What are different factors which help in photo-interpretation? Explain each factor briefly.
Photointerpretation deals with the examination of photographic images for the purpose of identifying objects and deduce their significance. Photointerpretation requires a solid background of training and experience, otherwise, he is liable to overlook or misinterpret small and subtle features on aerial photographs.
FACTORS HELPING IN PHOTO-INTERPRETATION
The process of photo-interpretation in forestry is made easier by taking advantage of the factors such as: field control, knowledge of area, knowledge of subject, field of view, and radial displacement.
- Field Control:
Field control simply means check a particular object on the ground which you find on an aerial photograph. The information collected from aerial photograph must necessarily be checked with the actual conditions in the field by paying visits to the field for observations. Such observations are not only important but also helpful to the interpreter to arrive at reliable conclusions.
- Knowledge of Area:
The interpreter should be familiar with the area and he should have adequate knowledge of the geographic and physiographic distribution of the tree species and the influence of site on the species association. The interpretation would be reliable if he has sufficient knowledge of the area he is interpreting. For instance, Chir (Pinus roxburghii) and Blue pine (Pinus wallichiana) may produce identical crown shape, shadow pattern, tone and texture, and therefore, it may be difficult to identify them unless their locations are known because these grow in definite elevation zones.
- Knowledge of the Subject:
It is one of the important pre-requisite for correct identification. A man who is able to identify a particular tree on a vertical photograph should necessarily be familiar with leaf shape, branch habit, and crown pattern of that particular tree, and its association and aspects.
- Field of View:
On medium scale photography, e.g.1:15,840, a simple lens stereoscope with double (2´) magnification permits the interpreter to view a considerable area of the photograph at one time. This not only helps in comparison of a number of photographs but also helps in the interpretation of photos.
- Radial Displacement:
It is a line or distance from the center of the photograph to any other point on the photograph.
Increase in radial displacement is directly proportional to the distance from the center of the photograph. This implies that radial displacement is maximum along margins of the photographs and near the Principal point or center it is minimum.
Radial displacement helps in the interpretation of the photo, for example, a tree near the principal point would appear as a dot but the same tree along the margin would appear more in side view due to radial displacement. Further it also helps in the identification of a particular spp (as shown in the figure below _ here radial displacement illustrates that the trees are conifers).
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