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Ladies and
legation of
RECENT DEVELOPMENT OF APPROXIMATE INSTRUMENTS, DISCUSSION 187
the Union of Soviet Socialist Republics wish to
bring greetings to the members of the Congress
and the members of the ISP, and wishes them
good health and future success. The Soviet
Delegation thanks the President and the Chair-
man of the Congress for the invitation to take
part in its work. Our Delegation has brought to
the Congress a number of papers which will be
handed over to the members of the Congress.
Today, the Commission will be given a summary
of the paper of Professor Drobyshev, Soviet
Stereophotogrammetric Instruments.
Various types of objectives are used in the
USSR for topographical mapping, including
extra-wide angle objectives, up to 150°. Extra-
wide angle objectives ensure a high accuracy of
determination of topographical points, and this
is of great importance for mapping hilly and
mountainous regions. However, the photogram-
metric processing of aerial photographs taken
with objectives with an angle of 150° cannot
be accurately carried out on the known all-
purpose stereophotogrammetric instruments
which are based on the principle of reproducing
the cones of projecting rays which exist at the
time of exposure.
By means of these instruments it is not pos-
sible to process aerial photographs with different
focal length with the same pair of cameras of
the stereophotogrammetric apparatus. There-
fore, Soviet scientists evolved a theory for the
construction of a terrain model which enables
them to transform the cones. This is the work of
Professor M. D. Konshin, Professor Drobyshev,
and others. New instruments have been devel-
oped on the basis of this theory, such as
Professor F. V. Drobyshev’s stereometer,
Profesor Romanofski's stereo-projector, and
Professor Drobyshev's stereograph. The stereo-
projector is a universal instrument of a mechan-
ical type. Its main feature is that the pair of
aerial photographs are always in a horizontal
position, and move in relation to the observa-
tion system by turning levers. The observation
system is of a simple design and permits of the
examination of the stereo-pair of photographs
with an invariable vertical position of the axis of
visual observation. The effect of the tilt angle
of the aerial photographs is taken care of by
means of two correction mechanisms which shift
the objectives of their observation systems. The
degree of this shift equals the required correc-
tion of the exposure tilt angle. In this way, a
transformed image of two corresponding points
is obtained in the plane of the photographs, and
from these points the point of the model is
determined by means of special levers.
Archives 4
The stereograph too is a universal stereo-
photogrammetric instrument of a mechanical
type. In this instrument, just as in the stereo-
projector, the aerial photographs are always in a
horizontal position. Therefore, the observation
system is fitted with a simple device similar to
that of the stereo comparator. The effect of the
tilt angle is taken into account by means of
correcting mechanisms which alters the focal
length. This length is altered by special levers
which are used for the construction of the
model. A feature of this instrument is its small
size which is achieved by combining spatial
aspects of the photograph with that of the
model.
In addition to the conventional movements
for the orientation of photographs, the stereo-
projector and the stereograph also make use of
displacement of the centre of the photograph,
depending on their tilt angle and the cone trans-
formation coefficient. This off-centre shift has
been introduced in order to place the zero error
point and the nadir in the required positions.
The former point is assumed at the starting point
of the coordinates in the transformation of a
pair of corresponding images, and the latter
point for the determination of the point of the
model in accordance with transformed images.
The vertical scale of the model built into the
stereo-projector and the stereograph is, in
general, as many times greater than the horizon-
tal as the instruments set focal length is greater
than the focal length of the aerial camera which
took the photograph. The instrumental accuracy
of the stereo-projector and the stereograph is
about 1 : 12,000 of the exposure altitude. These
instruments are used for the making of large-
scale maps.
Mr A. M. Wasser: I think we all agree that
plotting instruments can be grouped in two
classes: those which are based on exact geo-
metrical instruments; and others which are
based on approximate solutions. However, we
do realise that geometric principles can only be
approximately realised. For instance, space
rods or lineals which are supposed to represent
straight lines are no better than approximations
to the geometrical concept of a straight line. On
the other hand, an approximate solution is not
necessarily a bad one. The basic mathematical
tables are based on approximations of the
tabulated functions. Without it, mathematics
could never be put to any practica] use.
Following this line of thought, I have at-
tempted to develop the theory of approximation
in photography, first to see why not one of the
