8 
the line between the two camera stations. A high 
height estimate is indicated, where the image in 
the center of the held of view on the second dia 
positive is shown as P 0 (instead of the correct 
position, P). The image of the estimated point, 
P e , on diapositive 1 is displaced G z dZ from the 
image of P, and by GudZ from the image of P n . 
In practice it is expedient to make the correction 
using G H dZ at the first diapositive. The ortho 
photo signal is taken from the second diapositive ; 
the displacement of P 0 from P is ordinarily so 
small as to produce negligible error on the ortho 
photo. By assuming AZ = 1 (i.e., equal to the 
increment of the height counter), the required 
height corrections are readily found. Thus, 
dx - G x dX + GydY + G z 
and (5) 
dy = H x dX + HydY + H z 
In Eq. (5), dX and dY are found from the cor 
responding equations for the second diapositive 
by taking dx' = dy' = 0 ; that is, 
0 = G'xdX + G'ydY + G'z 
and (6) 
0 = H'xdX + H'ydY + H'z 
When Eq. (6) is solved for dX and dY and the 
results substituted into Eq. (5), the differential 
shifts, dx and dy, corresponding to a unit altitude 
error are determined. These values are outputted 
to the analog equipment. Because only a very 
modest accuracy is required, simple approxima 
tions are possible and only infrequent updating is 
required. The calculation represents a trivial part 
of the total computer problem. 
The computer also outputs a signal to control 
the intensity of the scanner exposing the altitude 
chart. For this purpose, the altitude is divided by 
three times the desired contour interval and the 
remainder, R, examined for assignment of the 
printout level. The level is assigned as follows: 
0 < R < 1/3 scanner off 
1/3 < R < 2/3 scanner medium 
2/3 < R scanner bright 
This creates a chart (shown on the first page) 
showing well defined altitude bands. The rotary 
sequence of densities permits an unambiguous 
assignment of altitudes, starting from a point of 
known altitude. 
REDUCTION OF V PARALLAX 
Creating an adequate stereo model for compila 
tion requires that both X and Y parallax be 
removed at a number of points in the stereo field 
so that coordinates of common points in the field 
can be accurately ascertained. The height-error 
sensor removes X displacements caused by height 
errors; therefore, it is directly applicable to the 
automatic removal of X parallax in the compara 
tor mode. The removal of Y parallax displace 
ments requires additional circuitry. 
Sensing of Y errors is achieved, in effect, by 
introducing an artificial Y parallax analogously 
to the X parallax provided by the delay lines in 
the height-error sensor (Figure 6). The Y paral 
lax is introduced on a time-sequential basis by 
shifting the scan in Y on one diapositive, posi 
tively for a defined time and then negatively for 
an equal time. The output of the associated corre 
lator is switched into a positive input of an inte 
grator during the positive shift, and to a negative 
input during the negative shift. If there is no Y 
parallax at the point, the inputs during the two 
time periods are equal, and the integrated output 
is zero. If there is any Y parallax, the correlator 
will have a larger output when the artificial par 
allax partially cancels the image displacement and 
a lower output for the other half cycle where the 
two components are additive; therefore, the 
integrated output provides a measure of the Y 
parallax. 
In the UAMCE, a positive and negative thresh 
old detector (similar to that used with the height- 
error sensor) provides digital error signals to the 
computer for reduction of Y parallax during com 
parator operations. For automatic compilation 
operations, the integrated output is used to deflect 
the center of one of the diapositive scans in a 
direction to reduce Y parallax; this ensures high 
sensitivity in the height-sensing circuitry, even 
with a poor model. 
SCANNING PATTERN 
It has been noted that the principal scanning 
for automatic height determination should be 
along a line parallel to the line joining the two 
camera stations (i.e., on an X-directed line). This 
must be augmented by a slow scan perpendicular 
to the fast scan (on a Y-directed line) to produce 
an effective area scan providing a TV-like cover 
age of each small area. This ensures continuity of 
the image for both the operator and for automatic 
tracking. The signal from one scanner is used to 
recreate the scene element for exposure of the 
orthophoto film sheet. 
For automatic operations the time required to 
sample a given area must be kept as low as pos 
sible. For this reason the Y scan is made in an 
unusual pattern. Every line on the positive side 
of the center of the scan is followed by a corre 
spondingly positioned line on the negative side. 
The scanned area is covered very coarsely at first, 
then gradually filled in. A total of 128 lines is