BASIC DESIGN AND PACKAGING
The Itek EC-5 Electronic Correlator, shown schematically
in Fig. 2, consists of two main sections: a flying spot scanning
system mounted on the Planimat and an electronics rack that
can be located up to 15 feet from the Planimat.
The scanning system comprises two cathode-ray tubes that
are integrated with the left and right viewing paths of the Plani-
mat. The scanning patterns produced by these tubes are imaged
onto the left and right diapositives, and the transmitted light,
modulated by the images, is converted into electrical signals by
two photomultiplier tubes. The two diapositives can be con-
tinuously viewed as a stereo pair through the eyepieces, using
the normal Planimat optical system. Dichroic mirrors are used
to separate the blue cathode-ray tube scanning light from the
yellow viewing light. The use of this system does not preclude
the use of color material, although the blue end of the spectrum
is lost. To maximize the stability of the system, an optical de-
magnification of 10 is provided between the cathode-ray tube
scanning pattern and the photographic plates. Alignment errors
or disturbances in the scanning pattern are thereby reduced by a
factor of 10.
The two video signals from the photomultiplier tubes are
fed into the electronics rack. Here the signals are correlated and
analyzed into the zero- and first-order error coefficients: x
parallax, y parallax, x scale, y scale, x skew, and y skew.
The x parallax signal, corresponding to the heighting error,
is applied to a servosystem which drives the motor controlling
the height of the z carriage in the Planimat, thereby correcting
the x parallax. The presence of terrain slopes causes x scale and
x skew distortions to be present. These distortions must be re-
moved to maintain good correlation between images, and are
corrected in the present system by changing the shape of the
scanning rasters by means of feedback to the scanning system.
Fig. 2 shows the scanning pattern and the method of correcting
distortion.
Ideally, no y parallaxes are produced in mapping photog-
raphy, but in practice small amounts of y distortion are usually
present. In this system, residual y parallax, y scale, and y skew
distortions are automatically corrected, again by changing the
position and shape of the scanning pattern.
SCANNING SYSTEM
The task of the scanning system is to convert the informa-
tion in the photographic images into time-dependent electrical
signals suitable for correlation. A flying spot scanner was chosen
as the best compromise between technical and economic factors.
Each flying spot scanner consists of a cathode-ray tube
with a short persistence P-16 blue-emitting phosphor upon
which a small, intense spot produced by the electron beam con-
tinuously traces out a scanning pattern. The spot is focused onto
the photographic plate by the optical system. The light trans-
mitted through the plate is modulated by the density variations
of the imagery, and is picked up by a photomultiplier tube
which converts the light energy into an electrical signal.
In the Automated Planimat, an optical duplexing system is
used to enable image scanning to be carried out without inter-
ference to the normal viewing of the photographic plates
through the eyepieces of the instrument. This is achieved by
means. of the system shown schematically in Fig. 4, which
represents one of the two optical paths in the instrument. The
viewing path originates with the illuminating lamp and con-
denser system. A dichroic mirror that transmits yellow light and
reflects blue light is mounted at a 45-degree angle between the
condenser and the photographic plate. An objective lens pro-
duces a magnified image of the plate at the eyepiece focal
plane where it is viewed by the eyepieces in the normal
manner.
A second dichroic mirror, transmitting yellow light and re-
flecting blue light, is placed between the objective and the eye-
piece. An auxiliary lens and a beam splitter provide the duplex
optical path in which the blue light from the cathode-ray tube
