What are fiducial marks used for in aerial photography?

Fiducial marks are critical reference points used in aerial photography that assist in accurately locating the principal point of the photograph. The principal point is essentially the point where the camera's lens is directed when the photograph is taken, and it forms the basis for measuring coordinates and understanding scale in aerial images. By providing a clear reference, fiducial marks help surveyors and photogrammetrists maintain the proper orientation of the images for further analysis.

In practical terms, once an aerial photograph is taken, these fiducial marks allow for precise calibrations and measurements needed for mapping, creating orthophotos, and other applications requiring high accuracy. They ensure that even if images are taken at various angles, they can still be aligned consistently based on these predefined marks. Such accuracy is fundamental in surveying and land use planning, making fiducial marks invaluable in aerial photography.