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which one intended tQ record are indeed captured by the digital terrain 
model [9], 
The fully automatic mode of digitizing implies the presence of 
image-correlating devices. Due to the automatic guidance facility 
already inherent in the construction of analytical instruments, they 
are well suited for incorporation of these devices. A variety of 
correlation methods and devices may be utilized in conjunction with 
analytical instruments for contouring, profiling and generation of 
digital terrain models. Besides the electronic flying spot scanner- 
correlators a number of others such as the coherent light optical 
correlators and the solid state correlators with matrices of photo- 
transistors as sensors are being used or developed [10] [11]. The 
processing of signals is either analog or digital. The predominantly 
applied correlation techniques are based on area correlation. An 
exception is the epipolar scanner-correlator which performs the scan- 
ning along epipolar lines by a laser scanner of high signal-to-noise 
ratio. The orientation of the epipolar lines and the forward motion of 
the carriages is controlled by the dedicated computer of the analytical 
instrument, on the basis of known orientation parameters. In that way 
a digital line correlation technique is achieved, which significantly 
simplifies the task of real-time preprocessing, shaping and matching of 
corresponding image elements. The speed of scanning requires parallel 
digital processing by the system's special purpose computer [12] [13]. 
Where used for generation of digital terrain models this scanner- 
correlator can process an average model in about ten minutes [14]. 
Epipolar scanning leads to a somewhat irregular grid of points, since 
points with different elevations that are in the same epipolar plane 
do not have their orthogonal projection on a straight line. But from 
the original high density scanning pattern a regular pattern of less 
densely spaced points can be derived. The scanning speed of other 
correlators that are presently incorporated into analytical instruments 
is about five to ten times faster than the scanning speed in man-machine 
systems. They operate under sustained human monitoring and assistance. 
Their cost-to-benefit ratio is not conclusively established for diffe- 
rent types of photogrammetric processes. 
5.3. Pictorial output 
  
The interest in pictoral outputs, such as rectified imagery, 
orthophotos and stereo-orthophotos, stems primarily from the fact that 
their generation techniques are presently the only practical way to 
automate the restitution of planimetry (i.e. natural and man-made 
features), while preserving to an adequate degree the quality of metric 
information. The entropy of semantic information in the course of the 
production of pictorial outputs is for most practical purposes not very 
significant. The wealth of information preserved, for example in ortho- 
photos and stereo-orthophotos, allows users from different disciplines 
to extract and interpret the parts of the information that are pertinent 
to their needs and still have the benefit of relating them to the common 
reference system. This definitely simplifies the correlation of infor- 
mation when the same pictorial output is used by several organizations, 
as for instance in multi-purpose integrated cadastral surveys [15]. 
Technically the differential rectification is performed by 
optical or electronic scanner-printers in on-line and off-line mode.