Map projection is a systematic drawing of parallels of latitude and meridians of longitude on a plane surface for the whole earth or a part of it on a certain scale so that any point on the earth surface may correspond to that on the drawing. The globe is true representative of the earth, which is divided into various sectors by the lines of latitude and longitude. The network of these is known as a graticule.
The need for map projections arises from the very fact that an ordinary globe is rendered useless for reference to a small country. It is not possible to make a globe on a very large scale. Say, if you want to make a globe on a scale of one inch to a mile, the radius will be 330 ft. It is difficult to make and handle such a globe and uncomfortable to carry it in the field for reference. Not only topographical maps of different scales but also atlas and wall maps would not have been possibly made without the use of certain projections. So a globe is least helpful in the field for practical purposes. Moreover, it is neither easy to compare different regions over the globe in detail, nor convenient to measure distances over it. Therefore for different types of maps different projections have been evolved in accordance with the scale and purpose of the map.
Classification of Map Projections. If you look at an atlas, or, any other set of maps, you will find a number of projections used therein. Map projection varies with the size and location of different areas on the earth's surface. While conical and zenithal projections are commonly used for mid-latitudes and Polar Regions, cylindrical projections are referred for equatorial lands. Not only that, projections also vary with the purpose of the map. While transferring the globe on a plane surface, the following facts should be kept in view,
(i) Preservation of area,
(ii) Preservation of shape,
(iii) 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. So the following groups of projections have been made according to the quality they preserve:
1. Equal area or homolographical projections.
2. Correct shape or orthomorphic projections.
3. True bearing or azimuthal projections.
In the first group of projections the graticule is prepared in such a way that every quadrilateral on it may appear proportionately equal in area to the corresponding spherical quadrilateral. It is, however, easier to make the area equal by ignoring the shape. For instance, a rectangle can be made equal in area to a parallelogram by keeping them on the same base and between the same parallels.
The second group of projections is known as conformal projection. It is relatively difficult to preserve the shape but for a very small area. Strictly speaking, only a few points of sphere can be projected in their true form over a plane surface. In order to achieve the quality of orthomorphism, certain modifications needed be made. The scale is changed from point to point; it is true at one point in all the directions. It is possible to make some of the meridians and parallels true, i.e., equal in length to the corresponding one on the globe. Meridians and parallels intersect each other at right angles on the globe. To make the projection conformal; certain devices are made so that they may cut one another at right angles over the graticule.
In the third group of projections, correct bearings or azimuths are preserved. This quality is well achieved in zenithal projections in which the sphere is viewed from a point lying either at the centre of the globe, or at the antipode of the central point, or at infinity. The line of sight in every case is normal to the plane of projection at the central point. If the map is required to show all directions correctly, then the rectangular quality of the spherical quadrilateral as well as the true proportion of its length and breadth must be maintained. In case, you want to show all distances correctly, no such map can be drawn on a plain sheet of paper.
Projections contain a network of parallels and meridians transferred from the globe to a sheet of paper. Such a transfer can be affected by perspective method by assuming that a paper is wrapped around the globe. If a light is imagined in the centre of the globe, the latitudes and the longitudes will cast shadows on the paper. This is the method of developing perspective projections. There are three ways in which such projections may be made. A paper cylinder may be wrapped around the globe with the paper touching the globe at the equator. Such a projection is called a cylindrical projection. In such a projection, all latitudes and longitudes are straight lines at right angles to one another. All latitudes are of same length equal to that of the equator. This means that away from the equator, the latitudes get exaggerated more and more; 60 degrees latitudes are exaggerated twice. Poles, which are points on the globe, are exaggerated as lines equal in length to the equator. The longitudes are shown correctly as lines of equal length. Cylindrical Projections are suitable for representing equatorial and tropical regions on either side of the equator. Cylinder Projections may be made equidistant or equal area. Cylindrical Orthomorphic Projection is called the Mercator projection, which is commonly used for navigation by ships. On this projection, the scale along longitudes is exaggerated to the same extent as exaggeration of latitudes at each point. Therefore directions are represented correctly. But areas are very much exaggerated in middle and high latitudes.
A paper cone may be wrapped around the globe so that the paper touches particular latitude and the grid of latitudes and longitudes may be projected on to the paper. Such projections are called conical projections. The parallel or latitude at which the paper is touching the globe is called the, standard parallel, which is correct in length. Other parallels are exaggerated. Meridians are also correct. The area on either side of the standard parallel is less distorted: Conical projections are useful for representing areas having a greater East to West extent. Conical projections can show only one hemisphere. Conical equidistant, orthomorphic and equal area projections may be drawn.
When a piece of paper is kept touching the globe a point such as the Pole and the latitudes and longitudes are projected on that paper. Such projections are called zenithal Projections or Azimuthal Projections. In the polar case, the latitudes are shown as concentric circles with the poles as the centre. Meridians radiate as straight lines from the pole. Polar areas are represented with minimum distortion. Away from the polar region, exaggeration of scale increases rapidly. Zenithal projections may be made equal area, equidistant or orthomorphic as the case may be. Oblique Zenithal projections with the centre at any point on the globe may also be drawn. The area around that point is represented with minimum distortion.
Non-Perspective Projections are those which are designed mathematically so as to have a desired pattern of latitudes and longitudes. Sinusoidal projection and Molleweids Projection are devised at equal area projections to represent the whole world. Equal area projections are used to world distribution of various features correctly. World distribution of forests cultivated lands, various crops, population etc. may be shown by Sinusoidal or Molleweid projections.
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The second group of projections is known as conformal projection. It is relatively difficult to preserve the shape but for a very small area. Strictly speaking, only a few points of sphere can be projected in their true form over a plane surface. In order to achieve the quality of orthomorphism, certain modifications needed be made. The scale is changed from point to point; it is true at one point in all the directions. It is possible to make some of the meridians and parallels true, i.e., equal in length to the corresponding one on the globe. Meridians and parallels intersect each other at right angles on the globe. To make the projection conformal; certain devices are made so that they may cut one another at right angles over the graticule.
A paper cone may be wrapped around the globe so that the paper touches particular latitude and the grid of latitudes and longitudes may be projected on to the paper. Such projections are called conical projections. The parallel or latitude at which the paper is touching the globe is called the, standard parallel, which is correct in length. Other parallels are exaggerated. Meridians are also correct. The area on either side of the standard parallel is less distorted: Conical projections are useful for representing areas having a greater East to West extent. Conical projections can show only one hemisphere. Conical equidistant, orthomorphic and equal area projections may be drawn.