Thus the hardware components as scetched in Fig.l will form a digital stereo- 
photogrammetric system. A scanner respectively a video camera is used to 
convert photographs into density matrices. Moreover the central computer is 
equipped with a magnetic tape unit to read already digitized image data, 
and with a disk drive for mass storage purposes. The image data to be pro- 
cessed actually are stored in the image memories and can be displayed stereo- 
scopically on a color monitor. This monitor, the x, y and z control and the 
terminal for communication with the computer form together the interactive 
working station to operate the system. The output may be plotted either in 
graphical form by means of a computer-controlled drawing table or in photo- 
graphic form (e.g. as orthophotos) by a raster plotter. 
3. Operational Aspects 
  
The operator working with a digital stereophotogrammetric system has to ful- 
fill pretty much the same functions as with an analytical plotter. But the 
system itself is based on quite different operation procedures. 
With regard to the interior orientation of the images two cases must be 
distinguished. The image data to be compiled may be digitized from photo- 
graphs taken with a metric camera or they may be acquired by a scanner or 
video system and directly recorded in digital form. In the first case the 
fiducial marks or the reseau crosses must be used to determine the interior 
orientation as usual. The reason for this is that the pixel matrix is not 
identical with the system of image coordinates. In the second case the inner 
orientation depends completely on the calibration of the data acquisition 
system. The pixel matrix itself serve as the system of image coordinates so 
that the interior orientation is determined without an additional source of 
error by just introducing the calibration data. 
  
To perform the relative orientation several solutions may be considered. 0f 
great promise is the transformation of one image data set in such a way that 
the pair of image data is reduced to the normal case of stereophotogramm- 
metry. The parameters for this transformation can be calculated by a least- 
Squares adjustment using a sufficient number of orientation points. In some 
special cases, e.g. for plotting convergent images, the transformation of 
both image data sets into an adequately defined system of model coordinates 
may be preferable. 
  
Absolute orientation of the stereo model based on ground control points is 
  
performed in the same way as in analytical plotters. 
After completion of the orientation procedures the operator uses the system 
in a way which is very similar to conventional stereoplotting. He moves the 
white (or black) cursor as a floating mark through the three-dimensional 
stereo model by means of the control units for x, y and z. These movements 
may either be plotted in real-time on the drawing table or recorded in digi- 
tal form. Furthermore data may be collected in profiles, rasters etc. There- 
fore also orthophoto production or similar tasks can be accomplished. 
À special feature of great importance is the fact that digital correlation 
techniques can easily be applied to image data in the normal case of stereo- 
photogrammetry. This means that stereocompilation may be performed in the 
usual way by an operator as well as by digital correlation procedures. The 
change from one mode to the other can be carried out at any time.