A METHOD OF RELATIVE ORIENTATION 
by 
George Poivilliers. 
Abstract: 'The paper concerns the adjustment of the settings in the relative 
orientation of a pair of perspective projectors by orienting with regard to any 
plane perpendicular to the air base, using the graphic intersection of chords in 
place of their arcs. The model is formed without successive approximations 
and the system is independent of the character of the model. 
In two former articles, I showed that the stereoscopic model could be 
formed at once by an orientation in a plane perpendicular to the air base, con- 
sidered as the x-axis. This orientation permits the correction of the relative 
setting about this axis of two perspective projectors in the form of a pair of 
aerial photographs. I showed also a graphic method of orientation in which 
two intersecting arcs were replaced by their tangents. 
The operations are simplified further if one replaces the tangents by their 
chords near the point of intersection. This method, which has been in use for 
two years at the French Institut Geographique National and the Belgian Car- 
tographic Institut Militare, is easy to teach to instrument operators and im- 
proves their work. This article applies to pairs of photographs having large 
cloud or water areas, and in general to the case of oblique of panoramic photo- 
graphs. 
The method of orientation need not be performed at the ends of the air 
base as is currently done, and which is a remnant of the former methods of 
successive approximation, but one can choose instead the crossection of the 
model where the profile of the ground differs as much as possible from the in- 
determinate circle condition. Three points, A, N, A’, are selected near this plane, 
N being taken essentially on the flight line, and A and A’ being selected as far 
apart and from N as possible. The parallax (y-parallax) is removed at N with 
the by motion, noting the setting, then the parallax is removed successively at 
A and A” with the bz motion of one projector, noting the two settings, without 
changing the by setting found at N. These values, by1, bz1 and by1, bz1, are the 
coordinates of two points in a plane located on circular arcs from AN and A'N 
(but the two points are actually plotted on a sheet of graph paper). The opera- 
tion is then repeated after having slightly changed the x-tilt (tilt, f, c») setting 
of one of the projectors. One obtains thereby two other points on the arcs having 
the coordinates by», bz» and bys, bz». The intersection of these arcs on the graph 
paper is replaced by straight lines passing through each pair of corresponding 
points. 
If the point of intersection does not fall in the vicinity of the points plotted 
on the graph, one can use another setting of the x-tilt. The two chords defined 
by the points are lines having a slope of y/z which intersect in a point whose 
coordinates represent values that will remove the parallax. 
The coordinates byo and bzo of the point of intersection are then set in the 
projector and the parralax 4 is removed with the x-tilt, / motion. 
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