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lack of precision in clearing the y-parallax. The accumulation of the errors that 
they induce act according to unforeseen laws. 
The failures caused by the imperfections of the adjustment of the instru- 
ments or constant deformations of the perspective projections are of a systematic 
character. This is more rigorous if the same conditions are applied to relative 
orientation. It is then possible to establish rules of accumulation that permit the 
correction of errors that are introduced. 
Eventually other failures appear from deformation anomalies of certain 
perspective rays of the photographs being used; they introduce breaks in the 
rules of accumulation of systematic errors. 
In recent articles we have shown how one can perform relative orientation 
without guessing and with very good accuracy and how one can diagnose and 
eliminate the erratic effects of the deformation anomalies of the perspective 
rays. 
The present article is relative to a method of stereotriangulation furnishing 
rigorous rules of the accumulation of systematic errors. 
Operating method, measurements. — The airplane is supposed to follow as 
straight a course as possible, that is, without sudden changes in heading (crab) 
or in altitude; the exposure interval is supposed to be constant. 
In the transition from model to model in the plotting instrument the base 
length along the x-axis is clamped at a constant value b, the by and bz compo- 
nents are set at zero. The pass points ANA" are marked on each photograph to 
help in visualizing the problem. The relative orientation is performed using ex- 
clusively the perspective rays corresponding to these same points or at points 
very nearby; images that have been found to be in an area of distortion must 
not be used in connecting one model to another. 
The measurements to be made are: 
The stereoscopic measurement of the x, y, z coordinates of the passpoints 
ANA’; 
The measurements of the angles a and : of the marked points (possible only 
on the Poivilliers stereotopograph type B) and the x, y coordinates of those 
points that have the same constant zo coordinate. 
In passing from one model to another, the angle ; of the point N is main- 
tained rigorously, or corrected in case of a y-distortion anomalie, or in case of 
a sudden change in heading (crab). 
Computation of the position elements of a model k. — These elements are 
seven in number, the relative orientation having fixed 5 of the 12 possible 
adjustments of the two projectors. In all those that follow we take the system- 
atic errors into account. For the definitions of the terms one is referred to the 
preceding articles. 
1. Length of base by — b — bi. — The difference in distance 
A 7x Ze = Zr 
of a passpoint between the (k — 1) model and the k model satisfies the relation 
Azy = z (^ br—A bia) t [&" —5/ + b(a” + «)]  y(f" —H)