Tilt displacement is the shift in an object's image position on a tilted photo from its theoretical position on a truly vertical photo. This results from the photo plane being tilted with respect to the datum plane at the time of exposure. This effect is demonstrated in the diagram below. Though the distances between A-B and C-D are identical on the datum plane, their corresponding representations on the photo plane are not (i.e. distances between a-b and c-d are not equivalent).