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for oblique trimetrogon photographs presented in a report at the Congress at
The Hague, that pairs of rectified oblique photographs could be used stereos-
copically. He supplied the basic ideas that accuracy is a function of the accu-
racy of performing rectification of the obliques to a horizontal plane: in
practice, it gives a correct planimetric rectification as far as the principal
point of the oblique photograph and an altimetric rectification giving in a
sense the relief valid locally with a correct equidistance of horizontal sections.
The same report gives first the geometric principles that guided the con-
struction of the apparatus; it discusses how these geometric principles were
used in practice, in particular how the stereoscopic relief effect of elevated
objects was exaggerated four times.
It told also of the apparatus for viewing the photographs and concludes
in indicating with interest what the instrument can offer for the rapid com-
pilation of a provisional survey at 1/100,000, for example, in regions where
the coverage with vertical photographs is insufficient to use ordinary map-
ping methods.
USE OF THE KELSH PLOTTER
by
Harry T. Kelsh, Photogrammetric Engineer.
The advantages and limitations of the Kelsh type of double projection
plotters and a resume of the applications of the instrument by various govern-
mental and commercial organizations.
All types of plotting instruments used in compiling maps from aerial pho-
tographs are designed to furnish a means of setting up a three dimensional
model representing a portion of the earth's surface, using as data the overlap-
ping part of two successive aerial photographs and means for graphically
recording any desired data appearing in the model.
Since all plotters are designed for the same general purpose, then, within
the limits set by our needs, or within the limts set by the quality of the data
we must use, comparative economic evaluation of the results is more impor-
tant than comparison of types of instruments based on personal preference.
Such economic evaluation necessarily must take into consideration the original
cost, the complexity of the instrument, for certain uses its portability, and
relative operator training time necessary to permit the securing of satisfactory
results.
There is no question as to the excellent degree of accuracy attainable
with well constructed plotters of the optical train type in the hands of skilled
and experienced operators. But to date, the good features have been accom-
panied by considerable complexity of design, lack of portability, and consider-
able cost. Operator training time is usually conceded to be longer than on pro-
jection type instruments.
The double projection type plotter is much more simple in construction.
It can be made reasonably portable. It is easier to use, and it is much less costly
to manufacture. Is is not as flexible in operation in that ordinarily it does not
