- 3 - 
in practical use, we shall abandon this principle in two respects: 
errors inherent in the photography and personal operator errors should 
have as little influence as possible in order to obtain more conclusive 
information on the instrument. To achieve this aim, simple aids, such 
as grid plates instead of aerial photographs and auxiliary eyepieces for 
improved observation are recommended. 
The test measurements should be evaluated by simple computation. If the 
results cannot be expressed in a few characteristic figures, they 
should, if possible, be plotted graphically in order to permit a quick 
and conclusive assessment of the operational condition of the instru 
ment. If the instrument has changed since the last test was performed, 
the user must be able to decide whether the instrument can be admitted 
for further work or whether it must be adjusted - perhaps with the aid 
of further measurements - or whether it should be repaired or complete 
ly overhauled. A computation of mean square errors of the corrected 
values should also indicate in how far corrections are useful. 
Instruments designed for different purposes may require different 
standard tests. However, even with instruments intended for one and the 
same purpose specially adapted standard tests are frequently necessary 
due to the use of different designs in the viewing, measuring or output 
system. 
The intervals between checks will also vary with the different tests. 
Some members representing the working group will submit "Presented Pa 
pers" to tne congress which will include recommended standard tests. 
These preliminary recommendations approved by the working group are an 
integral part of the results obtained by the working group. 
3) Preconditions and testing aids 
As was mentioned above, it may be assumed that the standard test was 
preceded by other, usually rather lengthy checks: technical inspection 
at the factory, initial calibration and adjustment of the instrument, 
inspection after installation at destination and final acceptance test 
performed by the manufacturer or user, as well as later the recalibra 
tion and readjustment generally prescribed by the manufacturer. 
1 ) 
Recommended aids for standard tests are high-accuracy grid plates as 
17 
As regards grid plates, standard plates with 2.0 or 2,5 cm intersec 
tions are considered appropriate; in the case of stereoplotters they 
are usually required for measurement in several planes. 
Such grid plates, for example with 2 cm intersections, 
a size 
of