be strong, whereas those with a human operator should not be excessive, but 
sensible. The latter should imply the principles of human engineering. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Analogue »| Digital 
Components | Components 
qe Operator [“ 
figure 1 
basic components with interactions 
The systems under consideration are provided with several channels for 
communication with human operator. The communication is bi-directional 
with each main component. The physical nature of signals is optical, mechani- 
cal, electrical and audial. 
The communication links with a photogrammetric instrument are 
provided by: observation system (sometimes supplemented by a separate 
location indicator), driving agents, means for orientation, various switches 
and alarm system. Operator may communicate with a computer via control 
console, a separate keyboard, teletype, tape/card reader, and various display 
units. 
The analogue and digital components of a system are linked by means 
of A/D (analogue to digital) or D/A (digital to analogue) interfaces, or both 
may be connected into digital servomechanisms. 
In different stages of a system operation different interactions are 
involved. Those active in preparation stage and in post-processing stage are 
not time constrained. Others have a real-time or a nearly real-time character- 
istic. Operator's functions in real-time operation should be adapted to his 
physiological limitations (i.e. time perception and response thresholds). 
An overall system configuration of a computer supported conventional 
instrument is represented in figure 2. Not all of these components and/or 
interactions are present in any system. Some of the connection with the 
computer represent alternative cases.