Relative Height (Monocular Cue)

A monocular cue in which we perceive objects higher in our field of vision as farther away.
Relative height also contributes to the illusion that vertical dimensions are longer than identical horizontal dimensions.

https://s-media-cache-ak0.pinimg.com/originals/1b/fc/ca/1bfccaccd787e07b225fa16ba38b8294.gif

In the picture, the horizon is indicated by the straight line that goes across the picture. The man located all the way to the left, closest to the top, seems farther away because he's higher in the field of vision. Meanwhile the one in the bottom right corner seems closer.