2 
usually measure such mechanical motions to permit recording of their 
magnitudes. However, neither the physical motion, nor recording are 
necessary in a digital system; the magnitude is what counts. It may 
be then processed through numerous steps before its impact appears in 
a numerical or physical output quantity. In their capacity as generators 
of magnitude inputs to digital systems, the encoders perform functions 
equivalent to any of the physical motions in an analog instrument. Al 
though the encoders are only one of several ways in which inpu£ data 
may be entered, they do provide a clear illustration of the replacement 
potential. 
Digital replacement of geometric transformations performed by analog 
means has been amply demonstrated by analytical triangulation methods, 
and by analytical plotters. This substitution goes beyond mere replace 
ment in that it offers undeniable advantages in accuracy and flexibility. 
This applies to stereo instruments, as well as to single photo devices. 
[1], [2], [3], [4] It also includes the geometric manipulations involved 
in the photogrammetric orientation function. This task, which is basic 
ally mathematical, gives an opportunity for a very advantageous replace-? 
ment: rigorous mathematical treatment replacing a "soft" empirical 
technique. 
As long as we work with photographs as inputs and mt.p manuscripts 
or orthophotographs as outputs, the digital system needs an element 
capable of transforming a digital magnitude to a physical motion. This 
is in effect the inverse of what the encoder does, and is efficiently 
performed by a digital servo. The function is a direct replacement of 
analog motions involved in scanning the photographs or plotting the manu 
scripts. It may be of interest to note that the servo usually includes 
an encoder as a feedback element; it tells the servo what is the magni 
tude of physical motion the servo has introduced. A comparison of this 
with the desired digital value gives the servo the information it needs 
to introduce additional motion. 
The list of replacement possibilities can be extended further to 
cover various control and display functions. It is more important, 
however, to recognize that the scope of digital functions in photogram- 
metjric instruments extends far beyond that: it includes totally new 
functions, such as post-processing of photograrametrically derived data. 
The "Automatic Draftsman" [5] is a case in point. It can improve the 
manuscript and thus perform a function not found in analog instruments. 
It is the responsibility of the designers of photogrammetric instru 
ments, particularly those who are concerned about digital components, 
to make sure that the photogrammetric instrument users will get full 
economic benefits from the state of the art in digital equipment field. 
The task is not easy. Digital technology advances at an extremely fast 
rate, driven by its immense economic potential. New devices are announced 
nearly every day, and complete new generations of implementation approaches