6 
Figure 8. Displacement-Error Measuring Unit 
HEIGHT-ERROR 
MEASURING UNIT 
The height-error signal must be converted to a 
digital measure of the error. This is accomplished 
in the height-error measuring unit (Figure 8). 
The signal from the height-error sensor is passed 
to an integrator whose output is monitored by a 
positive and negative threshold detector unit. 
When the integrated error signal exceeds one of 
the threshold limits, the detector steps a reversible 
counter in a corresponding direction and resets 
the integrator to zero. The counter, in turn, oper 
ates an elementary digital-to-analog converter 
that produces a voltage to deflect the center of 
one of the diapositive scans in a direction to 
reduce the observed time differential. If the alti 
tude error has not been completely compensated, 
the remaining error signal may again cause the 
threshold to be exceeded, resulting in a second 
count. Operation continues in this manner until the 
height-error has been compensated and appears 
as a corresponding count in the reversible counter. 
In operation, the counter position is then read by 
the associated computer and the integrator and 
counter reset to zero so-that a new measurement 
can be initiated. Note that the height-error meas 
uring unit is, in effect, an analog-to-digital con 
verter that provides a digital measure of any 
height error. Because of the closed-loop operation, 
the measurement is independent of the quality of 
the imagery in the field of view. However, the 
time required to make the measurement does 
depend on the image content. 
MATHEMATICS OF 
AUTOMATIC MAP COMPILATION 
Operation of automatic map compilation equip 
ment is limited to photography for which the 
geometry is sufficiently well defined so that the 
functional relationships 
x — G(X, Y, Z) 
and (2) 
y = H (X, Y, Z), 
which relate coordinates (X, Y, Z) * on the ground 
to coordinates (x, y) in the photographs, can be 
obtained. The functions, G and H, must be in a 
suitable form for computation by a digital 
computer. 
For ordinary photography the relationships in 
Eq. (2) take the well known form 
f I” UxX,. + u 2 Y r + u.,Z, 
X L w 1 X 1 . + w 2 Y,. + w 3 Z 1 ._ 
and _ _ (3) 
f ViX,. + v-.Y,. + v ;i Z,. 
y _ WjX,. + WoY r + w 3 Z,. _ 
In this case, f is the focal length of the camera, 
the coefficients u t , u 2 ...w 3 are functions of the 
orientation of the camera at the time a given 
photograph was exposed, and X r , Y r , and Z, are 
measured with respect to the position of the 
camera station. Equation (3) is augmented by 
simple equations providing corrections in the film 
coordinates, x, y, for distortion in the camera lens 
or film and for corrections in the geographical 
coordinates for the effects of the curvature of the 
earth and atmospheric refraction. For panoramic 
*For the present purposes a rectangular coordinate system is presumed. 
Where the curvature of the earth is significant, a suitable correction 
is used.