The réseau grid of a camera is a basis for using a camera as a measurement image camera . As reseau grid mostly glass plates with high precision etched crosses are called Réseaukreuzen , or fiducial marks used. For very simple requirements you can also use the four corners of a photo as grid points. The grid itself is mounted directly in front of the film level in the camera and is thus shown with every shot. Since the location of the réseau crosses is known very precisely, it is possible to mathematically compensate for falsifications of the images, such as film deformations during development, non-flat film during recording, scanner non-linearities or digitizing tablet discontinuities. This happens more precisely the more réseau crosses are measured.