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PHOTOGRAMMETRIC ENGINEERING 
sensitive element of a closed-circuit television 
system used here for producing orthophoto- 
graphs. During the memory-aided scanning 
portion in which the orthophotograph is ex- 
posed, the center of the sensitive surface of 
the Vidicon tube is automatically placed in 
the position previously occupied by the 
platen's measuring mark. 
The coordinatometer was given more pow- 
erful and versatile X-axis and Z-axis servo 
units, in addition to shaft encoders for the re- 
cording of profiles. 
There are now two X-Y coordinatographs 
driven in synchronism with the coordinatom- 
eter. One is for the drawing of a contour-plot, 
and the other for exposing film to produce an 
orthophotograph. The contour-plot (or line- 
drop) unit has a drawing head which is a six- 
position turret rotated in either direction by 
a stepping motor. In three of its positions the 
turret performs no inking. The intervening 
positions bear red, black, and green roller 
printing assemblies respectively. In operation 
the stepping motor is governed by a servo 
working a Z-shaft encoder. This addition of 
color-coding to the type of plot shown in 
Figure 1 provides a rapid and unambiguous 
indication of the direction of the ground's 
slope. An additional aid to contour compila- 
tion has been attempted by providing rollers 
with multiple lines to bring the printed line 
into scale correspondence with the chosen 
spacing between profiles. For this purpose, 
three sets of printing rollers are provided, 
which can be manually interchanged at the 
time the profile spacing for a model is chosen. 
The other X- Y coordinatograph is covered 
by a light-tight hood. It positions a high-in- 
tensity cathode-ray tube which exposes on a 
sheet of film the image being viewed by the 
Vidicon pickup of the closed-circuit television 
system. The decision to produce orthophoto- 
graphs remotely in this manner avoids the 
necessity for placing a heavy film-holding 
unit in the position of the measuring mark. 
This is important because memory-aided pro- 
filing requires the use of rapid, precise and 
rather delicate servo equipment to move the 
measuring mark. The penalty paid for using 
the television system is a slight loss in photo- 
graphic resolution, in addition to some elec- 
tronic complexity. 
Since it is desirable to produce orthophoto- 
graphs at scales other than the stereo model 
scale, interchangeable lenses between the 
cathode ray tube and the film provide for five 
different scale reductions in addition to 1:1 
reproduction. 
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Off to the side are cabinets containing the 
tape handlers, controls, amplifiers, etc. for the 
recording and playback of the profiles on the 
magnetic tapes. There are two tapes involved. 
One, which is called the ‘‘temporary”’ tape, 
stores only one profile at any time. Playback 
of the temporary tape provides information 
for memory-assisted scanning and for driving 
the contour-plot and orthophotograph equip- 
ment. The "permanent" tape stores all the 
profiles successively. Any portion of the 
permanent tape can be played back to drive 
the equipment in the same manner as the 
temporary tape. 
The same recording format is used for both 
tapes. The record for each profile consists of a 
series of Z readings, one for each 0.1 milli- 
meter increment of X. Each Z reading is an 
eleven-bit pure binary number giving the full 
value of Z, and occupying two characters on 
the tape. The Y value of each profile is re- 
corded at the beginning and end of the pro- 
file. The total length of the permanent record 
of one model depends on the profile spacing. 
At the closest spacing specified, which is 0.5 
millimeter, as many as four reels of tape may 
be required. In general, we expect profile 
spacing on the stereo model to be coarser 
than that. A wide choice of profile spacings 
has been made available on this experimental 
equipment. 
A clearer idea of the functioning of the sys- 
tem can perhaps be gained by going through 
the operating cycle. The first step in working 
with a new model is of course to set up the 
stereoscopic model and orient it to control in 
the ordinary manner. The operator then sets 
the correct profile spacing and contour inter- 
val, loads a sheet of film into the orthophoto- 
graph unit and sets the correct scale reduction 
factor. He then records the X and Y coordi- 
nates of the corners of the model, as read from 
the coordinatometer dials, and is ready to 
commence the first profile (the one across the 
front of the model). 
Starting at the left end of the first profile, 
the operator adjusts the X-drive servo to à 
comfortable speed. As the measuring mark 
moves in the plus X direction, he continu- 
ously uses his Z-drive servo control to keep 
the measuring mark in contact with the 
ground. The temporary tape automatically 
records the height of the measuring mark once 
each time the mark has been driven 0.1 mm 
in X. The drive stops when the right end of 
the first profile is reached. 
The temporary tape is now played back, 
causing the measuring mark to be moved in