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Figure 1. Geometrical object classes. 
As mentioned before, the concepts of the model, or 
modelling primitives, and operations on them are 
chosen so that they do not imply any semantic content 
which cannot be described formally. The semantics of 
objects are captured in the application specific database 
schema by naming the geometrical objects and 
characterizing of them with attributes. It is up to the 
database designer to decide which concepts of the 
model he wants to use, depending on the processing 
requirements of the application. For example, 
geographical reality could be modelled as a collection 
of isolated geometrical objects, with no spatial 
relationships between them, or the relationships could 
be taken into the model, thus permitting information 
on the context of an entity to be captured. 
4.1 STRUCTURAL FEATURES OF THE MODEL - 
OBJECT CLASSES 
The geometrical object model is represented 
graphically in Figure 1. In the figure, only the 
structural features of the model are shown, the 
Operations, that is, methods of object classes are 
459 
described in a later section. The following conventions 
are used in the figure. Circles represent abstract classes, 
and rectangles represent stored attribute values. The 
ISA relationships between objects are shown by 
unlabelled arrows. Labelled arrows denote attributes of 
an object class. Multivalued attributes have double 
arrowheads. As far as the instantiation of the model is 
concerned, note the two types of values of an attribute: 
the value may be stored as a simple value (rectangle), 
or it may be another object, that is, its identification. 
Grey arrows indicate the domain of the attributes. 
The model consists of abstract object classes, and 
operations, that is, methods defined for them. There 
are two types of classes in the model 1) Classes that 
serve for definitional purposes, which by themselves 
do not describe the geometry of an object. These classes 
provide a general abstraction mechanism. 
AssociationObject and AggregationObject are such 
classes. 2) Geometrical objects, that are subclasses of the 
classes mentioned above.