The Origin of Differential Photogrammetry 
We regard two image points at identical locations (x,y) in 
adjacent photographs, in a photogrammetric strip. 
  
  
  
  
  
  
  
  
ep oq 
(i ) Citi] 
Figure 2 
(see figure 2). These two points have systematic image 
deformations of the same magnitude. When forming the 
difference of their  projective relations (1), the 
systematic deformation terms F(x,y) are eliminated: 
8) o (i+1) 
óx xq! - xp" e pxU" (Ng, Eq, Hy) —- Px (Np,Ep,Hp), 
6) . vn ^b 
Sy yg? - yp? « py "D (Na,Eq,Ha) - Py" (Np,Ep,Hp). (2) 
We thus have a formulation of the photographic projection 
problem, and indeed for all projections, which is 
independent of the knowledge of the exact projective 
relationship. No assumption has to be made regarding 
systematic image deformation, or even on the nature of 
projection. 
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