CityGML ( City Geography Markup Language )

Geospatial information model (ontology) for urban landscapes base on the ISO 191xx
GML3 representation of 3D geometries, based on the ISO 19107 model
Representation of object surface characteristics (e.g. textures, materials)
Taxonomies and aggregations

Digital Terrain Models as a combination of (including nested) triangulated irregular networks
(TINs), regular rasters, break and skeleton lines, mass points
Sites (currently buildings, bridges, and tunnels)
Vegetation (areas, volumes, and solitary objects with vegetation classification)
Water bodies (volumes, surfaces)
Transportation facilities (both graph structures and 3D surface data)
Land use (representation of areas of the earth’s surface dedicated to a specific land use)
City furniture
Generic city objects and attributes
User-definable (recursive) grouping

Multiple representations in different LODs simultaneously; generalisation relations between objects in different LODs

Application Domain Extensions (ADE): Specific “hooks” in the CityGML schema allow to define application specific extensions, for example for noise pollution simulation, or to augment CityGML by

properties of the new National Building Information Model Standard (NBIMS) in the U.S

CityGML is an open data model and XML-based format for the storage and exchange of virtual 3D city models.
The aim of the development of CityGML is to reach a common definition of the basic entities, attributes, and relations of a 3D city model
CityGML not only represents the graphical appearance of city models but specifically addresses the representation of the semantic and thematic properties, taxonomies and aggregations. CityGML includes a geometry model and a thematic model.
CityGML differentiates five consecutive Levels of Detail (LOD), where objects become more detailed with increasing LOD regarding both their geometry and thematic differentiation

CityGML supports different Levels of Detail (LOD).
LODs are required to reflect independent data collection processes with differing application requirements
the same object may be represented in different LOD simultaneou sly, enabling the analysis and visualisation of the same object with regard to different degrees of resolution

LOD0 is essentially a two and a half dimensional Digital Terrain Model over which an aerial image or a map may be draped. Buildings may be represented in LOD0 by footprint or roof edge polygons.
LOD1 is the well-known blocks model comprising prismatic buildings with flat roof structures
LOD2 has differentiated roof structures and thematically differentiated boundary surfaces
LOD3 denotes architectural models with detailed wall and roof structures potentially including doors and windows
LOD4 completes a LOD3 model by adding interior structures for buildings