program. Of course, routing data from the disk drive of the host system, 
where the stereo images are stored, to the refresh memories is time con- 
suming and stresses the bus of the computer system. This means reduction 
of efficiency. 
At present this problem could be diminished by using four image refresh ' 
memories of an image processing system for storage of two partly overlap- . 
ping stereo pairs. Swapping between them leads to the advantage that re- 
loading one memory set can be handled in the background while the operator 
is observing the contents of the actual image memory set. 
Nevertheless it is necessary to provide a powerful facility for the inter- 
change of data between display memory and external data sources. Further 
developments must result in a system which is equipped with a private back- 
ground memory, especially a winchester disk drive which could be adressed 
as virtual memory by the image processing system and which allows high 
speed transfer rates. 
As in the previous chapter already described, the cursor of the image pro- 
cessing system is used as floating mark. For stereo compilation this floa- 
ting mark may be fixed in the centre of the display screen. In this case, 
which is analog to the traditional mode of stereoscopic measurements, the 
image movement is realized by the scroll capability of the image processing 
System. For accurate measurements by the operator a digital enlargement of 
factor two may be useful. If a large amount of details to be collected is 
already visible on the screen, optionally a second mode (moving floating 
mark and stationary image) is offered to the operator. In both cases one 
image has to be moved relative to the stereo partner in order to define the 
spatial position of the floating mark in the stereo model, which once again 
means scrolling of the images. The value of the x-parallax which could be 
determined by one-dimensional correlation indicates the hight of the object 
point. 
m 
9. Characteristics of a Digital Stereophotogrammetric System 
The concept of an entire digital stereophotogrammetric plotter leads to a 
system which offers a variety of advantages bejond those already achieved 
by the introduction of analytical plotters. 
Since the digital system works without moving parts the measurements are 
independent from mechanical devices. Furthermore no calibration problems 
arise because the coordinate systems, the flatness of the images etc. are 
determined perfectly by mathematical definitions. Limitations with regard 
to accuracy performance will only depend on the calibration of the data 
acquisition system. 
The flexible combination of stereocompilation by a human operator and digi- 
tal image correlation has already been mentioned. For many applications 
this combination will be extremely useful, particularly because tiresome 
and time-consuming monotonous measurements may be performed automatically. 
Moreover it must be stressed out, that the combination of a digital system 
with high resolution video cameras may lead to an entirely new concept of 
photogrammetry, which is "on-line photogrammetry" in the strongest sense: 
stereo measurements may be performed in real-time because no photographic 
process does interrupt the data flow from data acquisition to data proces- 
sing.