Depth perception is also limited in humans, particularly when presented with a uniform colour. True depth perception is only supported by binocular vision and works within the 50-75 foot range. Depth perception beyond this is much more reliant on the brain making a compensating calculation. It seems the eyes must be able to focus in on a variety of objects at different ranges to accurately determine height. This is particularly a problem in the winter when there is snow on the ground and a white sky. This is the basis of the phenomenon known as Whiteout. At the very least it causes problems with depth perception. Complete disorientation, similar to that encountered in cloud flying, may be a result.
While this is the most common example of problems with depth perception in flying another interesting phenomena occurs with inverted flight. The brain quickly loses the ability to make the compensating calculation of depth even when presented with good visual cues. Many pilots have had the experience in these circumstances of being able to identify a nearby aircraft and some characteristics of the occupants but having no idea of the distance the aircraft is from them. Experienced aerobatic instructors have indicated that the process to ‘learn’ how to judge distances while inverted can take up to several years to acquire. Along the way is that the best advice is to return to normal flight when encountering traffic if there are any doubts in this regard.
The shape of a field can influence our perceptions. Narrow fields are perceived to be longer than wider ones. As we can see from the above diagrams, pilots can be fooled by the true perceptions of the terrain below them as to the actual height they will be above the terrain ahead of them. Stereoscopic vision only gives accurate depth perceptions up to about 100 feet or so (30 m), everything beyond these distances is usually intellectualized based upon the persons experience with the width, length, and particularly the height of known objects. As such when a pilot flies over sloping terrain the tendency is to draw conclusions based on what is at hand rather than ahead. Only local knowledge, or experience at detecting other signs of the slope of the terrain, is useful in counteracting the negative effects on slope to our flight path.