Use of Airborne Devices for Recording Absolute Orientation |
Elements in Topographic Mapping ! |
by M. D. KONSHIN
U.S.S.R.
The main trend in the advancement of aerial photography in the US SR is aimed
at dispensing as much as possible with laborious topographic and geodetic ground work,
which should result in producing topographic maps mainly by photogrammetric means.
One of the essential steps in this process is the analytical extension of control, the number |
of photographs included in the photogrammetric net depending on the way in which the |
errors are accumulating and on the precision of the photograph itself. Therefore, to in- | |
crease the number of photographs to be included in the photogrammetric net without de- |
teriorating its accuracy, it is necessary to ensure a slower rate of error accumulation;
this is possible, provided the elements of exterior orientation are known beforehand. At
present, several techniques are used in the U S S R, which enable the elements of exterior Imi
orientation to be determined during the flight. These include the determination of dif-
ferences in height of flight by statoscope readings, the determination of the photograph
scale by radio altimeter readings, the determination of horizontal position of camera
stations by the radio geodetic data, and the elimination of tilt in the photograph through
using gyroscopic devices.
The principle underlying the construction of the statoscope is that of measuring the
changes in the outside air pressure. The instrument consists essentially of two manometer
tubes operating alternately. One end of each of the tubes is connected to a cylinder
containing a fixed volume of air. The displacement of the liquid in the manometric tube
caused by the changes in air pressure is recorded on a special film at the moment of B
exposure, The recorded difference in levels of the two columns of liquid in the two forks i
of the manometric tube enables the changes in the height of flight to be found out, with
the aid of the barometric formula, with the root mean square error — 1 m. The precision
of the performance of the statoscope is affected by tilts and warpings of isobaric surfaces,
as well as by vertical accelerations and changes of air temperature inside the cylinder.
To minimize some of these errors an aneroid height recorder has been developed: its
principle is the same, i. e. the measurement of pressure changes, but its receiving part is
an aneroid battery.
The radioaltimeter of the R V T D-A type is a radio rangefinder. At the moment
of exposure a special photorecorder registers the screen of the scope electron-beam tube,
with a cireular sweep having two pips — the trigger pip and the echo pip showing the
return waves reflected from the ground points nearest to the plane.
To locate the ground points the radio waves were reflected from, the photographs
are mounted in a stereometer or a universal stereophotogrammetric instrument and
oriented. About the centre of projection three-dimensional spheres are constructed having
different radii, along which the floating mark is shifted, or else the dot itself is of a
spherical surface. The points on the stereo model situated on the sphere having the
shortest radius are those the radiowaves were reflected from.
Then the ratio of the shortest radius to the vertical distance furnished by the
radar altimeter enables the scale of the stereo model to be obtained as well as, later
on, the altitude the exposure was made from. The mean square error in determining the
height of flight with the aid of the radar altimeter is 1.2.m.
Radiogeodetic stations of the RGSC type are based on measuring phase dif-
ferences between the outgoing and the return (re-transmitted) waves. The principle is
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