Parallax and Perspective during Aircraft Landings

The performance of successfully landing an airplane is generally supposed to be dependent on the ability to perceive visual depth and distance, or more generally space. The classical explanation of human ability to see space is that the eye provides a set of cues to the distance of a given object, and likewise differential cues to the different distances of different objects. Such cues, include (a) binocular parallax or disparity, (b) linear perspective, including the apparent size of objects whose real size is known, (c) aerial perspective, superposition, shading, and (d) motion parallax, i.e. the apparent movement of physically stationary objects relative to one another and to the observer. The last depends upon motion of the observer, usually locomotion, and the strength of the effect is in proportion to the velocity of the observer. Consequently it ought to be of particular significance for spatial judgments during aircraft landings, in which locomotion occurs at a considerabe velocity. The aim of this report is: (1) to examine the existing definitions of motion parallax; (2) to show that they are not sufficiently general to cover the visual situation during aircraft landing; (3) to formulate geometrically a somewhat different principle (called motion perspective); and (4) to apply this general principle to the problem of perceiving the ground and one's relation to it during descent.