to an object by comparing its apparent shape to its known shape. For
example, the greater the amount of detail you can recognize on an aircraft,
the closer you are to the aircraft.
(c) Vertical position in the field of vision. Objects tend to
be higher in the field of vision over distance. Therefore, you estimate the
distance to an object by its position in the field of vision. For example,
terrain features which are far away from the observer appear higher in the
field of vision than terrain features closer to the observer.
(2) Motion parallax: Motion parallax is the apparent motion of
stationary objects due to the motion of the observer. The speed with which
the object moves or whether it is stationary is dependent upon the distance
the observer is from the object. For example, fence posts close to the
observer will appear to be moving rapidly, trees at a greater distance will
appear to be moving more slowly, and mountains at even greater distances
will appear stationary. This cue to depth perception is considered
important because of its use during low-level flight.
(3) Retinal image size: The brain determines the distance to objects
by interpreting the size of the image focused on the retina. The brain is
able to determine the distance to an object by comparison of several
different factors. Those factors include the known size of the object,
increasing or decreasing size due to movement, terrestrial associations and
overlapping of contours.
(a) Known size of objects. The nearer an object is to the
observer the larger the retinal image is. By experience, the brain learns
to estimate the distance of familiar objects from the size of their retinal
image. A structure will subtend a specific angle on the retina based on the
distance from the observer. If the angle is small the observer knows the
distance is great. To do this the observer must know the size of the object
and have prior visual experience with it.
(b) Increasing and decreasing sizes of objects. If the retinal
image size increases, the object is becoming closer to the observer. If the
size decreases, the object is becoming farther from the observer. However,
if the size is constant, the object has remained a fixed distance from the
(c) Terrestrial associations. Comparing an object of unknown
size to an object of known size may be helpful in determining the relative
size of the unknown object and its apparent distance from the observer.
Ordinarily, the objects to be compared are judged to be at approximately the
same distance from the observer (Figure 9). For example, if you see an
aircraft (known size) in the vicinity of an airport (unknown size and
distance) you can judge the aircraft to be in the traffic pattern and that
the airfield is at approximately the same distance (determined from the
known size of the aircraft).