centre and can then be considered as just 
one more point in the main photo scheme, If 
the relationship is not known, then it can of 
course be readily established if there are 
enough common points, The angle of parallax 
Subtended by the two camera centres at the 
distant ground point will normally be completely 
negligible. 
  
  
If the heavenly body has such a low elevation 
that it is not convenient to take the intercept 
with the datum plane, then any convenient Xzconst 
or Y=const plane may be used; in this case, it 
probably will also be inconvenient to take the 
intercept with the photo plane of the main camera, 
This need cause no difficulty, but the full formula 
must be used when computing the constant term for 
the observation equations, 
  
  
  
  
  
  
  
  
  
  
  
  
  
(e) 
Compass or other Heading. It will be assumed 
that the bearing has been converted to its 
value in the ground coordinate system, In 
addition to the principal point, take a second 
"dummy" photo point at the end of the x-axis; 
there will now be (2p+1) observation equations 
for a photo with p normal photo points, If now 
the intercept with the datum plane of the ray 
through this second dummy point is (95:9): then 
the heading of the photo has tangent (T-R)/(S-Q), 
and one has the usual choice of including an 
additional observation equation to express the fact 
that this is the, given, tangent of the known 
angle, or of eliminating one of the variables 
to make the condition rigid, 
(f) Rounds of Theodolite Angles. These could be 
observed at a ground photo point to other photo points, 
or known triangulation points, they could include 
sun or star shots, or rays to air stations - the 
point occupied need not even be a photo point at 
all, In all cases, the most satisfactory procedure 
would be to have both horizontal and vertical angles 
observed, and to reconstruct the space (or "sextant") 
angles from point to point, After this, a space 
angle observed by a theodolite is on exactly the 
same footing as a space angle "observed" in an air 
photo, and the observation equation coefficients 
have exactly the same form in both c ases, It would, 
also, of course be possible to make two observation 
equations, by keeping horizontal ahd verti cal 
measures separate, but problems of weighting would 
then arise,