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thematic and physical characteristics, these aspects are not
discussed here. Since the rendering system visualises virtual
objects (real objects are considered in case of occluding), colour
and texture of real objects are not of interest and are not to be
maintained. More details related to physical properties of a
spatial object can be found in [15].
The new element in the 3D topological model is the organisation
of the data for positioning. The line features are encapsulated
with their co-ordinates and stored as a separate data set, i.e.
lines. Each line is considered as a strait line represented by two
sets of co-ordinates.
The IFO semantic data model is used to represent the spatial
relationships between all the objects. The motivation is based on
the fact that the IFO semantic model uses object-oriented
notations to represent objects and their relationships and
provides a mechanism to map them into a relational model.
Three kinds of object-types (abstract, printable and derived) are
distinguished by the model (see [1]). An abstract type is an
object that cannot be shown printed, mapped, etc., e.g. a
building, a street. A printable type of object is an object that can
be printed, shown, etc. e.g. co-ordinates of a building or a street.
The objects that are composed of some printable objects or
other atomic objects are derivable. From atomic (printable or
derivable) objects complex objects can be derived by
aggregation and grouping. The principle difference is that
aggregated objects contain elements from different types, while
grouped objects contain objects only from one type. IS-A
relationships are utilised to define sub-types (specialisation) and
super-types (generalisation) of objects. Figure 5 shows the
graphical notations for the objects, relationships and principles
of construction.
a) types o» objects:
o- C3 i i printable
b) functional relationships:
h»(1:m) has (1:1) is-a
Figure 5: Notations of IFO model
Applying these graphical notations we obtain the schema of the
model shown on Figure 6. The four abstract objects named
geometric objects (GO), one explicit spatial relationship (GR)
and two constructive objects (CnsO) attempt at representation of
all the objects.
Figure 6: The proposed 3D topological model
The topological model is similar to the ones presented in [3], [5]
and [9] but differs in number of construction elements, i.e. only
two. The 1 D-cell, often called arc or edge (see [5], [9], [10]), is
omitted. The arc in 2D space have the unique feature of defining
1:2 relationships with faces and nodes, i.e. an arc has two
neighbouring faces and nodes. This feature is only partially true
in 3D and therefore the explicit storage of arcs does not bring a
significant facilitation. Such representation, i.e. without arcs, has
shown speed acceleration in many data retrieval operations (see
[15]). Moreover the computational complexity of writing the
VRML file is largely reduced, since the representations of
polyhedron and surface are similar to description of irregular
shapes in the VRML node IndexedFaceSet (see [14]).
Figure 7: Modelling of complex spatial objects
4 DATA COLLECTION
The 3D reconstruction of urban areas is still a rather complex
process involving quite a lot of time and manual interactions.
Several different methods are applied to provide data with the
appropriate accuracy and resolution and to obtain the 3D
topology. The utilised methods can be subdivided into three
major groups: manual, semi-automatic and automatic. Whilst
manual and semi-automatic methods are used in the case of
relatively limited amounts of object elements with high accuracy
and 3D topology are required (i.e. to build the topological
model), automatic methods are applied for collection of many
elements with non-topological organisation (i.e. the line
features). Furthermore objects (e.g. buildings) with complex
shape and variety of details (balconies, overhanging roofs) are
modelled manually from images taken at street level. The
images are processed in a 3D modelling software (i.e.
PhotoModellei) that ensures accuracy and resolution within
required values. A snapshot from CosmoPlayer shows some of
the reconstructed objects of this group (see Figure 7).
Figure 8: Modelling of buildings without overhanging roofs
Simpler objects such as buildings with flat or gable roofs are
reconstructed applying semi-automatic methods. For example
the reconstruction of buildings with flat roofs and roof outlines
that can be projected onto the footprints (i.e. lack of overhanging
roofs) are reconstructed by a procedure utilising existing DTM
and manually digitised roof outlines (see Figure 8). The walls
and the corresponding 3D topology are obtained fully
automatically (see [16]). Terrain objects as streets, bicycle or
c) constructs:
/ X) ND
grouping
colouA texture
