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listed below: 
a. The device exploits the principle of self-calibration through designed 
redundance (that is, by utilizing software instead of hardware for certain functions); 
it is capable of determining coordinates within the full format size; it is rugged, 
compact, and readily transportable in a hand-held carrying case. In addition, it 
is fully self-checking; simple to operate at high measuring speeds; immune to 
effects of constant personal biases or even to systematically changing personal 
biases; provides sound estimates of the accuracies of final plate coordinates as 
well as other measures suitable as indexes for quality control; and it can be 
equipped with automatic digital readout. 
b. Images are viewed through a traveling zoom microscope. 
c. The glass diapositive plate is mounted on a special self-seating frame; 
measurements are made in four completely independent steps from separate 
directions. After all image points are measured and recorded in the first step, 
the plate frame is rotated 90° and the measurements repeated; the same proce 
dure is repeated with the frame set at 180° and again at 270° from the original 
position. This procedure automatically removes operator personal bias; pro 
vides detection of reading blunders; provides error estimates which can be 
propagated through subsequent reductions; and serves as an index for quality 
control. 
d. A transparent scale is used to pivot about a single point; measure 
ments are entered into a digital computer to determine the zero point of the 
measuring scale relative to the pivot point, and the x- , y-coordinates of the 
measured points, the coordinates of pseudo points, and the accuracies of the 
coordinates and parameters of the comparator. The resulting products (coor 
dinates of the points) are, in effect, thus determined by a four-station, two 
dimensional least squares trilateration. 
GENERAL MEASUREMENT RESEARCH COMPANY. 
The General Measurement Research Company, Berkeley, California, USA, 
has produced the Digital Linear Measuring Instrument (DIG) which is a single 
axis high resolution linear digitizer with a meter travel length, digital reference 
point preset, numerical indicator tube display, and coded decimal readout. The 
DIG has a relatively high least count capability. A companion device, the MILDIG, 
is of less precision by a factor of about 13; 1; the operating principle and many of 
the detail components are identical. The DIG system consists of three elements, 
namely, a precision glass scale, a compact reading head, and a remote display 
panel. A fine encoder (scale interval interpolator) and a coarse encoder are con 
tained in the reading head whose main requirement is to scan optically the scale 
and make absolute determinations of position, relative to a zero reference point 
on the scale. The fine encoder interpolates scale intervals digitally. Other char 
acteristics include all solid-state electronics; one of three measuring axes operable 
from one electronic package on a time-sharing basis; built-in calibration mode 
(calibration unaffected by power interruptions or fast traverses); coded decimal 
output for printout, storage or feedback purposes; multiaxis units with a single