faces could replace the planes. In case the measuring proce- 
dure is able to automatically detect break or structure 
lines also these could be embeded into this concept. Then 
the description of the "plane" will be more complex, then, 
instead of the description, possibly requiring a pointer to 
a separate description file. 
This data structure allows a stepwise growing of the 
graph. 
2.2 The Measurement Tree 
The hierarchical measurement procedure discussed in sect. 
1.1 in a natural way corresponds to a pyramidal structure of 
the image data available in each level. The image patch of 
one level contains the image patches of the next lower level 
leading to a tree type data structure. The measuring strate- 
gy analogously could follow a measuring tree. 
In our case the logical measuring unit is the arc in the 
wire model. Also, the scale factor between the levels is 
dictated by the pull-in range of the matching procedures 
(of. "table- 1). In a first step we moreover only aim at the 
precise measurment of the intersection of the given planes 
with the surface. Thus not the whole area within the image 
patches of the individual levels needs to be processed. 
These restrictions can be used to advantage when building up 
the actual measuring tree. 
The tree for the arc measurment procedure is shown in 
fig. 3. The vertical axis corresponds to the levels (bottom- 
up), the horizontal axis corresponds to the coordinate along 
the arc or to the time (left-right). The leaves of the tree 
represent results of the LSM, thus the final aim of the arc 
measurement procedure. The connections between the leaves 
realized by the branches demonstrate the impossibility of 
independently measuring the points on an arc. 
Figure 3: 
Measurement tree with path of procedure (cf. sect. 3.1) 
  
  
+ 616 +