parallel lines with some four thin rods extending perpendicular to the plane (Fig.=2). 
Oblique photography is taken from two camera positions with the tilt axis direction 
changed by some 90°, Neither of the tilt directions can be along the directions of the 2 sets of 
parallel lines, For each photograph the vanishing line VV is determined (Fig. 3). 
oblique 
photograph Oblique 
photograph 
  
  
  
  
  
  
  
  
Fig. 3 
The nadir point of the photograph is determined through the radial displacements of the 
thin wires perpendicular to the plane (Fig. 4). 
The locus of the principal point is the line perpendicular to the vanishing line VV and 
passing through the nadir point n. 
By changing the direction of tilt axis by some 90° and taking another exposure, another 
locus of the principal point is determined, and thus the position of the principal point can be 
deduced (Fig. 5). 
The principal distance can then be determined as : 
235 
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