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A map projection is any method used to portray all parts of a sphere or three-dimensional body(the round earth) on a plane or flat surface. A coordinate reference system(CRS) helps to define using coordinates how a  two dimensional projected map in a GIS is related to real places on the earth’s surface.Every flat map misrepresents the surface of the Earth in some way.Therefore the decision as to which map projection and coordinate reference system to use, depends on the regional extent of the area you want to work in, on the analysis you want to do and to a lesser extent on the availability of data. A basic knowledge of the properties of commonly used projections helps in selecting a map that comes closest to fulfilling a specific need.

Map Projections

Map projections are an important aspect of all mapping as all maps require the transformation of the surface of the spherical earth to a flat surface.       

The following are the importance of Projections to mapping and GIS:

• They are used to focus reader's attention
• Amplify and provide selective detail for map's message
• Most importantly,poorly chosen map projections can distort your thematic data

Families of Map Projections

The process of creating map projections can be visualized by positioning a light source inside a transparent globe on which opaque earth features are placed. Then projecting the feature outlines onto a two-dimensional flat piece of paper.The families of map projections depend on the kind of flattenable surface you are projecting the sphere onto.

Illustration 1: The three families of map projections. They can be represented
by a)planar projections , b) conical projections or c) cylindrical projections

 

Therefore there are three families of map projections. The azimuthal or planar,conical and cylindrical projections corresponding to projections onto a plane, a cone and a cylinder respectively.

Accuracy Of Map Projections

Map projection process always results in some kind of distortions. There are distortions of angular conformity, distance and area.

A map projection may combine several of these characteristics, or may be a compromise that distorts all the properties of area, distance and angular conformity, within some acceptable limit. Examples of compromise projections being the Winkel Tripel projection and the Robinson projection often used for world maps.

Map Projections with Angular Conformity 

Correct angular conformity can be maintained on a map. A map projection that retains this property of angular conformity is called a conformal or orthomorphic projection.They are usually used in situations where it is prudent to preserve angular conformity.Examples are the Lambert
Conformal Conic projection and the Mercator projection.It is useful in navigational or meteorological tasks.

Equal-Area Map Projections

A map projection in which areas on a sphere, and the areas of any features contained on it, are mapped to the plane in such a way that two are related by a constant scaling factor.Equal-area projections are also called equivalent, homolographic, homalographic or equiareal

Equal area projections result in distortions of angular conformity when dealing with large areas. Small areas will be far less prone to having their angles distorted when you use an equal area projection. Examples are  Alber's
equal area, Lambert's equal area and Mollweide Equal Area Cylindrical
projections.They are usually used in educational maps.

Equal Distance Map Projections

A map projection in which the distances between one or two points and every other point on the map differ from the corresponding distances on the sphere by only a constant scaling factor.A map is equidistant when it correctly represents distances from the center of the projection to any other place on the map. Equidistant projections maintain accurate distances from the center of the projection or along given lines.Usually used for radio and seismic mapping and navigation.Examples are Plate Carree Equidistant Cylindrical and Equirectangular projection.

 

I think this post is getting boring. I’ll continue later…………………….

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