Hempenius, Makarovid, Van der Weele, Tests of Restitution Instruments 
differences reveals corrections to the orientation elements which can be considered as 
corrections to the zero-points of the readings. 
Further, the question of the relation between the actual movements of the projectors 
and the corresponding readings, (of angular or linear elements) can also be a useful 
subject of examination. The simplest way will be to measure the variations of the coordi 
nates of at least one point (the principal point is frequently preferred) as a function of the 
readings of the orientation elements. 
An analogous reasoning is valid to check e.g. the readings of the elements in a 
rectifier. By measuring the coordinates of a projected grid in different positions of tilts 
and enlargements it is possible to compute the geometrical values of the settings which can 
be compared with the actual readings. This procedure is useful in those cases where pre 
computed data have to be set in the instrument. It is true that the accuracy cannot be 
higher than that of a graphical output but this does not alter the principle of the method 
and will effect only the tolerances for residual differences. 
II. 3. Correction-computers, (group B) 
General remarks. 
The title of this chapter should be understood as pointed out in II. 1. viz. to cover the 
application of grid-measurements for the test of the geometrical performance of those 
instruments, which are not based on a spatial reconstruction of the bundle of rays. 
All these instruments include, in some way or another, an analogous computation and 
it is this function that will be subject to test. 
Some instruments of this type are: 
Stereotop (Zeiss), Planitop (Zeiss), Stereomicrometer (Galileo), Stereometer S.D. 
(Drobysev) etc. 
Since the properties of those instruments show a wide variety it is impossible to 
include in this paper detailed recommendations on the way in which grid-plates can be used 
to check their performance as analogue computors. The principles of the methods are, 
however, simple and general. Assuming vertical photography and flat terrain, a grid 
model can be plotted to examine the errors involved under ideal circumstances. 
In addition each correction device of the instrument can be given a separate movement 
to check whether it has the effect which might be expected. 
The analysis of residual errors should be directed, in close relation to the design, to 
finding the sources of descrepancies and eventually the way to eliminate them. This could 
be by means of simple adjustment or by mechanical improvements (repairs). 
III. Test on the dynamic performance. 
III. 1. General Remarks 
Since the geometrical tests are dealing with the static properties of instruments 
as computers, the dynamic analysis concerns their ability and efficiency for a reliable 
continuous conformal reproduction of arbitrary figures, particularly small details. 
In cybernetics any plotting apparatus may be considered as a dynamic system. 
Conventional stereo-plotters are man-machine systems, in which the human operator takes 
part with his eyes, nerves, the memory and decision making capacity of his brain, and 
physically as a driving agent with his muscles. The dynamic performance is dealing with 
the flow of information through the dynamic part of an integrated plotting system. 
Little attention has been paid up to now by the users to the physical performance of 
instruments apart from their geometrical tasks. In the factories tests of individual com 
ponents of the plotting apparatus usually take place, while integrated dynamic tests have 
not been carried out even there.