CMRT09: Object Extraction for 3D City Models, Road Databases and Traffic Monitoring - Concepts, Algorithms, and Evaluation 
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Figure 1: The integrated model schema as a grUML diagram. Semantic entities are colored white with namespace ”sem”, appearance 
entities are colored gray with namespace ”app” and geometry/topology entities are colored dark gray with namespace ”geo/top”. 
TGraphs are directed graphs whose vertices and edges are typed, 
ordered and attributed. Their structure, types and attributes help 
to model the different aspects (topology, geometry, semantics, 
and appearance annotation) of urban objects in a common in 
tegrated data structure. TGraphs are supported by a powerful 
API (JGraLab 9 ) in combination with a graph query’ language 
(GReQL) and a corresponding UML-based metamodeling ap 
proach (grUML). grUML is a subset of UML class diagrams 
which allows the specification of classes of TGraphs on the 
schema level (Ebert et al., 2008). Figure 1 contains an example. 
2.2 Lightweight component model 
If all relevant data of the urban object model are stored in a 
TGraph, all processing of the model can be encapsulated in ap 
propriate components working on this particular TGraph. 
The work described here is based on a light-weight Java compo 
nent model which is employed for the different processing activ 
ities on the model (see section 4). The component concept is ba 
sically an extension of the well-known strategy pattern (Gamma 
et al., 1995). Every component has a definition in the form of a 
Java interface which describes its service and at least one imple 
mentation in the form of a Java class. 
9 http://j gralab.uni-koblenz.de 
Components are serializable and get their data to process as argu 
ments of their execute ()-methods. Further data that influence 
their work are handled as parameters which have a default value 
and are manipulated via getters and setters. For example some 
processing steps can be configured by parameters (like thresh 
olds). 
3 THE INTEGRATED MODEL SCHEMA 
The internal representation of urban object models by TGraphs 
has to be specified by a metamodel, called schema in the follow 
ing. This schema defines the set of compliant TGraphs. Classes 
define the possible vertex types, and associations define the edge 
types. The attributes of vertices and edges can be added accord 
ing to the well-known UML notation, as well. Edge direction is 
visualized by arrow heads, though it should be noted, that TGraph 
edges are traversable in both directions by algorithms. 
Figure 1 shows the main parts of an integrated schema which de 
fines a set of TGraphs for urban objects. (To improve readability 
all enumeration types and some semantic subclasses as well as 
attributes are elided.) This schema is inspired by and partially 
derived from the CityGML 1.0 schema. Especially it follows the 
idea to separate the four relevant aspects of an urban object model 
(namely topology, geometry, semantics, and appearance).