This section is from the book "Aerial Cameras, Aerial Films, And Film Processing", by Richard W. Underwood.
There are aerial cameras which contain in the optical path, usually adjacent to the focal plane or film surface, a glass plate with fine line markings (usually crosses or dots) which are positioned very accurately. These calibration marks, called reseau marks, appear on the exposed film. Generally, they are located at 1-centimeter intervals, with a central mark coincident with the optical axis of the lens system. These marks, to be of any significant value, should be at known positions and accurately located within ±0.0025 mm. The camera manufacturer should provide a very complete document regarding his calibration of the reseau mark positions and the precision capabilities of the measuring instruments used to generate such data.
It should be emphasized that a reseau calibration is not necessary for most work to be done with aerial cameras. However, if you require very accurate knowledge of the deformation of each exposure of film due to mechanical manipulation within the camera or film processor, or need to compute any movement of the emulsion with relationship to the film base during processing, or plan to perform very accurate aerial triangulation surveys, the reseau calibration is an invaluable asset.
Some aerial cameras have crosses, dots, or other identification marks in the corners or at the center of each side of the format. These fiducial marks are of great value in locating the optical axis of the system and in determining film deformation. For precise cartographic work, fiducial marks or a reseau are a requirement.