Hempenius, Makarovié, Van der Weele, Tests of Restitution Instruments 
system which, as has been stated already, consists either of coordinates of points or of 
maps. 
I. 5. In the case of coordinates we are specially interested in the geometrical performance 
of the instrument which is generally tested by using grid-plates as ‘input' informa 
tion. The observed images are thus well-defined and of high contrast and the observations 
are carried out with special care to avoid the influence of back-lash and elastic tensions 
in the mechanical parts of the instrument. The final aim is to have an impression (ex 
pressed in figures) of the accuracy with which the instrument as an analogue computer 
solves the equations for which it is designed (central-projection transformation for 
spatial instrument or other equations according to the construction principles adopted). 
In this context we want to repeat that an important characteristic of the observa 
tions is that they are carried out in such a way that the physical imperfections of the 
instrument have a minimum influence on the results. We mentioned already the influence 
of back-lash and elasticity ; but image-quality, inertia of moving masses, vibrations a. s. o. 
also belong this category. For this reason we are justified in saying that we are dealing 
with the static performance instead of the geometrical performance of the instrument. 
I. 6. When considering a map as the ‘output’ of the system we generally have to deal' 
with a continuous restitution of the images and hence the dynamic performance of 
the instrument is of utmost importance. The dynamic performance deals with the relative 
accuracy in the transfer of straight and curved lines from the photographs to the plotting 
table. This aspect is, at first glance, still of a purely geometrical character, but it is 
obvious that the influences of dynamic properties of the instrument cannot be eliminated 
here. The most important of these are already mentioned before and can be distinguished 
in backlash, vibrations, friction etc. which are partly influenced by the speed of move 
ment, partly by the mass that is moved during the scanning and partly by the physical 
condition of the instrument and its maintenance. It will be clear that the influence of 
these phenomena depends also on the skill of the operator and that, in testing the dynamic 
performance of the instrument, care has to be taken to evaluate this influence. 
Some of the factors mentioned just now are far more pertinent in an electrical or 
electronical circuit than in a mechanical one (e.g. noise as a consequence of a digital to 
analogue conversion in the ciruit) but our main aim at present is to sho^ that this ap 
proach is applicable to the ‘classical’ instruments as well as sufficiently general in nature 
to be useful in the future also. 
In the next chapter special attention will be paid to the geometrical and the dynamic 
performance of instruments. For the optical performance we will limit ourselves to the 
remarks made in this chapter. 
Optical performance will not be elaborated further. 
II. Test on the geometrical performance of instruments 
II. 1. General remarks 
As has been stated already in section I. 5. the aim of a test on the geometrical 
quality of an instrument is to find out with which accuracy it performs its task as an 
analogue computer. This description of the aim has been formulated in such a general 
way to include as many procedures, as possible, that are or can be used to test instruments. 
The computer functions of a photogrammetric instrument depend on its design and 
construction which in its turn is related to its purpose. In this respect we can distinguish 
two main groups of instruments, namely: 
A, Instruments which have, as in major function, the execution of the projective trans 
formation.