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surveyed and utilized as "vertical extraction”, namely from an altimetric 
point of view, then the conditions for the photointerpretation can be es- 
tablished with the consequence to furtherly reduce the execution times for 
a thematic cartography with favourable effects on the relative costs. In 
other words, in such a case, the orthophotomap would represent either the 
instrument for the photointerpretation or the final document, namely the 
thematic map. 
In Canada, in this connection, has already been set up a technique 
known as stereoscopic orthophotography which consists in the production 
of two stereoscopic orthoprojections. 
Regrettably such a methodology which takes for granted the availa- 
bility of two orthophotomaps turns out to be vary expensive and therefore 
it can be only scarcely used. 
However starting from this idea, we have operated with the aim to 
find a system which enables to stereoscopily observe the orthophotomap ; 
such a system would not have required long execution times and would not 
have implied too high costs. Accordingly we have thought that the. tri- 
dimensional vision of the orthophotomap could have been executed trough a 
second image of original stereoscopy which at any rate, retain its paral- 
lax, namely which is not rectified by the deformation of a central-type 
projection. 
At this stage the ways which could be followed, were two: 
a) utilize two images having a different scale; this process however 
would have required the use of an instrument which enables toequalize the 
scales in the observation system. 
b) utilize two images having scales rather equal or, in any case, 
with differences in scale not higher than 15 - 207. 
We have chosen this second solution owing to the greater simplicity 
which it presents, especially from the viewpoint of the equipments re- 
quired to carry out the work. 
Practically this method consists in coupling the orthophotomap with 
the photography of original stereocouple which has been not utilized in 
the execution of the orthophotomap itself. The photography's scale has 
been brought to that of the orthophotomap. 
This process of equalization of scales which generally is reduced to 
a photographic enlargment (rarely in a reduction), can sometimes require to