Table 1: Levels of a hierarchy for semiautomatic matching on
an analytical plotter with CCD-cameras
Oper. - operator, requ. - required
level|algo-| pixel| window size pull-in range| precision
rithm| size | [dx] [mm]! [dx] [mm] | [dx] [mm]
dx
0 LSM 20 um| 16 0.32 2 0.04 0.4 .002
1 FBM 20 um| 40 0.8 12 0.24 1.0 .02
2 FBM | 100 um| 40 4.0 12 1-2 1-0 „12
3 oper. 20° 10° requ. +5
4 oper. 230. 230 requ. 5.
In order to arrive at a (nearly) fully automatic measu-
ring system the levels 3 and 4 have to be replaced. We use a
sequential procedure for our solution. Thus the measuring
head scans the model. The selection of the scan path has to
be adapted to the specific task.
1.2 Surface Description
There are various ways to describe a surface in space
(cf. Bóhm et. ^al. 1984, Henderson/Bhanu 1985). Mathemati-
cally oriented descriptions essentially have a parametric
form, "e. 5g. (x(u,v),y(u,v),z(u,v)), where u and v are sur-
face coordinates. The specific role of the Z-ccordinate for
describing topographic surfaces reduces this triple to
2(x,y) implying x = u and y s v. This is reasonable as, ex-
cept for a few places, z(x,y) gives a one to one relation
between the computer representation and the terrain points.
Changing to an automatic measuring system this reduced
form z = z(x,y) is not powerful enough any more, even in
topographical applications as without using the interpre-
tation capabilities of the operator the system has to cope
with objects as e. g. houses or trees where at least verti-
cal surface parts are visible in one or more images. This
obviously also holds for close range applications. But also
the general parametric form is easily applicable only for
simple surfaces, where the parametrization is clear in
advance.
In CAD-systems this problem has been solved efficiently
by a finite element tessalation of the surface. There are at
least two ways to represent this tessalation. Both are based
on the idea of describing it as a graph with nodes, arcs and
areas, i. e. surface patches. One either stores the nodes
and arcs, i. e. the boundary lines of the surface patches,
which allows an efficient manipulation for interactive gra-
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