2.8. Definition of the CRS for a 3D City Database instance¶
The definition of the CRS of a 3D City Database instance consists of two components: 1) a valid Spatial Reference Identifier (SRID, typically the EPSG code) and 2) an OGC GML conformant definition identifier for the CRS. Both components are defined during the database setup (see Section 1.3) and are further stored in the table DATABASE_SRS (see Fig. 2.28).
The SRID is an integer value key pointing to spatial reference information within Oracle’s MDSYS.CS_SRS table or PostGIS’ SPATIAL_REF_SYS table. Both DBMSs are shipped with a large number of predefined spatial reference systems. At setup time, the SRID chosen as default value for the 3D City Database instance must already exist in the mentioned tables.
The GML conformant CRS definition identifier is used as value for the gml:srsName attribute on GML geometry elements when exporting database contents to CityGML instance documents. It should follow the OGC recommendation for the Universal Resource Name (URN) encoding of CRSs given in the OGC Best Practice Paper Definition identifier URNs in OGC namespace [Whit2009]. At setup time, please make sure to provide a URN value which corresponds to the spatial reference system identified by the default SRID of the database instance. Since CityGML is a 3D standard, the URN encoding shall always represent a three-dimensional CRS which, for example, can be denoted as compound coordinate reference systems [Whit2009]. The general syntax of a URN encoding for a compound reference system is as follows:
Authority, version, and code depend on the information authority providing the CRS definition (e.g. EPSG or OGC). The following example shows a possible combination of an SRID (here referring to a 2D CRS) and CRS URN encoding (3D) to set up an instance of the 3D City Database:
The example SRID is referencing a Projected CRS defined by EPSG (DHDN / 3-degree Gauss-Krüger zone 2; used in the western part of Germany; EPSG-Code: 31466). The URN encodes a compound coordinate reference system which adds a Vertical CRS as height reference (DHHN92 height, EPSG-Code: 5783).