approaches: An analytical stereoplotter can either be converted to a 
hybrid system or it can serve merely as an epipolar 
scanner/digitizer (Makarovic, 1982). 
Analytical stereoplotters permit simple digitizing of relatively 
oriented image pairs directly in the epipolar geometry. The 
geometric condition is that the fictitious air-base b is parallel to 
the X-axis in the virtual model space (figure 7). Thus the relative 
orientation should use only the rotation parameters (hence, 
by'=by"=bz'=bz"=0). 
Fig.7: Direct digitizing in epipolar geometry 
A fictitious XY-plane at Z=c, where c is the principal distance, is 
parallel to b and , e.g., perpendicular to the epipolar plane 
containing the principal axis of the left image. In the XY-plane, 
equidistant lines parallel to X are laid out covering the whole 
fictitious model area. By projecting these lines on the two images, 
we obtain the conjugated pairs of epipolar lines. 
For the AD image conversion, a fictitious orderly tracking of the 
lines in the XY-plane is programmed, e.g., from the left-upper 
corner to the right-lower corner of the model area. The real-time 
program of the analytical stereoplotter accordingly transforms the 
lines from the XY-plane into the two image planes (x',y'; x",y"), 
which are recorded. Such a direct digitizing in the epipolar format 
makes resampling unnecessary, and image matching is reduced to one 
dimension. 
Hybrid systems permit terrain relief modelling from stereoimages in 
real-time, which is not necessary for production of DTM. The 
rigorous time constraint prevents the use of complex and 
sophisticated processing strategies and algorithms. The information 
throughput is great, though the required storage capacity is modest. 
In some systems with adaptive scanning, the analogue images are 
rescanned/digitized in each matching cycle, i.e., to avoid digital 
resampling and the corresponding loss of image quality. Moreover,