2. Image Coordinate System

Metric photogrammetric cameras use a 9 inch square film format. The frame surrounding the negative at the focal plane contains fiducial marks located at the middle of each side; some cameras include corner fiducials, Figure B-3.

Figure B-3
Fiducial Marks

Connecting opposite fiducial marks defines the image's coordinate system. The axes should intersect at the principal point, o. The principal point is at the perpendicular intersection of the optical axis and focal plane, Figure B-4. 

Figure B-4
Principal Point, o