The 'principal distance' (f) of a camera in photogrammetry is:
Choose the correct answer
The distance from camera to ground target
Focal length of the camera lens — perpendicular distance from lens centre to photo plane; determines photo scale
The spacing between adjacent flight lines
The lap percentage of photos
Correct Answer
B. Focal length of the camera lens — perpendicular distance from lens centre to photo plane; determines photo scale
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Principal distance (f) = perpendicular distance from perspective centre (lens) to principal plane (photo plane). In vertical aerial photography: ground coverage = (f × H)/f × format size = H × format/f where H = flying height above ground. Scale of photograph = f/H. Larger f (longer focal length) → larger scale, less coverage per photo.
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