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be applied in a differenced way, namely taking into consideration  every- 
time portions of photography instead of the entire photogram. This is the 
case of areas having strong differences in height which bring about scale 
alterations in the same photogram; otherwise one can run the risk to have 
scale-differences higher than the limit of 15 - 20Z, and therefore imply- 
ing a "waste" in the stereoscopy. 
It has been possible to experimentally verify that the same result 
can be achieved both varying the scale of the photography, as above illus- 
trated, and by varying that of orthophotography. In our case we have pre- 
ferred the first solution. 
It is just as well to furtherly point out that both in the case a) and 
in the case b), above illustrated, remains, in all points, a certain ver- 
tical parallax which requires a continuous fitting of stereoscopy, area 
by area. 
In other words a continuous stereoscopic observation of the whole 
orthophotography is not possible, and it is necessary, depending on the 
nature of the observed area, to remove the photograph which stereoscopily 
pairs with the orthophotography so that to eliminate the parallax "by". 
The advantages which this accomplishment (or methodology) offers, 
are: 
- the stereoscopic observation is possible with a simple table ster- 
eoscope 
- no auxiliary equipment is required 
- the procedure is simple, rapid and little expensive 
- the observation, so effected, considerably improves the legibility 
of the details present in the orthophotographs; in fact it must be re- 
minded that some details could be in the shade on the orthophotography 
whereas on the other photography of the couple (photograph taken from ano- 
ther point of view) they could be more visible. 
The vision of the soil thus obtained is more natural, the third ai- 
mension is perceived as in the reality and not "exalted" and "exasperated" 
as generally occurs in aerophotographs. 
Finally it is to be reminded that the stereoscopic vision exists be- 
cause a given parallax exists as a function of the distance between the 
projection centers and of the difference in height of the projected point. 
As a consequence also the amplitude of the area whichcan be stereoscopily