3.1.12. Tunnel model¶
The tunnel model is closely related to the building model. It supports the representation of thematic and spatial aspects of tunnels and tunnel parts in four levels of detail, LOD1 to LOD4. The UML diagram of the tunnel model is shown in Fig. 3.21. The pivotal class of the model is _AbstractTunnel, which is a subclass of the thematic class _Site (and transitively of the root class _CityObject). _AbstractTunnel is specialized either to a Tunnel or to a TunnelPart. Since an _AbstractTunnel consists of TunnelParts, which again are _AbstractTunnels, an aggregation hierarchy of arbitrary depth may be realized. Both classes Tunnel and TunnelPart inherit the attributes of _AbstractTunnel: the class of the tunnel, the function, the usage, the year of construction and the year of demolition. In contrast to _AbstractBuilding, Address features cannot be assigned to _AbstractTunnel.
The geometric representation and semantic structure of an _AbstractTunnel is shown in Fig. 3.21. The model is successively refined from LOD1 to LOD4. Therefore, not all components of a tunnel model are represented equally in each LOD and not all aggregation levels are allowed in each LOD. An object can be represented simultaneously in different LODs by providing distinct geometries for the corresponding LODs.
Similar to the building and bridge models (cf. Section 3.1.5 and Section 3.1.6), only the outer shell of a tunnel is represented in LOD1 – 3, which is composed of the tunnel’s boundary surfaces to the surrounding earth, water, or outdoor air. The interior of a tunnel may only be modelled in LOD4.
In LOD1, a tunnel model consists of a geometric representation of the tunnel volume. Optionally, a MultiCurve representing the TerrainIntersectionCurve can be specified. The geometric representation is refined in LOD2 by additional MultiSurface and MultiCurve geometries. In LOD2 and higher LODs the outer structure of a tunnel can also be differentiated semantically by the classes _BoundarySurface and TunnelInstallation. A boundary surface is a part of the tunnel’s exterior shell with a special function like wall (WallSurface), roof (RoofSurface), ground plate (GroundSurface), outer floor (OuterFloorSurface), outer ceiling (OuterCeilingSurface) or ClosureSurface (see Fig. 3.22). The TunnelInstallation class is used for tunnel elements like outer stairs, strongly affecting the outer appearance of a tunnel. A TunnelInstallation may have the attributes class, function and usage.
In LOD3, the openings in _BoundarySurface objects (doors and windows) can be represented as thematic objects. In LOD4, the highest level of resolution, also the interior of a tunnel, composed of several hollow spaces, is represented in the tunnel model by the class HollowSpace. This enlargement allows a virtual accessibility of tunnels, e.g. for driving through a tunnel, for simulating disaster management or for presenting the light illumination within a tunnel. The aggregation of hollow spaces according to arbitrary, user defined criteria (e.g. for defining the hollow spaces corresponding to horizontal or vertical sections) is achieved by employing the general grouping concept provided by CityGML (cf. Section 3.1.2).
Interior installations of a tunnel, i.e. objects within a tunnel which (in contrast to furniture) cannot be moved, are represented by the class IntTunnelInstallation. If an installation is attached to a specific hollow space (e.g. lamps, ventilator), they are associated with the HollowSpace class, otherwise (e.g. pipes) with _AbstractTunnel. A HollowSpace may have the attributes class, function and usage whose possible values can be enumerated in code lists. The class attribute allows a general classification of hollow spaces, e.g. commercial or private rooms, and occurs only once. The function attribute is intended to express the main purpose of the hollow space, e.g. control area, installation space, and storage space. The attribute usage can be used if the way the object is actually used differs from the function. Both attributes can occur multiple times.
The visible surface of a hollow space is represented geometrically as a Solid or MultiSurface. Semantically, the surface can be structured into specialized _BoundarySurfaces, representing floor (FloorSurface), ceiling (CeilingSurface), and interior walls (InteriorWallSurface). Hollow space furniture, like movable equipment in control areas, can be represented in the CityGML tunnel model with the class TunnelFurniture. A TunnelFurniture may have the attributes class, function and usage.