A marker used to locate the geometric center or principal point of a photograph is called what?

• A. Principal Point
• B. Spot height
• C. Fiducial Mark
• D. Vanishing Point

Answer: (C)

In photogrammetry, fiducial marks provide fixed reference points on an image that allow you to determine its geometry, orientation, and scale. They are intentionally placed markers with known coordinates so software can identify the image’s frame of reference and locate the principal point—the intersection of the optical axis with the image plane—within the photo. This makes it possible to calibrate the camera model and rectify distortions. A principal point is a theoretical location dictated by the camera, not a physical marker. Vanishing points arise from perspective geometry of parallel lines and are not markers, and spot height is a terrain measurement term, not a marker used for locating the principal point.