. Its content is part of a JSON schema (explained below), or an empty object.

8. one member with the name "extraRootProperties". The value must be a JSON object. Its content is part of a JSON schema (explained below), or an empty object.

9. one member with the name "extraSemanticSurfaces". The value must be a JSON object. Its content is part of a JSON schema (explained below), or an empty object.

{
  "type": "CityJSONExtension",
  "name": "Noise",
  "description": "Extension to model the noise",
  "url": "https://someurl.org/noise.ext.json",
  "version": "0.5",
  "versionCityJSON": "2.0",
  "extraAttributes": {},
  "extraCityObjects": {},
  "extraRootProperties": {},
  "extraSemanticSurfaces": {},
}

Figure 58

"items": {
  "oneOf": [
    {"$ref": "geomprimitives.json#/Solid"}
  ]
}

14.3.  Case 1: Adding new properties at the root of a document

It is allowed to add a new member at the root of a CityJSON file, but if one wants to document it in a schema, then the member’s name must start with a "+". Imagine we wanted to store some census data for a given neighbourhood for which we have a CityJSON file, then we could define the extra root member "+census" as follows:

"extraRootProperties": {
  "+census": {
    "type": "object",
    "properties": {
      "percent_men": {
        "type": "number",
        "minimum": 0.0,
        "maximum": 100.0
      },
      "percent_women": {
        "type": "number",
        "minimum": 0.0,
        "maximum": 100.0
      }
    }
  }
}

Figure 60

And a CityJSON file would look like this:

{
  "type": "CityJSON",
  "version": "2.0",
  "extensions": {
    "Census": {
      "url": "https://someurl.org/census.ext.json",
      "version": "0.7"
    }
  },
  "CityObjects": {...},
  "vertices": [...],
  "+census": {
    "percent_men": 49.5,
    "percent_women": 51.5
  }
}

Figure 61

14.4.  Case 2: Defining attributes for existing City Objects

It is also possible to add, and document in a schema, specific attributes, for example if we wanted to have the colour of the buildings as a RGBA value (red-green-blue-alpha):

{
  "type": "Building",
  "attributes": {
    "storeysAboveGround": 2,
    "+colour": {
      "rgba": [255, 255, 255, 1]
    }
  },
  "geometry": [...]
}

Figure 62

Another example would be to store the area of the parcel of a building, and also to document the unit of measurement (UoM):

{
  "type": "Building",
  "attributes": {
    "storeysAboveGround": 2,
    "+area-parcel": {
      "value": 437,
      "uom": "m2"
    }
  },
  "geometry": [...]
}

Figure 63

For these two cases, the CityJSON Extension object would look like the snippet below. Notice that "extraAttributes" may have several properties (the types of the City Objects are the possibilities) and then each of these has as properties the new attributes (there can be several).

An extra attribute must start with a "+"; it is good practice to prepend the attribute with the name of the Extension, to avoid that 2 attributes from 2 different Extensions have the same name.

The value of the member is a JSON schema, this schema can reference and reuse JSON objects already defined in the CityJSON schemas. Thus, the keywords of the member values are defined by the JSON Schema specification. For instance, "additionalProperties" is a JSON-schema keyword stating that one is not allowed to add properties to this JSON object, beyond the ones defined in the schema (eg "value", "uom").

"extraAttributes": {
  "Building": {
    "+colour": {
      "type": "object",
      "properties": {
        "rgba": {
          "type": "array",
          "items": {"type": "number"},
          "minItems": 4,
          "maxItems": 4
        }
      },
      "required": ["rgba"],
      "additionalProperties": false
    },
    "+area-parcel": {
      "type": "object",
      "properties": {
        "value": { "type": "number" },
        "uom": { "type": "string", "enum": ["m2", "feet2"] }
      },
      "required": ["value", "uom"],
      "additionalProperties": false
    }
  }
}

Figure 64

14.5.  Case 3: Defining a new Semantic Object

It is possible to define a new Semantic Object (besides the ones prescribed, see Clause 9.3), and document it in the Extension.

New Semantic Objects must have a "+" as their first character, and other attributes/properties can be defined.

"extraSemanticSurfaces": {
  "+ThermalSurface": {
    "type": "object",
    "properties": {
      "type": { "enum": [ "+ThermalSurface" ] },
      "azimuth": {"type": "number"}
    },
    "required": [ "type", "azimuth" ],
    "additionalProperties": false
  }
}

Figure 65

14.6.  Case 4: Creating and/or extending new City Objects

The creation of a new City Object is done by defining it in the CityJSON Extension object in the "extraCityObjects" member:

"extraCityObjects": {
  "+NoiseBuilding": {
    "allOf": [
      { "$ref": "cityobjects.json#/_AbstractBuilding" },
      {
        "properties": {
          "type": { "enum": ["+NoiseBuilding"] },
          "attributes": {
            "properties": {
              "buildingLDenMin": {"type": "number"}
            }
          }
        },
        "required": ["type"]
      }
    ]
  }
}

Figure 66

"extraCityObjects": {
  "+NoiseBuildingPart": {
    "allOf": [
      { "$ref": "cityobjects.json#/_AbstractBuilding" },
      {
        "properties": {
          "type": { "enum": ["+NoiseBuildingPart"] },
          "attributes": {
            "properties": {
              "buildingLDenMin": {"type": "number"}
            }
          }
        },
        "required": ["type", "parents"]
      }
    ]
  }
}

Figure 67

Since all City Objects are documented in the schemas of CityJSON (in cityobjects.schema.json), it is basically a matter of copying the parts needed in a new file and modifying its content.

A new name for the City Object must be given and it must begin with a "+".

Because City Objects can be of different levels (1st-level ones can exist by themselves; 2nd-level ones need to have a parent), we need to explicitly define that the "parents" member is mandatory for 2nd-level objects.

Please note that since JSON schemas do not allow inheritance, the only way to extend a City Object is to define an entirely new one (with a new name, eg "+NoiseBuilding"). This is done by copying the schema of the parent City Object and extending it.

14.7.  Rules to follow to define new City Objects

The challenge when creating Extensions to the core model is that we do not want to break the software packages (viewers, spatial analysis, etc) that already read and process CityJSON files. While one could define a new City Object and document it, if this new object does not follow the rules below then it will mean that new specific software needs to be built for it (and this would go against the fundamental ideas behind CityJSON).

1. The name of a new City Object must begin with a "+", eg "+NoiseBuilding".

2. A new City Object must conform to the rules of CityJSON, ie it must contain a member "type".

3. Existing City Objects cannot be extended and have new types as children, eg it is not allowed to add a new City Object "+Balcony" to a "Building". Instead, a new type, eg "+FancyBuilding", should be created and it can have a "+Balcony" as a potential child.

4. All the geometries must be in the member "geometry", and cannot be located somewhere else deep in a hierarchy of a new member.

5. The Geometry object’s boundary must be one of the eight types described in Clause 9. Similarly, the geometry appearances and templates must follow the core specification. This ensures that all the code written to process, manipulate, and view CityJSON files will be working without modifications.

6. The reuse of types defined in CityJSON, eg "Solid" or semantic surfaces, is allowed.