tion 
the 
tion 
sur- 
ons 
ble 
ly 
the 
‘ces 
ing 
ole- 
  
or 
el 
  
for the first digit. In this manner, it can be printed as any other figure, whereas 
on the other hand, its high peripheral speed is eliminated. 
The general mechanical stability of the stereoplanigraph is constantly checked 
with the greatest care. The reaction of the structural elements which have a parti- 
cular bearing on the accuracy during the motions of the carriage, is tested by 
means of auto-collimation methods. For instance, the inclination of the large z column 
in the y—z plane and its rotation about its axis is controlled during the plotting 
of the y coordinate just like the rotation 
of the z carriage about the parallels 
with reference to the x and y axes 
during the plotting of the z coordinate. 
The observations and tests made until 
this date provide conclusive evidence 
for the assumption that the structural 
design of the stereoplanigraph, which 
is much heavier than that of other in- 
struments of this order, results in an 
extraordinary stability of adjustment 
and calibration. 
Finally, a few remarks on the pro- 
blem of the measuring mark will be 
opportune. Experiences which were for- 
merly made in the design of rangefinders 
have shown that the brightness and 
colour of the measuring mark in con- 
trast to the terrain are essential for 
stereoscopic plotting. On the basis of 
Fig. 3. Printing counter of Model C8 this experience, the designers of the 
Stereoplanigraph. Above g  hand- stereoplanigraph have adopted a lumi- 
wheel, 3 control buttons for d, g nous mark whose colours may be chang- 
and z. At right: pressure lever. “ed within a certain range. In an effort 
“gto obtain initial data permitting a quan- 
titative evaluation of the problems raised by the introduction of the luminous 
marks, we have run several test series with four different operators. The elevation 
of four points whose location was determined by contrasts in photographs with 
varying degrees of definition was plotted 10 times each on the orthoscopic and 
pseudoscopic model with either a well-defined or poorly defined contrast of the 
luminous mark. A small green dotted mark was used for these test runs. The 
problem was to determine the systematic and accidental errors of elevation which 
might be expected with either large or slight contrasts. 
The following table lists the results of these initial tests and tabulates the 
errors in the readings of elevations on the orthoscopic and pseudoscopic model 
(making allowance for the dead travel of the counter mechanism, which was 
ascertained by another method). The figures printed in parentheses in each column 
are the average values of the mean error of the elevation of a point for ortho- and 
pseudo-effect.