Geography Reference
In-Depth Information
CityGML
CityGML (
City Geography Markup Language
) is an open data model and
XML-based format for sets of 3D urban objects. It is implemented as an application
schema for the GML3 (
Geography Markup Language version 3
). Both are extend-
ible international standards for spatial data exchange issued by the OGC (
Open
Geospatial Consortium
). CityGML is used for the representation, storage, and
exchange of virtual 3D city models. Generalization hierarchies among thematic
classes, aggregations, and relations between objects are also included in this
application schema (Kolbe
2009
). CityGML provides a standard model and mech-
anisms for describing 3D objects with respect to their geometry, topology, seman-
tics and appearance; furthermore, it defines five levels of detail (LoD). Individual
levels of detail contain, for example, following elements:
• LoD0—2,5D model of terrain, building footprints,
• LoD1—extruded building footprints,
• LoD2—3D buildings with simplified modelling of roofs,
• LoD3—3D buildings with modelling of exterior details,
• LoD4—3D buildings with modelling of interior details.
CityGML provides much more than simply 3D content for visualization, as it has
many diverse applications. CityGML supports class definitions, regulations, and
explanations of the semantics for geographic features in 3D city models, including
digital terrain models, buildings, vegetation, transportation objects, and city furni-
ture. Practical applications often require the storage and exchange of data which do
not belong to any of the predefined classes. CityGML provides two different
methods of extension for such cases. The first is the use of generic features and
attributes; however, this method reduces semantic interoperability. A concept
called
Application Domain Extension
(ADE) is used to eliminate this problem. In
contrast with generic objects and attributes, ADE has to be defined within a separate
XML Schema definition.
CityGML includes a relatively general data model, whose content can be linked
to a topographic database. Handling specific issues that are associated with the 3D
modelling of buildings with interiors requires the extension of this structure. And,
for this purpose, various specialized ADEs are used (Kolbe
2009
; OGC City
Geography Markup Language Encoding Standard
2012
).
ADEs, which describe in detail the interiors of buildings and technical facilities
inside the building, are for example CAFM (
Computer Aided Facility Management
)
ADE, GeoBIM ADE and UtilityNetworkADE. Class Buildings is through CAFM
ADE extended to other descriptive information used for facility management. This
ADE is designed for LoD 4 and adds to existing classes (e.g.
FloorSurface
),
abstract subclasses, and attribute data characterizing a manufacturer, condition, a
responsible person, or a year of construction (Bleifuss et al.
2009
).
GeoBIM ADE combines information contained in the IFC format with
CityGML and designs standard GeoBIM. Like the previous ADE, this ADE extends
