primitives user defined 
  
SS De 
ua 
  
  
  
  
Figure 1. Basic and user defined primitives 
4.2. Basic primitives 
The basic primitives that are used are the classical 
boxes, spheres, cones and prisms. These different 
primitives contain more specific primitives with a more 
restrictive topology : a box contains a cube, a cone 
contains a cylinder, a prism contains a pyramid. 
The topological definition of the basic primitives is taken 
from a library of predefined shapes. 
An extension of these primitives involves new basic 
element types : 
Bands, Traces, Faces,  H(eight)-Summits, 
H(eight)-Contour Points , H(eight)-Points 
   
projection 
  
considered as a regular surface limited by its 
surrounding shape. 
It exists two different ways to define such a Band : 
- the first way consists to acquire the superior shape of 
the Band and to project it onto a reference surface, 
- the second way consists to acquire the shape directly 
on the ground surface (intersection between the Band 
and the ground surface) and to elevate it to a specific 
user defined level or elevation. 
e A Trace is a linear based surface that is the 
equivalent to the surface based volume called Band 
described before. In this case, the acquired element is 
a line. The acquisition exists also in the two same 
ways as for Bands. 
e H(eight)-Summits / -Contour Points / -Points are 
different types of points that are possible to be used 
in the model. Like with Bands and Traces, these 
different types of points make it possible to increase 
the dimension by one for an object that is acquired as 
a point (its projection on the ground surface is a 
point). Its linear dimension is defined between the 
measured summit and his projection onto the ground 
surface. 
Like the Bands and Traces, the H-Elements can be 
acquired directly on the ground surface and then laid out 
to a user defined level or elevation. 
e A Face is a surface that is contained in and that 
belongs hierarchically to another ground surface. 
4.3. Other complex primitives and modelling 
If a primitive does not exist in the library of basic 
primitives, then it can be created through its topology by 
using the different tools of a CAD-system (Figure 1.). 
The different elements that compose the new objects are 
horizontal 
vertical 
parallel 
10 
\ verf? 
o 
CN 
4 
Figure 2. Tools and new element types 
e A Band is an object that is acquired in form of a 
surface shape and that will be used as a volume. The 
volume is obtained by means of the projection of the 
acquired and measured surface on another surface 
which is the ground surface. The surface is 
assembled in this way according to the topology of 
construction (right angles, elements in line, projection, 
extension, object membership...). 
At time of the building of the CAD-primitives, the specific 
points used for the acquisition and the topology of these 
462 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
  
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