Notes
Outline
Distance Estimation and Depth Perception
Cues
Binocular.
Depend on the slightly different view each eye has of an object.  These cues are only of value when the object is close enough to be viewed differently by each eye.  Therefore, since this is often not the case within aviation, binocular cues are not discussed.
Monocular…
Cues
Aerial Perspective
F
L
P
Motion Parallax
Geometric Perspective
L
A
V
Retinal Image Size
K
I
T
O
Geometric Perspective
Linear Perspective
Parallel lines, such as railroad tracks, tend to converge as distance from the observer increases.
Geometric Perspective
Apparent Foreshortening
The true shape of an object or terrain feature appears elliptical when viewed from a distance.  As the distance to the object or terrain feature decreases, the apparent perspective changes to its true shape or form.
Geometric Perspective
Vertical Position in the Field
Objects or terrain features that are farther away from the observer appear higher on the horizon than those that are closer to the observer.
Retinal Image Size
Known Size of Objects
The nearer an object is to the observer, the larger its retinal image.  By experience, the brain learns to estimate the distance of familiar objects by the size of their retinal image.  A structure projects a certain angle on the retina.  If this angle is large, the observer “sees” the object as being closer, and vice-versa.  To use this cue, the observer must know the actual size of the object and have prior visual experience with it.  Without this experience, the observer must rely on motion parallax  to judge an object’s distance.
Retinal Image Size
Increasing or Decreasing Size of Objects
If the retinal image of an object increases in size, the object is moving nearer the observer.  If the retinal image decreases, the object is moving farther away.  If the retinal image size is constant, the object is at a fixed distance.
Retinal Image Size
Terrestrial Associations
Comparison of one object, such as an airfield, with another object of known size, such as a helicopter, will help to determine the relative size and apparent distance of the object from the observer.  Objects ordinarily associated together are judged to be at approximately the same distance.
Retinal Image Size
Overlapping Contours or Interposition of Objects
When objects overlap, the overlapped object is farther away.  An object that is partly concealed by another is behind the object that is concealing it.  This is especially important to us when making landings at night…lights flickering or disappearing in the landing path should be treated as obstructions to the flight path, and the path should be adjusted accordingly.
Aerial Perspective
Fading of Colors or Shades
Objects viewed through haze, fog, or smoke are seen less distinctly and appear to be at a greater distance than they actually are.  If atmospheric transmission of light is unrestricted, an object is seen more distinctly and appears to be closer than it actually is.
Aerial Perspective
Loss of Detail or Texture
The farther from an object an observer is, the less apparent discrete details become.  For example, a cornfield at a distance becomes a solid color.
Aerial Perspective
Position of Light Source and Direction of Shadow
Every object will cast a shadow if there is a source of light.  The direction in which the shadow is cast depends on the position of the light source.  If the shadow is cast toward the observer, the object is closer than the light source to the observer.
Motion Parallax
Often considered the most important cue to depth perception.  Motion parallax refers to the apparent, relative motion of stationary objects as viewed by an observer who is moving across the landscape.  Near objects appear to move past or opposite the path of motion; far objects seem to move in the direction of motion or remain fixed.  The rate of apparent movement depends on the distance the observer is from the objects.  Objects near the aircraft appear to move rapidly, while distant objects appear to be almost stationary.  Thus objects that appear to be moving rapidly are judged to be at a greater distance.  For example, grass along a road goes past very rapidly because it is close…trees a little farther out seem to go by a little slower, and mountains in the far background of your view seem to either sit still or move with you.  From this, the mountains are judged to be at a greater distance from the observer.