The wire model also fits to the more specific task of 
determining the intersections of as surface with a set of 
given planes. Moreover, also edges, boundary lines or struc- 
ture lines can be included. The wire model, seen as a graph 
can in addition to line elements contain single points, such 
as spot heights or relative maxima and minima. 
2. Data Structure 
2.1 The Wire Model 
The main requirement for the data structure of the wire 
model is that it must allow updating. This then also allows 
to build up the model during the measuring process, an im- 
portant feature, as the structure of the graph is not known 
in beforehand. In our special case we also need a connection 
of all arcs belonging to the same plane for constructing 
profiles. Finally also single points should be taken into 
account. This leads to a data structure with three inter- 
connected lists, one for arcs, one for nodes and one for 
planes. 
The basic element of the wire model is the arc. The list 
of arcs for each arc contains its starting and its end node 
and its description. It is a connected list, where a pointer 
to the next arc of the same plane is attached to each arc. 
As the graph itself should be independent of the arc 
description, the description is not stored in core memory, 
thus only a pointer to a description file is attached to 
each arc. This, in an easy way, allows to use the same data 
structure for profiling or contouring, but also for covering 
the surface with a band of irregularily filled surface 
patches (cf. fig. le). Thus the list of arcs has the follo- 
wing form (p - pointer): 
L.arc = {p(startnode),p(endnode),p(nextarc),p(description)} 
The nodes are collected in a separate list. For each node 
it contains the three coordinates in object space, the three 
components of the normal vector and a list of pointers to 
the planes where the node belongs to. Thus the list of nodes 
has the following form: 
L.node - íx, y, z; cos a, cos B, cos t; {p(plane)}} 
The degree of the node, i. e. the number of arcs being 
connected at a node can vary between 0 and 6. The degree in 
our context is specific for each point (cf. figure 2): 
0: .. isolated point. |No,.arc.is tied to it, e. g. at a spot 
height. 
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