288 
(Figure 1) which not only instantly process the information, but also use it 
in a feedback mode to monitor and control the operation. The main element of 
this control is a continuous, computer controlled, real-time positioning of 
images, The interface between the computer and the photo-carrying hardware 
allows for a two-way communication. On-line photogrammetric systems of this 
type are called analytical plotters. 
Existing analytical plotters differ widely in their design, computer support 
and interface implementation (PERS, 1982). However, there is not a 
significant difference in their basic functions and the operator interacts 
with all these systems in a very similar manner. This unifying factor of the 
operator-system interaction and control structure makes it possible to analyze 
the potential of computer simulations of analytical plotter operations in a 
relatively general approach. In order to do that one should first look at the 
overall structure of data processing control in individual systems. From the 
viewpoint of the processing control there are essentially two types of 
systems, with central processing and distributed processing. 
Central Processing 
On-line systems with central processing rely on a single computer in a 
configuration corresponding to a general sketch of the closed-loop approach in 
Figure 1l. More detailed block diagram in Figure 2 replaces the simple block 
[ PHOTOS] by [ANALYTICAL PLOTTER] showing all its individual components 
important for both the internal and external control structure. The operator 
supplies manual input to coordinate registers, some of which are then serviced 
by | SERVO CONTROL] and this module in turn positions photo stages with respect 
to OPTICS], thus providing an optical feedback to the operator, as needed for 
his control of measurements. The inner servo-loop which involves [REGISTERS], 
[ SERVO CONTROL], [ PHOTO STAGES], usually operates independently of the 
computer. Additional input from the operator comes through console switches 
and affects the function of registers. Display counters can be available on 
the console to provide a readout from some registers. The computer 
communicates directly with the analytical plotter through registers and 
switches. Coordinate values are transferred to and from registers, while 
switches are computer interrogated to affect the control of operations. Both 
functions are serviced by special interface subroutines. Finally, the 
operator communicates with the computer directly, through a standard video 
terminal. The above description is conceptual and if reflects well the 
function of some analytical plotters, such as NRC Anaplot, Zeiss C-100, Wild 
AC 1. The number and function of console switches differ from case to case. 
  
  
  
  
  
  
  
  
  
  
      
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
ANALYTICAL PLOTTER 
COMPUTER PLOTTER 
Mj OPEN LOOP iiy COMPUTER PHOTO STAGES 
Perm REGISTERS | 
VIDEO : i vz | | couvress | | orrrcs | 
TERMINAL P seres | | INPUT XYZ C 
COMPUTER PLOTTER ; 
! y CLOSED LooP } A | 
OPERATOR OPERATOR 
  
  
  
  
  
Fig. l On-line control Fig. 2 System with central processing