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THE STEREOSCOPIC EXAMINATION OF AIR PHOTOGRAPHS
SPATIAL RECONSTITUTIONS AND ITS DEFORMATIONS
(Summary of the paper to be presented by General HURAULT to Commission I)
The author explains in a preamble that his paper is a summary of several chapters
of his recent book (1) which was written especially for the use of interpreters and covers
the subject more fully.
He begins by showing that stereoscopic examination has such great advantages
that it is the only method of examination which should be used for interpretation.
He reminds his readers of the physiological mechanism of natural binocular vision,
which is a differential sensation, and gives its characteristics.
He next considers artificial binocular vision, in the examination of "normal" ste-
reograms, which are defined as having the axes of the airs cameras parallel to each
other and at right angles to the base; he discusses the permissible tolerances in setting up
which will ensure binocular fusion.
He indicates the forms that anamorphosis of the ground can take during the exami-
nation of these "normal" stereograms, considering only the geometrical aspects of the
problem at this stage. Two kinds of deformation occur : one concerns the vertical direc-
tion and can be denoted by a coefficient of stretch; the other has the effect of making the
vertical converge to a point ; a movement of the stereoscope across the stereogram bends
allthe verticals over sideways. There is a distance at which the reconstitution of the
low parts of the ground has no deformation in the vertical direction (that is, its coeffi-
cient of stretch is equal to one), but this distance is often less than the minimum distance
of distinct vision.
The author next discusses mental interpretation in stereoscopic examination. Accor
ding to his tests, the convergence of the lines which fix the position of a point does not
affect the spatial reconstitution which is made; there is a tendency to position the low points
of the ground at about the minimum distance of distinct vision.
He reviews successively the case of examination with a stereoscope, that of ana-
glyphs and that of projection in relief on to a screen.
He gives an indication of the alteration in spatial reconstitution that occurs due to
errors in the direction of the axes of the air cameras (that is, lack of parallelism of their
axes and lack of perpendicularity with the base), and due to any rotational errors in set-
ting up the stereogram. He evaluates the permissible tolerances and discusses the way in
which spatial reconstitutions are deformed geometrically and how we perceive these de-
formations.
He makes a similar examination of all the other factors which lead to the intro-
duction of residual parallaxes.
He ends with a study of the various problems connected with the taking of air
photography and draws conclusions concerning the proper execution of this task.
(1)" L'examen stéréoscopique des photographies aériennes (théorie et pratiques)"
by GénéralL. Hurault - 1960, This work, published by the Institut Géographique National,
136 bis, rue de Grenelle, Paris, consists of 540 pages of text, in 2 volumes, with 250
figures in the text, an Annex I containing 102 plates separate from the text, and an
Annex II containing 108 air photographs printed on bromide paper. Page size : 21 x 27 cm
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