Full text: The 3rd ISPRS Workshop on Dynamic and Multi-Dimensional GIS & the 10th Annual Conference of CPGIS on Geoinformatics

ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001 
description of the scene (objects, geometry, colours), which 
is very appropriate for the rendering system. 
Accuracy. The accuracy of the re-constructed 3D model 
plays an important role in the entire process. All the objects 
that are approachable by the user within few meters 
distance have to be reconstructed with an accuracy that 
corresponds to the accuracy of real-time extracted lines 
from the video images. The user might get as close to an 
object as one centimetre may be of significance. Indeed, 
aiming at centimetre accuracy of the objects is a very 
expensive and practically an unrealistic requirement. 
However, certain parts, elements or section of real objects 
have to ensure accuracy ranging from few centimetres to a 
decimetre. Doors and windows at street level, balconies at 
lower floors, statues, paths, tiles (see Figure 9), etc. are 
examples of such elements. The re-construction methods 
applied to achieve such precision are given in section 4. 
i .;. 4> 
Figure 2: Geometry details represented by line features (the 
façade in the middle) 
Analysis of the requirements reveals the complexity of the issue 
leading even to a contradiction, i.e. high level of details and 
maintenance of 3D topology. Apparently, the construction and 
maintenance of complete 3D topological model (including 
windows, doors, etc.) is time and effort expensive and therefore 
inappropriate. One approach (utilised in our project) is a clear 
discrimination between the types of data needed for the two 
subsystems. The line features (expected from the positioning 
subsystem) are kept as straight lines (see Figure 2). The 
outlines of the 3D objects are considered a separate data set 
and organised in a 3D topological data structure (see Figure 3). 
Thus the amount of data to be topologically maintained can be 
significantly reduced that contributes to the simplification of 
many spatial operations and thus to the speeding up the data 
retrieval. Furthermore, the two data sets are linked by explicit 
reference between line features and façades. The number of 
lines is expected to be rather large (for one façade, it may rise to 
300-400) and therefore the 3D reconstruction process takes 
care relationships “a line belong to a face” to be created and 
explicitly stored in the database. 
The research in 3D data structuring attracts a lot of attention in 
the last several years. The focus is basically on an extended 
conceptual model capable of integrating geometric (position, 
shape and size) and thematic characteristics of objects, and 
mutual spatial relationships. One stream of investigations 
emphasises on formalism (structure, ordering and operators) to 
construct a geometric object regardless of the dimension (see 
[10]). Such models aim at the complete representation of all the 
topological relationships among the objects from different 
dimensions. The models can be referred to as an implicit 
representation of objects, i.e. the relationships are stored and 
the description of the objects can be derived out of them. The 
disadvantage is the size of the database that grows 
tremendously with the complexity of the model. Many reported 
3D models give priorities to topological models that maintain 
objects (i.e. an explicit description of objects). More details on 
data structures of this group can be found in [3], [5], [9], [15]. 
The major problem of such 3D models is that a few of them are 
tested on large data sets under real-time requirements. 
An intensive work on clarifying guidelines for developing GIS 
and database software (maintaining 2D, 3D topology) is carried 
out as well. OpenGIS specifications are one of the commonly 
accepted standards (see [6]). The approaches proposed there, 
however, are based on separate maintenance of geometry and 
topology objects, which in practice leads to large duplications. 
Furthermore, the most of the topological models proposed for 
implementation consider mostly the 2D world. 
Figure 3: Outlines of 3D objects 
Bearing in mind the requirements to the model delineated in the 
previous section and utilising recent achievements in 3D GIS 
research, we propose a 3D topological model extended to 
provide data to augmented reality application. The proposed 3D 
model is a typical boundary model with explicit description of 
objects. To represent its geometric properties related shape, 
size and position, a spatial object can be associated with four 
abstractions namely point, iinestring, surface and polyhedron 
(see Figure 4). The notations of the four abstract objects 
correspond to the ones accepted in the OpenGIS specifications. 
A point is an object that does not have shape or size but 
position. A linestring is a type of an object that has length and 
position. A surface is an abstraction of object that has position 
and area. A polyhedron has a position and a volume. These 
objects are built of smaller, simpler elements, called node and 
face. Nodes describe spatial objects that can be represented as 
linestrings (e.g. pipe lines) and points (e.g. trees, lampposts). 
Nodes are constructive elements of faces as well. The order of 
the nodes in the face is maintained as wheel. The orientation of 
the faces is anticlockwise looking at the objects (e.g. buildings) 
from outside. Faces are to be used for the reconstruction of 
objects that are associated with surfaces (e.g. streets, parking 
lots) and polyhedrons (e.g. buildings). 
Figure 4: Examples of spatial objects to be supported 
Every 3D object can be represented by its thematic and physical 
characteristics as well. For example the spatial object building 
has a year of building, owner and usage that is referred to as 
thematic characteristics. Physical properties are related to 
surface reflectance that determines the colour and texture of the 
objects. Although the model is potentially capable of maintaining

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