PHOTOGRAMMETRIC ENGINEERING 
the photographic interpreter was noted in the 
1956 Commission VII Report. This trend has 
continued through the present reporting 
period. This technique is particularly used by 
organizations turning out topographic maps 
which are annotated or overprinted with in- 
formation derived from photographic inter- 
pretation. In these cases, projection or direct 
viewing plotting equipment is used alter- 
nately by the photogrammetrist and the 
photographic interpreter. Further, interpre- 
ters are in many cases, acquiring and using 
small stereo-compilation devices for photo- 
graphic interpretation work, particularly in 
forestry and geology. 
[n addition, the family of instruments 
specifically designed for the photographic 
interpreter has increased greatly. These have 
ranged from plastic scales and templates to 
high-magnification  stereo-viewing — micro- 
scopes. Some of the more important types of 
instruments are listed below: 
1. Light tables and other devices for trans- 
parency viewing. While for many types of 
interpretation work prints are still the sim- 
plest and most economical answer, a greater 
and greater amount of work is done each year 
using photographic transparencies viewed 
with rear illumination. Consequently the ap- 
plication of light-tables and magnifier-light 
table combinations has been increased. Some 
of the advantages of transparencies as a 
source material are: 
a. Since much of the detail to be inter- 
preted is often rendered in dark tones on a 
positive photograph, the use of rear-lighted 
transparencies permits better image illumina- 
tion than obtainable with reflected light on 
paper prints. 
b. The absence of paper grain in the case of 
film transparencies permits higher powered 
magnification devices to be used without 
losing the image. 
c. The use of connected roll transparencies 
avoids the sorting and filing problems in- 
herent in handling separated paper prints. 
2. Rear-projection viewers. The use of 
transparencies has led in a natural fashion to 
the screening of positive transparencies on 
rear-projection viewers. These instruments 
usually provide for a standard magnification 
of the image during projection (four, eight, or 
as high as twenty times). Many of these view- 
ers have a measuring capability through the 
use of rectangular or polar coordinate reading 
devices which operate on the projected image. 
These measuring devices are easily fitted 
with magnetic readout heads, to permit meas- 
urements to be recorded on punched or mag- 
4 
netic tape for automatic plotting or elec- 
tronic computation. Since most of these in- 
struments are of a comparatively low order of 
accuracy, they fall more specifically into the 
field of photographic interpretation than that 
of photogrammetry. Some of these viewers 
have a stereo-projection capability. 
3. High magnification equipment. Careful 
processing and use of positive transparencies 
has permitted the employment of higher mag- 
nification devices by the photographic inter- 
preter. Some of the more successful of these 
are microscopes, modified for stereo viewing 
of photographs. These have varying magnifi- 
cations, usually starting at about eight times, 
and extending, through the use of turret 
lenses or the zoom magnification principle, to 
a magnification of forty times or more. 
4. Electronic plotters coupled with analogue 
or digital computers. The photographic inter- 
preter frequently has the requirement of mak- 
ing a planimetric drawing of his data, either 
to an arbitrary scale on plain paper, or on an 
existing map. While the electronic computer, 
in its application to photography, has been 
thought of primarily as a tool of the photo- 
grammetrist, it is now gaining acceptance as a 
photographic interpretation instrument. In 
the digital computor system, the photograph 
is laid on an x-y coordinatograph equipped 
with magnetic readout heads, so that the suc- 
cessive positions of the courser, as it traces 
out the detail interpreted from the photo- 
graph, are punched into computor tape. The 
programmed operation of the computor may 
be designed, for example, to rectify data from 
the oblique photograph to the vertical. Com- 
putor output tape, after the calculation is 
complete, is fed into a second x-y plotter, 
which plots out the new point positions. 
The analogue system works in much the 
same way, except that both plotters are con- 
nected directly to the analogue computor 
without a punched tape step intervening. The 
use of an anologue computor permits line ma- 
terial to be traced in rectified position in a 
single continuous operation. 
5. Image enhancement equipment. These 
rather recent innovations are still in experi- 
mental stages. However they show excellent 
prospects of being of value in photographic 
interpretation work. In electronic image en- 
hancement equipment, the photographic 
image is first scanned electronically, and con 
verted into a signal, which is then re-dis- 
played on a cathode ray tube. Added elec- 
tronic circuitry enables adjustments to be 
made to density and contrast levels of the 
image to bring out whatever detail is desired.