PROPOSED STANDARD TEST OF STEREOSCOPIC
PLOTTING INSTRUMENTS
by Joun T. PENNINGTON, Alexandria, U.S.A.
Precise calibration and adjustment of equipment used in each step of a
photogrammetric mapping operation are required if the highest practicable order
of accuracy is to be obtained, regardless of the type of equipment employed. The
use of inferior or poorly adjusted equipment in one step of the process, can adver-
sely affect results so that much, if not all, of the potential advantage gained by
good equipment in proper adjustment in other steps of the process is lost. Often,
resultant accuracies are not as good as they could be because through wear or
reasons unknown, the plotting instrument is not in proper adjustment, and in-
accurate conclusions are often drawn regarding the capabilities of a particular
type of equipment because they are based on tests made with equipment in poor
adjustment.
In order to promote a better understanding of the capabilities and limitations
of photogrammetric processes and equipment, it is considered highly desirable that
4 standard test for stereoscopic plotting instruments be developed. With such a
test used universally, the performance of plotting instruments and the state of
their repair and adjustment can be expressed in terms which can be universally
understood. Such a test would also be valuable as a periodic check of the adjust-
ment and calibration of plotting equipment and would provide a uniform basis
for specifying the accuracy and precision of equipment for procurement purposes.
To serve the purposes indicated, the test should be designed so that all the
possible sources of error, not due to the instrument itself, that are inherent in the
usual application of the instrument are eliminated. It should be simple and easy
to apply so that it can be made by a trained instrument operator in not more than
4 few hours and should be applicable to all types of plotting instruments to pro-
vide a basis for comparison between instruments. The test should be designed so
that measurements are made in the same manner and under the same condition
as they are made in operation with the plotting instrument, and should give the
information desired in a convenient condensed form so that it can be readily
evaluated.
The requirement that the test be such that all possible sources of error not
due to the instrument itself are eliminated, precludes the use of terrain photo-
graphy, in which there are potential sources of error in the camera, photographic
materials, photographic quality, ground control, etc. To meet this requirement
the test must be made by making measurements of some form of a master grid
plate, for which the dimensions are precisely known. In order that the measure-
ments are made in the same manner as they are made in operation with the instru-
ment, stereoscopic grid plates should be used and all measurements should be made
stereoscopically.
At the request of Captain O. 5. Reading, President of the International
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