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THE DESIGN OF PHOTOGRAMMETRIC PLOTTERS, HELAVA 125 
Electronic means for transferring the data are the most versatile and flexible. 
However, the photogrammetric source information is in a form that is not immediately 
suitable for electronic transferring. The coordinates of photographs, for example, are 
measured as mechanical translations. Therefore, these translations are first converted to 
voltages by using potentiometers connected to an accurately-maintained reference voltage. 
The data may then be transferred to different parts of the process by wires. This mode 
of data transfer is essential to electronic analog computers. The method itself is of extra- 
ordinary flexibility but difficulties are encountered in the conversion phase. For accurate 
results, the potentiometers must be carefully calibrated and padded [7]. A further con- 
version is necessary to get the data back to the form of mechanical translations when 
this form is needed using a servomechanism. The servoing problem is of great impor- 
tance for a plotter based on mathematical methods and it will be treated in more detail 
later on. 
Digital transferring is the most advantageous method of transferring the dimen- 
sional data. The source data is “digitized”, that is, converted to pulses using special 
devices. The conversion can happen either directly from mechanical translations or in- 
directly from voltages. The latter alternative is not recommended for the conversion of 
source data, but is necessary if a hybrid system of electronic analog — digital methods of 
computation is intended. 
There are numerous devices available for analog to digital conversion. These devices 
are often called digitizers, and their operation may be based on purely mechanical prin- 
ciples or an a combination of optical, mechanical, electrical, and electronic means. Most 
of them are designed to digitize shaft rotation and so must be connected to a lead screw 
when applied to the digitizing of translations. However, there are also “linear” digitizers 
'apable of digitizing the translations directly without the use of a lead screw. It is ex- 
pected that these devices will find increasing use in photogrammetry. 
The technical details of digitizers vary greatly. For example, some digitizers may be 
read only when there is no motion, others many thousand times in a second; some devices 
divide one full shaft revolution into 128 parts, some to 500,000 parts; some indicate the 
results as decimal figures, others in binary system, ete. From the point of view of the 
photogrammetric application it is important that the digitizer be reliable, that it has suf- 
ficient resolving power, and that it fits the computation method used in the plotter. Obvi- 
ously, for a general purpose computer we need a digitizer which gives the values of the 
coordinates in a suitable code in full and which may be read, in motion, perhaps one 
hundred times a second, if not more frequently. For an incremental type of computer a 
simpler digitizer may be used. It is only necessary to indicate changes of variables. In, 
this case the digitizer produces a number of pulses that is proportional to the change of 
the variable. These pulses are counted by the computer and may be recognized directly 
in the calculations. 
Conversion of data from digital to analog form is necessary for utilizing the results 
of the computations. For this purpose mechanical translations equivalent to the digital 
values must be produced. This is done by using digital servomechanisms or stepping 
motors, and will be discussed in more detail in a later section devoted to the servoing 
problems. 
3.8.2. Data for interpretation. 
The information on which the interpretation is based is given in the form of density 
variations in the original photographs. This data may be transferred using optical or 
electronic means, or a combination of' both, depending upon the application. Optical pro- 
jection through a lens and a telescopic stereoscope are examples of the optical method 
used very widely in photogrammetry. An open and closed loop television [8], the elec-