a great economic impact in massive production. 
The analytical transformation systems with analogue electronic image 
transfer by means of CRTs represent another variant. Examples are 
UNAMACE, Bunker & Ramo (Bertram, 1965) and GPS, Gestalt (Kelly, 
1977). These systems imply processing of the video signals by 
analogue electronic circuits. 
Hybrid systems for image transformations have been bypassed by the 
recent development of the digital systems. 
Digital systems allow image transformations in geometric and 
intensity/colour domains. Initially such systems were used for 
transforming low-resolution remotely sensed digital images. 
The geometric transformations are as in analytical systems. The 
image window, however, can be represented by a square or rectangular 
matrix of pixels, or even by a single pixel. If the corresponding 
DTM is detailed enough, accurate geometric transformation is 
possible for any type of terrain. 
Digital systems have several potential advantages, as they can 
process both geometric and intensity/colour information. Examples of 
the latter are image enhancement and edge matching at automatic 
mosaicking. Moreover, graphs and text can be easily overlaid on 
images, e.g., to create image maps. 
A transformed digital image can be composed of fragments from 
different images of the same terrain. These fragments can be from 
new and old input images (for updating), or they can represent 
sections of different terrain slopes facing different camera 
stations. Moreover, occluded areas in one image can be replaced by 
the visible segments from the adjacent overlapping images, etc. 
These potential advantages are impeded by the AD conversion of 
images. Relatively large pixel sizes used at present (20 to 50 pm) 
reduce the image quality substantially. The resolution of the 
analogue images varies between 50 and 150 lp/mm, which corresponds 
to pixel sizes between 2 and 7 pm. Hence, the amount of the required 
digital image information would be 25 to 100 times more than at 
present. We should increase the storage capacity, the speed of 
storage/retrieval, communication, processing, and of the output 
accordingly. 
Image quality is further reduced by the resampling which is 
associated with the geometric transformation of digital images. To 
meet a specified ground resolution in the transformed digital images 
with relatively large pixel sizes (at present), the scale of the 
input images has to be increased accordingly. This, however, impairs 
greatly the economics of production.