In visual perception, foot points are of no small importance. On a plane lying in front of the viewer, base points of objects at different distances appear at different heights. In particular, the same distances can be easily recognized. Unrecognizable differences in height of the terrain can appear to simulate the same or different base positions of the objects with the consequence of size delusions.
- Max Koecher, Aloys Krieg: level geometry. Springer, Berlin Heidelberg 2009, ISBN 978-3-540-49328-0 , p. 113.