true vertical photograph

true vertical photograph

true vertical photograph
P in this case is photo nadir, ground nadir and the principle point of the photograph.
An aerial photograph taken with the optical axis of the camera perpendicular to the ground. In such a photograph, the principal point, the ground nadir, and the photo nadir coincide. Whereas the principal point is always the geometric center of a photo, the photos other than the true verticals—the ground and photo nadir—will fall away from the principal point.