3.2.8. Generics schema¶
3D city models will most likely contain attributes, which are not explicitly modelled in CityGML. Moreover, there may be 3D objects that are not covered by the thematic classes of CityGML. Generic objects and attributes help to support the storage of such data.
GENERIC_CITYOBJECT
For generic objects the full variety of different geometrical representations known from other tables is offered. Explicit (LODx_BREP_ID, LODx_OTHER_GEOM) and implicit geometry (LODx_IMPLICIT_REP_ID, LODx_IMPLICIT_REF_POINT, LODx_IMPLICIT_TRANS-FORMATION) as well as terrain intersection curves (LODx_TERRAIN_INTERSECTION) (all with 0 ≤ x ≤ 4).
Fig. 3.46 GenericCityObject and generic attributes database schema
CITYOBJECT_GENERICATTRIB, CITYOBJECT_GENERICATT_SEQ
The table CITYOBJECT_GENERICATTRIB is used to represent the concept of generic attributes. However, the creation of a table for every type of attribute was omitted. Instead a single table CITYOBJECT_GENERICATTRIB represents all types and the types are differentiated via the values of the attribute DATATYPE.
The table provides fields for every data type, but only one of those fields is relevant in each case. An overview of the meaning of the entries in the field DATATYPE is given in Table 3.16. The relation between the generic attribute and the corresponding CityObject is established by the foreign key CITYOBJECT_ID.
DATATYPE
|
attribute type
|
1
|
STRING
|
2
|
INTEGER
|
3
|
REAL
|
4
|
URI
|
5
|
DATE
|
6
|
MEASURE
|
7
|
Group of generic attributes
|
8
|
BLOB
|
9
|
Geometry type
|
10
|
Geometry via surfaces in the table SURFACE_GEOMETRY
|
Note
Please note that the binary and geometric data types (incl. geometry via surfaces) are not supported by CityGML and cannot be exported using the CityGML Importer/Exporter tool.
Generic attributes can also be grouped using the CityGML class genericAttributeSet. Since genericAttributeSet itself is a generic attribute, it may also be contained in a generic attribute set facilitating a recursive nesting of arbitrary depth. This hierarchy within a genericAttributeSet is realized by the foreign key PARENT_GENATTRIB_ID which refers to the superordinate genericAttributeSet (aggregate) and contains NULL, if such does not exist. The foreign key ROOT_GENATTRIB_ID refers directly to the top level (root) of a genericAttributeSet tree. In order to select all generic attributes forming a genericAttributeSet one only has to select those with the same ROOT_GENATTRIB_ID.
The next available ID for the table CITYOBJECT_GENERICATTRIB is provided by the sequence CITYOBJECT_GENERICATT_SEQ.