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124 THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, HELAVA 
Another way to improve the slewing rate is to employ the principle of variable in- 
crements. A variable increment computer processes the data in the incremental fashion, 
but the sizes of the increments are automatically selected in such a manner that the 
changes of variables are “digested” at a fast rate. 
The time required to calculate the entire program through once may be easily esti- 
mated for a given computer and for a given formulation. However, the computation time 
of different computer designs varies greatly, and therefore it is possible only to give 
speed estimates of a very general nature. It appears that an iteration rate of about 100 
times per second can definitely be achieved by using the entire-word-transfer principle. 
A serially-working fixed increment computer can perform the iterations at rate of up to 
1000 times per second, while the parallel mode of operation would raise this figure to 
100,000 or more. The variable increment computer may be considered in general operation 
to have a speed that is of the same order as that of a fixed increment computer, but in 
cases of a rapid change in variables it is faster by an order or two. These speed estimates 
are rather conservative, being based on the performance of instruments that are now in 
use. In view of the latest developments in electronics, these speeds may be expected to be 
greatly improved in the near future. 
Any one of the three principles explained above may be used in a photogrammetric 
plotting instrument intended for a human operator, or at a comparable speed, for an 
automatic operating device of some kind. However, a need for higher speed is inevitable 
with the development of automatic image correlation and recording systems, and there- 
fore the most economical total solution will probably be based on the incremental prin- 
ciple. 
3.3. Transferring of data. 
There are three different groups of information that need to be transferred during 
the process of plotting: 
1. dimensional data (c.g., coordinates) ; 
2. data for interpretation (e.g., visual image), and 
3. parameters. 
The means of transfer are generally different from group to group, although some 
overlapping occurs. We shall now study, in general terms, the problems and means of 
transfer in each group. 
3.31. Dimensional data. 
Dimensional data may be transferred in an analog or digital form. Optical, mechan- 
ical, electrical, and electronic means may be used. An example of the optical transfer of 
dimensional data is the optical projection used in a number of photogrammetric instru- 
ments. In this application the optical projection must be very accurately controlled as to 
its geometry; otherwise only visual data would be transferred. This causes insurmount- 
able difficulties in any application that calls for flexibility. 
Mechanical data transfer is widely used in photogrammetry. The system of shafts 
and gears which connects the instrument to the plotting table is one mechanical data 
transferring system. Other similar devices are common in photogrammetric plotters. The 
mechanical means are very reliable but not sufficiently flexible when complicated trans- 
ferring problems are encountered. However, they are very useful for specific purposes 
and undoubtedly will be widely used in plotters of any kind. 
The electrical data transferring system uses synchromotors. When used alone, this 
method is capable of transferring shaft rotations only. In this form it may be used, e.g., 
to run the pointers of an indicator clock. When combined with mechanical and electronic 
means, the electrical method offers some design advantages, e.g., it permits replacement 
of mechanical linkages by electrical wires.