ique photo-
ruments are
lirections of
1t of a set of
is to image
graphs. The
1 tilted table
arms of dif-
. bar free to
The left, or
arent mate-
(at its lower
to the photo
nown as the
t a point at
onds to the
let arm gen-
plane which
ibed by the
e oblique ex-
's from the
s photo arm
n a common
igure 9, was
FrG. 9. Stereoblique plotter.
developed by J. G. Lewis in 1944 to draw
planimetry from overlapping oblique pho-
tographs in orthographic projection. The
device is a stereoscopic plotting instru-
ment containing two revolving index lines
which combine in the stereoscopic model
to form a vertical line piercing the ground
at a definite point, the map position of
which is determined by two revolving rods
intersecting in a plotting pencil. The oper-
ator actuates the floating line by directly
sketching with the pencil on the map.
This apparatus required special treat-
ment of the viewing stereoscope in order to
reduce eyestrain resulting from the diffi-
cult perspective conditions, and to meet
the definite orientation requirements im-
posed by the oblique photographs. These
conditions were met by a stereoscope, de-
veloped by J. L. Buckmaster, which pro-
vided the necessary adjustments.
PHOTOGRAMMETRIC ENGINEERING
Fic. 10. Topoangulator.
Topo-Angulator!?
The Topo-angulator, Figure 10, is a
mechanical device developed by David
Landen in 1944 for use with trimetrogon
photography, and which projects vertical
angles to photographic detail into the
principal plane for elevation determina-
tion. This is accomplished by means of a
transparent plate called the projector
etched with a hairline; the projector and
hairline are actuated by means of a slide
operating parallel to the principal plane
of the photograph.
Trimetrogon Method of Aerial Photogram-
metr y?
The trimetrogon method was developed
jointly, in 1941 and 1942, by the Geological
Survey and the Army Air Forces. Impor-
tant contributions were made by the follow-
ing of the Geological Survey: Gerald Fitz-
Fic. 11. Model showing trimetrogon method.
