They are used to locate the principal point or geometric center.

• A. Viewfinder
• B. Fiducial map
• C. Principal point
• D. Focal length

Answer: (B)

Fiducials give fixed reference points with known positions, which lets you recover the image’s coordinate system and locate the principal point—the point where the optical axis intersects the image plane and the geometric center of the image. By detecting these fiducial marks in the image and comparing their measured coordinates to their true coordinates, you can solve for where the center lies and for the overall camera geometry, enabling accurate alignment, rectification, or georeferencing of the map. A viewfinder is only for framing and does not provide these geometric references, and focal length describes how the lens projects scene points onto the image plane but doesn’t by itself determine the principal point.