3.2.1. Mapping rules and metadata

3.2.1.1. Mapping of classes onto tables

In general, every class of the UML diagram is mapped onto a separate table; the name of the table is identical to the class name (leading underscores indicating abstract classes are omitted). If multiple classes are contained in an orange box in the UML diagram though, these classes are mapped onto a single table in the relational schema.

Scalar attributes of the classes become columns of the corresponding table with identical name. The types of attributes are customized to corresponding data types of the target database systems PostgreSQL/PostGIS and Oracle as shown in the following Table 3.1.

Table 3.1 Data type mapping (excerpt)

UML	PostgreSQL / PostGIS	Oracle
String, anyURI	VARCHAR, TEXT	VARCHAR2, CLOB
Integer	NUMERIC	NUMBER
Double, gml:LengthType	DOUBLE PRECISION	BINARY_DOUBLE
Boolean	NUMERIC	NUMBER(1,0)
Date	DATE or TIMESTAMP WITH TIME ZONE	DATE or TIMESTAMP WITH TIME ZONE
Complex Types (Color, TransformationMatrix, CodeType etc.)	VARCHAR	VARCHAR2
Enumeration	VARCHAR	VARCHAR2
GML Geometry, textureCoordinates	GEOMETRY	SDO_GEOMETRY
GML RectifiedGridCoverage	RASTER	SDO_GEORASTER & SDO_RASTER
Texture (only reference of type anyURI in CityGML)	BYTEA	BLOB

3.2.1.2. Explicit metadata about feature classes

The central metadata table OBJECTCLASS contains all feature classes supported by the 3D City Database. Every CityGML feature class is assigned a unique and stable ID in this table. For example, Building is assigned the ID value 26 and Bridge has the value 64. In addition, the name of the feature class is stored in the attribute CLASSNAME. The name of the table onto which the feature class has been mapped is provided by the TABLENAME column.

The SUPERCLASS_ID attribute references the direct superclass of the feature class and, thus, maps the class hierarchy. The additional BASECLASS_ID attribute points to the root class of the hierarchy and can be used to quickly understand whether an entry in OBJECTCLASS represents a GML feature type, object type or data type without having to traverse the entire class hierarchy. If a feature class represents a CityGML top-level feature, the IS_TOPLEVEL flag is set to 1 and 0 otherwise.

All city objects stored in the 3D City Database are registered in the root table CITYOBJECT. This table has an attribute OBJECTCLASS_ID which references an entry in OBJECTCLASS. This way, the class of the city object can be easily and efficiently identified. If required, its class name, its feature table, whether it is a top-level feature, and even its (transitive) superclasses can also be queried. In addition to CITYOBJECT, all tables that are used to store CityGML features provide an OBJECTCLASS_ID attribute.

Note

Registering CityGML ADEs in the 3D City Database leads to additional entries in the OBJECTCLASS table for each class defined in the ADE. The OBJECTCLASS table has two more attributes IS_ADE_CLASS and ADE_ID which are used to manage and identify ADE classes. More information is provided in Section 3.2.16 for more information.

Table 3.2 Excerpt of the OBJECTCLASS table

ID	CLASSNAME	SUPERCLASS_ID
0	Undefined
1	_GML
2	_Feature	1
3	_CityObject	2
4	LandUse	3
5	GenericCityObject	3
6	_VegetationObject	3
7	SolitaryVegetationObject	6
8	PlantCover	6
9	WaterBody	105
10	_WaterBoundarySurface	3
11	WaterSurface	10
12	WaterGroundSurface	10
13	WaterClosureSurface	10
14	ReliefFeature	3
15	_ReliefComponent	3
16	TINRelief	15
17	MassPointRelief	15
18	BreaklineRelief	15
19	RasterRelief	15
20	_Site	3
21	CityFurniture	3
22	_TransportationObject	3
23	CityObjectGroup	3
24	_AbstractBuilding	20
25	BuildingPart	24
26	Building	24
27	BuildingInstallation	3
28	IntBuildingInstallation	3
29	_BuildingBoundarySurface	3
30	BuildingCeilingSurface	29
31	InteriorBuildingWallSurface	29
32	BuildingFloorSurface	29
33	BuildingRoofSurface	29
34	BuildingWallSurface	29
35	BuildingGroundSurface	29
36	BuildingClosureSurface	29
37	_BuildingOpening	3
38	BuildingWindow	37
39	BuildingDoor	37
40	BuildingFurniture	3
41	BuildingRoom	3
42	TransportationComplex	22
43	Track	42
44	Railway	42
45	Road	42
46	Square	42
47	TrafficArea	22
48	AuxiliaryTrafficArea	22
49	FeatureCollection	2
50	Appearance	2
51	_SurfaceData	2
52	_Texture	51
53	X3DMaterial	51
54	ParameterizedTexture	52
55	GeoreferencedTexture	52
56	_TextureParametrization	1
57	CityModel	49
58	Address	2
59	ImplicitGeometry	1
60	OuterBuildingCeilingSurface	29
61	OuterBuildingFloorSurface	29
62	_AbstractBridge	20
63	BridgePart	62
64	Bridge	62
65	BridgeInstallation	3
66	IntBridgeInstallation	3
67	_BridgeBoundarySurface	3
68	BridgeCeilingSurface	67
69	InteriorBridgeWallSurface	67
70	BridgeFloorSurface	67
71	BridgeRoofSurface	67
72	BridgeWallSurface	67
73	BridgeGroundSurface	67
74	BridgeClosureSurface	67
75	OuterBridgeCeilingSurface	67
76	OuterBridgeFloorSurface	67
77	_BridgeOpening	3
78	BridgeWindow	77
79	BridgeDoor	77
80	BridgeFurniture	3
81	BridgeRoom	3
82	BridgeConstructionElement	3
83	_AbstractTunnel	20
84	TunnelPart	83
85	Tunnel	83
86	TunnelInstallation	3
87	IntTunnelInstallation	3
88	_TunnelBoundarySurface	3
89	TunnelCeilingSurface	88
90	InteriorTunnelWallSurface	88
91	TunnelFloorSurface	88
92	TunnelRoofSurface	88
93	TunnelWallSurface	88
94	TunnelGroundSurface	88
95	TunnelClosureSurface	88
96	OuterTunnelCeilingSurface	88
97	OuterTunnelFloorSurface	88
98	_TunnelOpening	3
99	TunnelWindow	98
100	TunnelDoor	98
101	TunnelFurniture	3
102	HollowSpace	3
103	TexCoordList	56
104	TexCoordGen	56
105	_WaterObject	3
106	_BrepGeometry	0
107	Polygon	106
108	BrepAggregate	106
109	TexImage	0
110	ExternalReference	0
111	GridCoverage	0
112	_genericAttribute	0
113	genericAttributeSet	112