I ERES 
Roland Billen 
  
logico - physical at the end. These explanations are very simplified, more information can be found in Pantazis (1996). 
Our opinion is that the influence of the 3D introduction is either conceptual or technical. 
The technical influence can be considered as the adaptation of tools and techniques to 3D referential, e.g. : - 3D 
formalisms; - 3D data structures; - 3D processing; - 3D acquisition modes; - 3D visualisation modes; - etc. This 
influence is more related to the physical implementation and conceptual model of data and process. But it is also 
important for the descriptive level, for example a 3D visualisation can help the user to think in 3D. All these tools and 
techniques are widely studied, for example in Breunig (1996) and Flick (1996). 
The conceptual influence is the core of our thought. It is directly related to the descriptive and the conceptual levels. It 
can be viewed as all the means which can help the user to spatialise his urban interests in 3D. These means can be 
derived from cognition models of urban environment (by to specialist or layman). Note that the other abstraction levels 
are influenced. Indeed, this new 3D way of thinking must be taught to the users at the end of the conception method in 
order to insure a complete 3D use of the GIS. 
The rest of the article is dedicated to a primary mean to handle the 3D thinking in urban studies. 
4 URBAN OBJECT SPECIFICATIONS 
A way to help the user to express his needs is to provide him a basic 3D object framework. This basic framework is 
conceptual and is not related to any data structures. The experience acquired in the field of 2D GIS, show that a 
complete definition of the object and of its metadata is the necessary basis for a proper GIS conception. These concepts 
are extended to the 3D urban GIS, and stand in a user perspective, i.e. define object specification. The fact that the 
proposed 3D data fit on the specification is not taken into account in this primary study. Indeed, the needs of the users 
would be expressed in an ideal virtual 3D model without any technical constraint. From then on, the user would create a 
definitive conceptual model, and would use it to evaluate the new technologies (structure or acquisition modes) which 
could appear in the future. 
The idea is to fix some basic 3D urban objects. At an upper level, other objects can be defined and are related to the 
lower ones by a generalisation relationship. Conversely, it should be possible to go from an upper level object to a 
lower one (figure 2). 
  
  
  
  
  
  
  
  
E E. 
1. Cell 2. Building is an aggregation of cell(s) 
  
  
Pd A 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
3. Building ensemble is an generalisation of building(s) 
Figure 2. Some potential urban objects 
For example, a cell (ex: an apartment) is a basic 3D urban object which composes an other object, the building. The 
generalisation relationship between these objects is only an aggregation, so the 3D geometry of the building is directly 
derived from the geometry of the cell. An upper object, the building ensemble, is composed by buildings but its 
geometry is the generalisation of the geometry of the building. The aim of these subdivisions is to conceptualise a part 
of the urban environment, and to identify the object of interest. For example, for a price analysis of flats in connection 
with the height and their view, the cell object is of primary interest. On the other hand, a study on the propagation of 
GPS signal in urban area would need the building ensemble object. 
There are several ways to obtain the 3D geometry of an object. : - measurement; - estimation; - generalisation; - 
desaggregation; 
Measurement 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 81