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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