8 
The equation 
p - [< Rl - h x ) - (r 5 - h 5 )] , 
d 
expresses the difference in the inclination of the horizon line between the 
reference photos and the measured photos. In equation 2, d is the distance 
between fiducials and R5, and equals approximately 22 mm. 
To investigate the accuracy of computing tip and tilt by means of 
( 
equations 1 and 2, let us assume that the standard error of one parallax 
observation is mp. Applying the formulae from the theory of errors to 
equations 1 and 2, we get the standard errors of tip and tilt as follows: 
(a) using equation 1 
nic( = - 128 m p 
(b) using equation 2 
mp c = "t 582 nip 
where mp is expressed in mm. 
From the above, it is evident that the determination of tip and 
tilt by equation 1 is more accurate. Therefore, we are using it in our 
computations. 
If all the four horizon photos are available, the averages of the 
results obtained from the forward and aft, and from the left and right photos 
are taken as the final values of ' < f > and to respectively. The differential 
tips and tilts are measured between a reference photo and each consecutive photo % 
of a flight line, in the manner illustrated in the figure below: 
o 
2 
LjJ 
or 
Ixl 
Lx 
Ixl 
OC 
Fig. 8