Hot Spots and Shadow Points
What are hot spots? What causes them? How can these be avoided?
Write a note on the factors which hinder Photo-interpretation.
FACTORS HINDERING PHOTO-INTERPRETATION:
The process of photo-interpretation is hindered by factors such as shadow point, area of maximum shadow, the effect of sun altitude, and effect of light.
All these factors are explained as:
Shadow Point/ Hot Spot:
Shadow point is also called no shadow point or hot spot. “This is the point where the relief displacement of objects exactly matches the shadows with the result that no shadows are visible”.
Cause of Hot Spot:
The case happens when aircraft and sun rays fall exactly in the same line; the shadow of the aircraft may often be registered on the photograph for low altitude. Under such conditions, sun rays after passing through the nodal point of the camera lens strike the ground and a hot spot is created on the photograph.
If a hot spot is created, the photographic image appears very light and over-exposed. The detail in such an area because of halation gives less detail as compared to any other part of the negative. (Halation: a blurred bright patch around a light source on a photographic image. It is caused by light being reflected off the film base and back onto the light-sensitive layer.) Furthermore, the image has a washed-out appearance and is, therefore, difficult to interpret stereoscopically. See the figure (say an aerial photo) (From right: Arshad, Imran, Ahtizaz, Nazeer, Lal, Kamran, and Sher Jan and an elephant are in the background _ Mohenjo Daro)
Since Electronic Printers have become available, the effect of uneven density can greatly be reduced in the prints. The process is known as “Dodging” (See the Dodged photograph of the above one.)
Area of Maximum Shadow:
Photographs are relatively darker on the western, southern and eastern sides, depending upon the time of photography. The area of maximum shadow is sometimes called as the solar reflection point. The solar reflection point and the no shadow point are equidistant from the nadir (N) in opposite direction along a course that is oriented with the shadow of a vertical object. See fig:
Effect of Sun Altitude:
In vertical photographs, the objects appear differently near the center (Nadir point) as compared to those which are at the corners of the photographs having radial displacement due to variation in sun altitude.
For instance, a tree near the center appears as a dot as compared to the corner. (See fig)
Effect of Light:
The angle of the sun also plays an important role in altering the appearance of the same objects from one photograph to another. A picture taken at different angles reveals a difference in the appearance. Such changes are the result of fore, side, and backlighting.
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