* E 
GV-10 PHOTOGRAMMETRIC ENGINEERING 
ing to the celestial coordinates of the principal point, and the y fiducial line of 
the negative oriented on the line drawn from the principal point to the ob- 
server's position. This is illustrated in Figure 4. 
The coordinates of the observer corresponding to RA and à are LST and 9. 
The line connecting the plotted position of the principal point and the observer 
should define an angle with the meridian passing through the observer equal to 
the observed azimuth, approximately. The star images selected for definition 
and geometric distribution are easily identified when the position and orientation 
of the negative on the star chart are established. 
After the stars are identified, the camera coordinates are measured referred 
ZENITH 
ps 
eo" 
v 
ee 
we 
x on 
c LC 9^ 
No? J A 
\ d 
S NS 
~~ + 
+ + 
LA) 
>> 
<] 
F1G. 5. Geometry of refraction on an oblique exposure. 
to the fiducial axes, and the apparent places of the stars for the instant of ex- 
posure are computed. The terminals of the star trails are measured in x and y. 
The mean time of opening and closing is used in the apparent places computation 
and the mean coordinates of the terminals in photogrammetric computations. 
The coordinates of any point are: 
xb, + xe, 
— xp = x, 
> p 
ybi + Ve; * * 
| yp m 
2 yp =: 
where b and e denote beginning and ending, and p, the principal point. 
The x and y values are corrected for refraction. The correction is applied so 
as to increase the absolute values of x and translate all y values away from the 
zenith. 
The equations for the refraction corrections are derived from the cosine for- 
mula illustrated in Figure 5. 
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