Earth is a spherical (not exactly but let’s consider!) surface. This spherical surface is also termed as spheroid or ellipsoid. It has always been a challenging job to specify the location and shape of a particular feature on the earth surface. In order to solve this issues the geographer and cartographer came up with an idea of coordinate system–where a point is described by a single x-y coordinate pair, popularly used as x and y coordinates. This 3D representation of spheroid is called Geographic Coordinate System. The GCS system has several disadvantages–some of them are: confusing units (eg. DD, MM, SS), inconsistent comparison, inappropriate for spatial measurements, and so on.
Hence a need of more comprehensive, easy-to-use, and understandable coordinate system was felt. A method of converting the Geographic Coordinate System (3-D) to Projected Coordinate System (2-D) system was devised and was termed as Map Projection.
Based on shape of the surface onto which GCS loactions are projected, the projections are grouped into 3 major classes:
- Cylindrical Projection: uses a cylindrical surface that lies tangent to (touches) the earth at the equator along a great circle. Cylindrical Projections are of 3 types:
- Equilateral: surface tangent to the earth at equator
- Transverse: rotate the cylinder sideways making it tangent along a line of longitude
- Oblique: places tangent at an angle
- Conic Projection: uses cone on the sphere
- Tangent: cone is tangent to the globe along the line of latitude
- Secant: cone is places through the sphere touching two places
- Azimuthal Projection: a plane is placed tangent or secant to the sphere. This projections are used for displaying the earth’s poles, and for that reason they are sometime called polar projections. Other names for this method are stereographic and orthographic projection.
Projection is mainly done to avoid distortions: Area, Distance, Shape, and Direction. Usually map projections reduce or eliminate certain distortions at the expense of others. Some preserve shape, while some preserve distance. Others preserve area or direction.
Maps based on cylindrical projections typically preserve direction and shape at the expense of distance and area. Notice that the longitude lines point north-south, indicating correct direction, but Alaska and Greenland are enlarged.
Conic projections typically preserve area or distance at the expense of direction and shape. The longitude lines no longer point north-south, but the areas are preserved better.
Maps based on azimuthal projections preserve area or distances.
It really important to be careful while choosing a projection system. Ask these 3 questions:
- Which map properties you want to preserve?
- What is the size and shape of the study area?
- What is used by convention? … or regulation?
- Map projection and coordinate system
- Spatial unit
- Standard parallels
- False easting and northing