The Earth’s surface is curved but as it must be shown on a flat sheet there is need for a projection. A projection is a method by which the curved surface of the earth is represented on a flat surface. It involves the use of mathematical transformations between the location of places on the earth and their projected locations on the plane (flat surface e.g. paper). A map projection is a mathematically described technique of how to represent the Earth’s curved surface on a flat map. In other words, it is the mathematical process of transforming the spherical earth into a flat earth (Figure 4.0). It is also the representation of parts of the surface of the Earth on a flat paper map or on a computer screen. Every map we see on paper has been projected; it is the projected version of the spherical earth
Classification of Map Projections
There are several types of map projections, as well as several methods used to achieve these projections. Basically, however, there are three classes of map projections; they are cylindrical, conical and azimuthal (Figure 4.1). The Earth’s reference surface projected on a map wrapped around the globe as a cylinder produces a cylindrical map projection. Projected on a map formed into a cone gives a conical map projection. When projected directly onto the mapping plane it produces an Azimuthal (or Zenithal or planar) map projection. Figure 4.1 below shows the surfaces involved in these three classes of projections.
A map projection without distortions would correctly represent shapes, angles, areas, distances and directions, everywhere on the map. Unfortunately, any map projection is associated with scale distortions.
There is simply no way to flatten out a piece of ellipsoidal or spherical surface without stretching some parts of the surface more than others. The amount and which kind of distortions a map will have depends largely – next to size of the area being mapped – on the type of the map projection that has been selected.
Projections can be identified by the distortions which they avoid – in general a projection can belong to only one of these classes:
- Equal area or Equivalent projections preserve the area of features by assigning them an area on the map which is proportional to their area on the earth – these are useful for applications which require measuring area.
- Conformal projections preserve the shape of small features, and show directions (bearings) correctly – they are useful for navigation.
- Equidistant projections preserve distances to places from one or two points.
There are several types of map projections, as well as several methods used to achieve these projections. Each projection is most accurate at its center point and becomes more distorted the further away from the center that it gets. The projections are generally named after either the person who first used it, the method used to produce it, or a combination of the two. Some common types of map projections include:
- Transverse Mercator
- Lambert Azimuthal Equal Area
- Miller Cylindrical
- Sinusoidal Equal Area
- aAlbers Equal Area Conic.