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CityGML

CityGML is an XML standard for describing 3D models of cities. It's based on another XML standard called Geography Markup Language (GML). Both standards are developed by the standardization consortium OGC.

The basics of CityGML are very intuitive and easy to understand. The document's root node is a CityModel, which contains a list of CityObjects. These city objects can be of type Building, Bridge, Vegetation, Tunnel and a few others. Each city object can have some attributes to describe its characteristics, and will contain some GML features to define its geometry.

Imagine two buildings each with walls and a roof:

The corresponding CityGML would contain a single CityModel and two Buildings. Each building could have attributes like "yearOfConstruction" and "measuredHeight" and would contain a so called lod1MultiSurface. This entity would contain the necessary GML geometry to define the hull of the buildings, for example a list of polygons.

The raw, simplified XML would look something like this:

<CityModel>
    <cityObjectMember>
        <bldg:Building>
            <bldg:yearOfConstruction>1940</bldg:yearOfConstruction>
            <bldg:measuredHeight>16.4</bldg:measuredHeight>
            <bldg:lod1MultiSurface>
                <gml:MultiSurface>
                    <gml:surfaceMember>
                        <gml:Polygon>
                            <gml:exterior>
                                <gml:LinearRing srsName="CH1903" srsDimension="3">
                                    <gml:pos>683278.754 247335.818 409.3</gml:pos>
                                    <gml:pos>683286.378 247338.829 409.3</gml:pos>
                                    <gml:pos>683286.378 247338.829 425.7</gml:pos>
                                    <gml:pos>683278.754 247335.818 425.7</gml:pos>
                                    <gml:pos>683278.754 247335.818 409.3</gml:pos>
                                </gml:LinearRing>
                            </gml:exterior>
                        </gml:Polygon>
                    </gml:surfaceMember>
                    [...]
                </gml:MultiSurface>
            </bldg:lod1MultiSurface>
        </bldg:Building>
    </cityObjectMember>

    <cityObjectMember>
        <bldg:Building>[...]</bldg:Building>
    </cityObjectMember>
</CityModel>

The polygons defining the walls of the buildings contain a bunch of 3D coordinates (gml:pos). Each coordinate consists of three numbers: longitude, latitude and height. The coordinates are expressed in a specific coordinate-system (spatial reference system SRS), in this case the Swiss coordinate system "CH1903" as is defined with the attribute srsName="CH1903".

The commonly used coordinate system is "WGS84". The same polygon expressed as coordinates in WGS84 would look like this:

<gml:LinearRing srsName="WGS84" srsDimension="3">
    <gml:pos>8.541258 47.3715983 409.3</gml:pos>
    <gml:pos>8.5413594 47.3716244 409.3</gml:pos>
    <gml:pos>8.5413594 47.3716244 425.7</gml:pos>
    <gml:pos>8.541258 47.3715983 425.7</gml:pos>
    <gml:pos>8.541258 47.3715983 409.3</gml:pos>
</gml:LinearRing>

CityGML gives us a way to describe a set of buildings (and other city objects) and the exact geometric shape of their walls and other exterior parts.

3D Tiles

3D Tiles is a specification from Cesium to stream and render 3D meshes. One of its use cases is to progressively load 3D buildings on a map, when zooming in.

A 3D Tiles dataset is described in a file called tileset.json. This file defines the area for which it contains data (longitude, latitude and height) and a URL where that data can be loaded:

{
  "root": {
    "boundingVolume": {
      "region": [
        0.14900111790831405, 0.8267774614090546,
        0.14908570176240377, 0.8267920546921999,
        409.3, 425.7
      ]
    },
    "content": {
      "url": "buildings.b3dm"
    }
  }
}

The tileset metadata file can also define how to load higher-resolution data for the same region when the client zooms in. But for our purpose we'll ignore that functionality and stick with the bare basics.

Our data URL references a file called buildings.b3dm which is a file in the Batched 3D Model format. This file contains the raw data to render all models in our scene in the GL transmission format (glTF). The two houses from before would be declared and rendered as 36 triangles. Each vertex of each triangle will have a numeric identifier to indicate which model it belongs to ("batchId").

The B3DM file also contains a so called Batch Table, which stores custom properties for the models in the scene. For our two buildings that could look like this:

{
  "id": [1, 2],
  "yearOfConstruction": [1940, 1902],
  "measuredHeight": [16.4, 20.3]
}

Together these two pieces of information (models' geometry and models' properties) allow to render buildings and to visualize the values of properties in different ways.