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31 
There is much less experience so far with the 
optical image transfer. The designers of these systems 
claim the following main advantages. 
"flexibility in choosing a large range of. scales. 
- working in illuminated rooms. 
- comfortable viewing and profiling operations 
- higher. accuracy in profiling 
- adaptability to existing plotters 
= production of color or. black-white orthophotos 
- ease. in production of orthophotos or stereo- 
orthophotos in a single production step. 
A certain disadvantage is the necessity of intro- 
ducing image corrections due to the tili .and tip of the 
original photographs. This necessity does not exist in 
direct projection type of instruments since the record- 
ing of the image elements takes place in the model space. 
As to the image quality, I do not have comparative 
data. However, there are no reasons to assume difficulties 
(when using an optical image transfer system) in achiev- 
ing a quality comparable to that of viewing systems in var- 
ious,plotters. ..In addition one can claim a uniform quality 
disregarding the elevation range in the stereo-model, 
which is not the case if direct projection instruments 
without auxiliary focusing devices are used. In large 
scale work in mountainous country this is an important con- 
sideration which seriously limits the capability of the 
direct projection type instruments. 
Electronic image transfer has so far been associa- 
ted with instruments using the automatic image correla- 
tion technique. This is due to the technology involved 
and a unique flexibility in handling the geometry of the 
elementary images. 
The quality of electronically transferred images 
is comparable to the quality of other systems as long as 
no magnification in the image transfer is contemplated. 
This.is.one of its. limitations... Another drawback is the 
difficulty in: producing. colour pictures of a quality com- 
parable to the: current. photographs, Whether these diffi- 
culties are of a temporary nature remains to be seen. 
Each method of the image transfer has its advanta- 
ges: and limitations, which: of course can be discussed as 
such. However, the results of such a discussion are only 
meaningful if they are considered together with the respec- 
tive orthophoto system, its basic concept, performance,