value for vertical photography is base-height ratio = 0.63. This doubling of the
base-height ratio is one of the important advantages of convergent low-oblique
photography. It can be shown that when the base-height ratio is doubled, the
errors in relative orientation and the deformations in the resulting stereoscopic
model are only half as great. Furthermore, the increase in the base gives a
more exaggerated stereoscopic impression of relief, making vertical measure-
ments easier to determine. Because of the stronger intersection of conjugate
image rays, the operator of a stereoscopic plotting instrument using this photog-
raphy is more positive that his "floating mark is on the ground." These advan-
tages, attributable to the increase in the base-height ratio, are strong contrib-
utors to the C-factor (ratio of flying height to the least contour interval that may
be plotted accurately), the contribution being such that it can be safely said that
the C-factor of a precise instrument using convergent photography is substan-
tially increased (nearly doubled) over its C-factor when using vertical photog-
raphy.
Another very important advantage of convergent photography is the extent
of the ground area covered. Convergent exposures provide working-model
coverage which is 2.2 times the area of a corresponding model from vertical
photography, assuming equal flying heights; however, since the C-factor of con-
vergent photography is greater, the comparison of ground covered should take
into account the higher flying height possible with convergent photography. On
the basis of a 1000 C-factor for the convergent System as compared to a 600 C-
factor for a vertical system, the ground area covered by à conve: gent-photog-
raphy model is 6 times as great. The advantages of having to compile 1/6 as
many models are obvious; more important still is the considerable Saving in
time and money resulting from the requirement of only 1/6 as many control
points.
Full advantage of convergent photography can only be realized if it is uti-
lized in effective stereoplotting instruments. In this respect, the situation today
is very favorable. Through the addition of a simple inexpensive bracket, the
present Multiplex projectors have been adapted to compile convergent photog-
raphy; Kelsh plotters have likewise been modified without major design changes
to accommodate convergent photography. A new instrument, the Twinplex,
(Fig. 2) especially designed to use convergent photography both for compilation
and stereotriangulation has been developed at the Geological Survey under the
supervision of Mr. R. K. Bean. The prototype model of the Twinplex utilized
standard Multiplex projectors; however a special type of projector known as the
ER-55 is now being developed and tested, The lighting system is greatly im-
proved through the use of a special ellipsoidal reflector which is designed to in-
crease the overall light passing through the lens and to intensify the light rays
which travelthe farthest, that is, to the corners of the model. The diapositive
in the ER-55 is four times the area of the present Multiplex diapositive, yet the
projected model is twice as brilliant using the same light source. This projec-
tor can be used with either vertical or convergent photography. The projection
distance is 500 mm as compared to 360 mm for Multiplex.
