The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
103 
Objects’ features and relations have been set using a geometric 
and topological model suited to the theory known as Boundary 
Representation (Foley, 1995). 
Real objects are represented using 0, 1, 2, 3 dimensional 
primitives (e.g. node, edge, face, toposolid)' each primitive of a 
higher order is made of primitives belonging to the previous 
order. 
Geometric primitives of 0, 1,2 dimension (e.g. point, curve, 
surface) correspond to topological primitives of the same order. 
Solids are bounded at least with 4 surfaces connected to each 
other; surfaces must be flat and oriented in order to mark the 
inner and the outer side of each plane. 
Each surface is bounded with an ordered set of edges (outer 
ring); empty areas are bounded with further inner rings. Each 
edge must be a straight line, bounded with oriented nodes. Only 
nodes are defined through x, y, z coordinates. 
-H ■ 1 
‘ tort node ' 
6 . enttnmte , 
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ftëinîPfiïpvnï' "■ ài 
Figure 1. The spatial model 
In the development of a spatial model it is necessary to detect a 
‘main’ class, whose properties are transferred to the others; 
Giant Root can restrict properties of geometric and topological 
classes. 
groups of geometric objects with similar features; thematic 
layers act in a similar way, since they collect geographical 
objects linked with same-level relationships {_Construction, 
Artefact, Transportation, etc.). 
GIANT_Feature is a ‘type’ class to represent geographical 
objects in the model. 
The purpose is to manage composite geometric objects as if 
they were geographical objects within the thematic structure of 
the model (buildings, rivers, roads, etc.) 
Geographical objects inherit the features of their thematic class 
and the features of the corresponding geometric class, defined 
in GML specifications (CompositePoint, CompositeCurve, 
CompositeSurface, CompositeSolid) (Figure 2). 
GlANTJheme 
GWNT_Surf»ceCompcnent 
ft.* 
GtAWTjSurveC&mponent I GIANT^PansComponen! 
Figure 2. Relations between geometric and thematic classes 
‘Construction’ is the class of ‘ _Building’ and ‘_Artefact'; the 
objects of this class are objects of the GIANT model, whose 
properties are defined according to the GIANT Root class. 
The fBuilding’ class includes urban objects classified as 
buildings; his attributes are inherited from the class ‘'Building' 
and from ‘BuildingPart'. 
The main class is a subclass of JTopoPrimitive and of 
_GeometricPrimitive, due to the constraint assigned to the 
model that imposes the two classes are coincident. Such 
structure can manage data and processes both topological and 
geometric (Figure 1). 
In the proposed model objects can be primitives or primitives 
collections, but not complex objects. 
The model uses the 'Xlink' concept from GML3 specifications. 
It allows to define the spatial attribute of an object only once: if 
an attribute is part of a higher order object, or of objects 
belonging to different thematic classes, it can be linked. 
A ‘gmT.id’ identifier has been attributed to each primitive; such 
identifier can be recalled using the 'href attribute. Such 
constraint removes redundancies and makes the database 
simpler. 
The model’s thematic layers {GIANTJTheme) act as the 
‘composite’ geometric classes {CompositeCurve, 
CompositeSurface, CompositeSolid). Geometric classes collect 
A building can be divided in several parts {Building Part) 
having their own identity. 
A block represents an element of town planning scheme, but 
several parts (buildings) usually constitute a block (Figure 3). 
Building units inside each construction are defined through the 
class ‘orizzontaipartitioning', which is an attribute of the 
abstract class ‘_Building'. 
The abstract class ‘ Building' allows the representation of 3D 
geometric elements that constitute buildings’ bound. The model 
uses the ‘bounded' geometric attribute, that is a 
‘ _BoundedbyType'. 
< ;. —— ■ ■ - ~> 
<xs:complexType name="_BuildingType" abstract="true"> 
<xs:complexContent> 
<xs:extension base="_ConstructionType"> 
<xs:sequence> 
<xs:element name="bounded" 
type="_BoundedbyType" minOccurs="07> 
</xs:sequence>