6 
} 
if 
+ 4 
+ 
i/V 
+ 4 
-f 
fei 
~t~ 4 
Fig. 5 
f + 
gfi 
+ 
4- 
4- 
111 
4- 
t + 
4 
Fig. 
5a 
Fig. 6 Wild Stereomicroscope Used 
for Measuring Horizon Photographs 
r 
2.3 Measurement of the Horizon Photographs 
Let us assume that Figures 5 and 5a represent the horizon lines 
corresponding to the vertical photographs i and i + 1 respectively. Let us 
assume further that horizon photographs i and i + 1 were taken in the 
direction of flight. The difference in the position of the horizon line in 
photos i and i + 1, relative to their fiducial marks, is proportional to the 
difference in tip of the corresponding vertical photographs. If we place these 
horizon photos under a stereoscope, so that the corresponding fiducial marks in 
both photos overlap in the optical model, we can observe the horizon in three 
dimensions. The fiducial marks will form a reference plane and the horizon 
line will appear in three dimensions: either above, below, or intersecting the 
reference plane. This stereo effect is caused by the displacement of the 
horizon line in photo i 4 1 relative to photo i. This displacement or 
parallax is measured using a Wild stereo-microscope, and the relative tip and 
tilt are computed. 
A reference horizon photo is placed in the left frame of the 
stereo-microscope so that the horizon line is approximately perpendicular to 
the instrument base. The other horizon photos of the same strip are placed one 
by one in the right frame in the same position. r 
The horizontal parallaxes of the central fiducial marks (crosses 
R-^, R2, R3, R4, R3) and the horizontal parallaxes of the adjacent points on the