2. Point Marking Instrument PMG 1 x 
  
  
   
     
   
    
   
    
   
   
   
   
   
   
   
    
   
  
  
  
  
  
     
   
  
     
     
As previously remarked, the basic requirement for application 
of a monocomparator for analytical aerotriangulation is that 
all "modelpoints" to be measured are clearly identifiable in 
each separate photoplate where it occurs. In case of natural 
details or signalized terrain points being used, a relatively 
coarse marking of the position is adequate. More critical, how- 
ever, is the case where points have to be used which are arti- 
ficially marked on the photography. Here it is essential that 
the artificial points in each separate photograph really re- 
present the same corresponding terrain point. This can only 
be achieved by thorough preparation using stereoscopy. 
  
The Kern PMG 1 has been designed to fulfill the requirements 
for the following functions: 
a) Selection and creation of artificial photopoints in the: 
threefold overlapping area in a strip. 
b) Transfer of existing artificial photopoints to adjacent 
strips or adjacent photographs in the strip. 
c) Marking of the position of an artificial or natural detail 
or signalized terrain point for quick identification. 
Because of the first function, the creation of artificial photo- 
points in the threefold overlap, the PMG 1 has been equipped 
with three stages so that three transparencies can be handled 
simultaneously. The threefold overlap can be observed stereoscopi- 
cally in photopair 1-2, 2-3 and 1-3 alternatively. A suitable 
point can be selected by scanning the threefold overlap by means 
Of a quick X- and Y-motion simultaneously for all three plate- 
carriers against a fixed three channel optical system. After 
this, the floating mark is put accurately in the right position 
by means of a relative X- and Y-correction of the two outer e 
platecarriers for which either a quick motion drive or a fine 
setting screw can be used. The measuring marks are situated in 
a yoke above the three plateholders. Attached to each measuring 
mark unit is a saphire needle. By pressing an electrical foot- 
switch, the yoke is coming down to the emulsion where the needles 
make a circular impression in all three photoplates at the same 
time. Essential here is that no relative motion between measuring- 
mark and needle is possible, as they are both mounted near to 
each other in the same moving unit. Fig. l shows the build up of e 
the instrument.