3/ ths introduction of the correct plotting scale 
II. Character of the present methods of orientation of the pairs 
The observer is obliged to make the three operations mentionned above 
several times for each stereogram; be operates with successive approxima- 
tions, according to the well-known methods. There are several methods ta 
king more or less time to reach the orientation. Each consecutive repeti- 
tion aims to bring the model to the horizontal position and to the proper 
scale. The more air photographs deviate from the ideal scheme, the more 
repetitions are necessary. As an average one can admit three approxima 
tions for each of the cycles mentionned above. Besides, if we want to 
orient the stereogram as accurate as possible /by the methods in use till 
/ 
now/, we must repeat the cycles between themselves and this condition 
prolongs the time of orientation considerably. It is wellknown too, that 
before we can begin the second cycle, for instance, we have not only to 
finish well the first cycle, but we must also know with adequate approxi 
mation the scale of the model, because the proper rotation of the model 
depends also from the scale. It is clear, that this relation is valid in 
the reverse direction too, it means, that to obtain the correct scale of 
the map /in the plane of x y/ one must have the stereogram well levelled 
already. 
Therefore, from the application of the proper sequence of cycles de 
pends very much the number of the successives approximations not only in 
the cycles but also between them. The observer who starts to orient the 
stereogram is not yet aware, what factor may afflict mostly the number of 
repetitions, and this is why he acts usually by haphazard. In the first 
cycle already, that is by the relative orientation he is in this situa 
tion, notwithstanding with the method he uses for this orientation, For 
instance, the sequence of reaching proper relative angles of inclination