this may be expressed by including two, one or three 
condition equations stating that the final values of 
the relevant X,Y,Z coordinates should have, as nearly 
as possible, their known ground control values, xx 
Other external conditions will be considered later (Para 5). 
  
x Unidentified, since the photo coordinates in this 
photo are known (0,0), and are not observed in any 
other photo, It may be noted that the excess of 
observation equations over unknowns is unaffected, 
Since each is increased by two when the principal 
point is included, and that this is the one point 
which need never be rejected as being a bad 
observation?! 
xx This is known as a flexible control condition, 
mostly to be used for height control, or when an 
abundance of, perhaps unreliable, plan control is 
available; the alternative, rigid control, is simply 
to hold the corresponding coordinates rigidly at 
their known values, and reduce the number of variables 
accordingly. 
  
3. Provisional Coordinates 
For the method to work ecomomically, it is essential 
that the provisional coordinates are sufficiently close 
to the final coordinates to neglect second order terms, 
so that the linear observation equations do in fact apply. 
With near-vertical photography, very approximate 
coordinates are first obtained by the methods of Appendix C1 
or C2 (C2 is preferable if threex points are known), and 
are then improved by a simple method such as that of 
Appendix D, iterated i? necessary. 
With oblique photography, the most fruitful approach 
seems via the method of Appendix C2 (again assuming threes 
points known): having obtained accurate coordinates (U,V,W) 
of the air station, which may take s everal iterations if 
there is no prior information as to tilt, and having levelled 
the ground plane xx(para 3,ibid.), one can substitute U,V 
for X,Y in the "exact equations" (para 1,ibid.), and solve 
for x,y, the photo coordinates of the plumb point. If one 
now relevels the photo plane so as to make this plumb point 
the new "principal point" (0,0), one can then repeat the 
whole of Appendix C2 with the assurance that the new levelled 
ground plane will in fact be parallel to the datum plane. 
Improvement by Appendix D follows as in the vertical case.