2. Photo Coordinate System

Measurements can be made on the photo positive instead of the negative because they share the same geometry, including focal length. Connecting fiducial marks defines the photo coordinate system, Figure C-5.

Figure C-5 Photo Coordinate System

The positive x-axis is in the direction of flight with positive y-axis to the left. The principal point, o, is at the intersection of the two axes, coinciding with the coordinate system origin. For basic measurements, the axes are assumed perpendicular. Camera calibration data may indicate otherwise, in which case axes non-orthogonality may need to be accounted for.