A map projection is a technique for taking the curved surface of the earth and showing it on something flat, similar to a PC screen or a bit of paper. maps makers have conceived techniques for taking points on the curved surface of the earth and “anticipating” them onto a flat surface.
Different projections are used for different types of maps because each projection particularly suits certain uses. Different projections are developed for different purposes. Some projections minimize distortion at the expense of increasing distortion of others. The choice of a map projection for a map depends on the purpose for which the map will be used, the area being mapped, and the map’s scale.
Metric Properties of Map Projection
The metric Properties of Map projection are :
Each projection preserves or compromises or approximates basic metric properties in different ways. The purpose of the map determines which projection should form the base of the map.
Choosing a projection surface
If a surface can be transferred onto another surface or unrolled into a sheet of paper without stretching, tearing or shrinking is called an applicable surface. The sphere and ellipsoid do not have developable surfaces, so any projection of them onto a plane will have to distort the image. The cylinder, cone and the plane are all developable surfaces as they all can be transferred onto another surface without any distortion.
An aspect of the projection
Once a choice is made between using a cylinder, cone, or plane, the aspect of the shape must be specified. The aspect describes how the surface is placed relative to the globe. The aspect of the projection can be normal (in line with the Earth’s axis), transverse (at right angles to the Earth’s axis) or oblique (any angle in between).
Choosing a model for the shape of the earth
Map Projection also depends on, how the shape of the Earth is approximated. In the following section on projection categories, the shape of the earth is taken as a sphere in order to simplify the discussion. However, the Earth’s is also modeled as an oblate spheroid or ellipsoid.
To Select the Earth shapes model it involves choosing between the advantages and disadvantages of a sphere vs an ellipsoid. For small scale maps such as atlases and globes, Spherical models are useful, since the error at small scale is not usually noticeable or important enough to justify using the more complicated ellipsoid. For large or medium scale maps ellipsoidal model is used such as topographic maps that need to accurately depict the land surface.
A third model for the shape of the earth is – geoid, it is a model of global mean sea level that is used to measure precise surface elevations.
A projection classification is based on the type of projection that is used. When the earth’s surface projected on a map wrapped around the globe as a cylinder produces the cylindrical map projection and when it is wrapped into a cone it gives a conical map projection. If it is projected on a planar map then it produces an azimuthal or zenithal map projections.
Projection is classified according to properties of the model they preserve. Some of the categories are:
- Preserving direction – azimuthal or zenithal.
- Preserving shape – conformal or orthomorphic.
- Preserving area – equal area projection.
- Preserving distance – show the distance between one or two points and every other point.
- Preserving shortest route – Preserved only by the gnomonic projection.
The Azimuthal projection emphasis on preserving the distance and direction accurately from the center point of the earth, but have distortion in shapes as well in sizes from elsewhere. The central meridian is drawn as a straight line and others are drawn as curved lines. A straight line drawn through the center point is on a great circle. Orthographic projections are used for perspective views of hemispheres. Area and shape are distorted. Stereographic projections are commonly used for navigation in polar regions. Directions are true from the center point and as the scale increases away from the center point, distortion in area and shape also. Below are the some Azimuthal Projection are :
- Azimuthal equidistant
- Equidistant conic
Conformal map projections emphasis on preserving angles, Perpendicular graticule lines intersecting at 90-degree angles on the map. The area enclosed by a series of arcs may be greatly distorted in this process. Below are some Conformal Projections:
- Transverse Mercator
- Lambert conformal conic
Equal Area map projection preserve area of displayed features, generally to do this distorting shape, angle, and scale. Equal-area maps are also called equivalent or authalic. The meridians and parallels may not intersect at right angles. These are some projections that preserve area:
- Albers Conic
- Peters Projection
- Gall orthographic
- Lambert azimuthal equal-area
- Lambert cylindrical equal-area