analogous image 
HHH © HHH measuring system 
  
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| 
operator 
  
  
  
  
  
X, Y 
  
  
  
Fig. 2.3 Measurement principle of a 
digitizing tablet 
iL a digital image 
image proces- 
sing hardware 
  
  
  
  
  
  
X, y 
  
  
  
Fig. 2.4 Principle of measurement on a 
monitor 
  
stereoscopic 
measurement 
devices 
  
  
  
  
  
  
  
STC ISP OSP 
  
  
  
  
  
  
  
  
  
  
  
  
   
image carriers 
with P devices 
image image 
m yc 
N EN microscopes 2n E 
with measuring marks 
  
  
  
  
  
  
  
  
  
  
  
— 1| operator E ; 
image : image 
carrier exa = xe ye [P carrier 
control control 
  
  
  
  
digitizing computer image 
tablet monitor carrier 
  
  
  
  
  
  
  
  
  
Fig. 3.1 Devices for stereoscopic image 
measurement 
3.1 Stereocomparators 
Stereo comparators are the simplest instruments 
of this category. In contrast to the analytical 
plotters there is no computer controlled 
preservation of the image (relative, absolute) 
orientation during the data acquisition 
process. Therefore for every point measurement 
both image carriers have to be positioned 
manually in x and y (Fig. 3.1.1). The measured 
image coordinates serve as input for the 
calculation of spatial coordinates. 
  
  
Fig. 3.1.1 Priciple of data acquisition 
with a stereocomparator 
3.2 Analytical Stereoplotters 
Analytical stereoplotters are the more 
expensive stereoscopic systems. During the data 
acquisition process the image orientation is 
preserved computer controlled. Therefore only 
three image carrier movements have to be 
carried out manually. If the orientation is 
performed using a bundle adjustment, a 
simultaneous camera calibration is possible 
[WESTER-EBBINGHAUS 1987]. In contrast to 
stereocomparators, the output of analytical 
plotters are three dimensional object 
coordinates. There are two categories with 
different kinds of positioning values, the 
image-space-plotters and the  object-space- 
plotters. 
3.2.1 Image-Space-Plotters 
Image-space-plotters work similar to 
stereocomparators. In addition they are 
equipped with an computer controlled y-parallax 
compensation, which is calculated several times 
per second, to preserve the image orientation 
(Fig. 3.2.1.1). Therefore only x and y of one 
image carriers and the x-parallax between both 
have to be controlled manually, while the 
Stereoscopic viewing is preserved. From the 
measured image coordinates and the parallaxes 
spatial coordinates are calculated, using the 
predetermined orientation parameters. 
3.2.2 Object-Space-Plotters 
Object-space-plotters work in the same way as 
the well known analogous stereoplotters. Four 
computer controlled servo motors are required 
for their realization in contrast to image- 
space-plotters, which only need a computer 
controlled y-parallax adjustment. The measured 
variables are spatial object coordinates 
(X,Y,Z). That means that the measurments are 
directly done in the object space reference 
system. The x,y image carrier movenments of 
both photos to preserve the image orientation 
are calculated several times per second, using 
the predetermined orientation parameters (Fig. 
3.2.2.1].