(8) 
motor torques that accelerate the point horizontally back to the 
contour with an acceleration given by equation (20). 
For any given value of plotting error AD we clearly want 
the tracking point to accelerate back to the contour as fast as 
possible, Hence we want the number K defined in equation (21) 
to be as large as possible, In equation (21), Kp is a physical 
property of certain electric circuits and of an electrical resolver. 
It can be fixed at any desired value. M is the mass associated with 
the tracking point and is fixed. Jm is the rotational inertia of 
the servomotor and is fixed, S is the magnitude of the terrain slope 
at any point and this number varies over a large range as a topographic 
track is traced. K, is a physical property of the electronic track- 
sensing equipment. It is a function of the properties of the local 
electronic scan used to detect  x-parallex and is a strong function 
of the photographic characteristics of the images on the stereo plates. 
The net result of these facts is that the number K in equation (20) 
may vary over a large range of values as a topographic track is traced 
over the terrain defined by the stereo pair. The quickness with which 
the system of Figure 2 nulls plotting errors therefore varies very 
widely, This characteristic is intrinsic to automatic plotters of 
the projection type as automated by Hobrough. The same character- 
istic may be expected in other types of machines. 
In the analysis leading to equation (20) we neglected the fact 
that the track-sensing equipment cannot sense plotting error instantly. 
The equipment takes some time to act so that in Figure 2 the error 
signal e at any instant is actually proportional to the plotting 
error AD at some earlier instant. Similarly, there is a time delay 
in the generation of torques Ty and T, in the servomotors that 
are controlled by the signal e . This Seley is natural to the 
electrical equipment used to drive the motors. To compensate for 
these delays and the effects of friction in the mechanical linkages, 
electrical circuits of suitable design are connected between the error 
signal e and the servomotors. These circuits are not shown in Figure 
2. When these circuits are present, it can be shown that there is a 
maximum value of the parameter K that can be used if the plotting 
errors are to be corrected in an acceptable manner by a control system 
such as that blocked out in Figure 2. If X is too large, plotting 
errors will be over-corrected and the tracking point may oscillate 
seriously about the contour and indeed may move erratically away from 
the contour altogether. If K is too small, tracking errors are cor- 
rected too slowly. Since K may vary greatly as any given track is 
traced, it is quite possible for automatic tracking to be acceptable 
over one part of the track and to be quite unacceptable over another. 
For the above reasons, if optimum performance is to be obtained 
from automatic Blottine gachines, it is necessary that their control 
systems be adaptive ^»2,6. That is, the control system must automat- 
ically take account of the intrinsic variation in system parameters 
caused by changing terrain and image characteristics.