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Projection System
Classification based on methods of construction
Mathematically the term 'projection' means the determination of points on the plane as viewed from a fixed point. But in cartography it may not be necessarily restricted to 'perspective' or geometrical projection. On the globe the meridians and parallels are circles. When they are transferred on a plane surface, they become intersecting lines, curved or straight. If we stick a flat paper over the globe, it will not coincide with it over a large surface without being creased. The paper will touch the globe only at one point, so that the other sectors will be projected over plane in a distorted form. The projection with the help of light will give a shadowed picture of the globe which is distorted in those parts which are farther from the point where the paper touches it. The amount of distortion increases with the increase in distance from the tangential point. But only a few of the projections imply this perspective method.
The majority of projections represent an arrangement of lines of latitude and longitude in conformity with some principles so as to minimize the amount of distortion. With the help of mathematical calculations true relation between latitude and longitudes is maintained. Thus various processes of non-perspective projections have been devised.
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Classification based on preserved qualities
While transferring the globe on a plane surface some facts should be kept in view:
- Preservation of area,
- Preservation of shape,
- Preservation of bearing i.e. direction and distance.
It is, however, very difficult to make such a projection even for a small country, in which all the above qualities may be well preserved. Any one quality may be thoroughly achieved by a certain map projection only at the cost of others.
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