Size constancy is the perception of an object as having a fixed size, despite the change in the size of the visual angle that accompanies changes in distance. That is, we have a tendency to see an object as the same size regardless of the size of its image on our retina. When the aircraft flies for a long period over tall tress and suddenly flies over smaller trees, the pilot's brain will assume that their size has remained the same. Consequently, this may create an illusion of obtaining greater height above the trees (the pilot will assume that the aircraft has climbed away from them).