Every spatial object in this earth need to be geo-referenced. It is the process of giving or assigning real world co-ordinate system to the object.
Geospatial data should be geographically referenced (called georeferenced or geocoded) in a common co-ordinate system.
A coordinate system provides a frame of reference for measuring locations on the
surface of the earth. It defines the origin and orientation of latitude and longitude
Very often a country has its own coordinate system which serves the entire country but is not valid for other (neighbouring) countries.
Three important globally used coordinate systems are:
- Geographic coordinate (latitude, longitude)(degree, minute, second: DMS)
- UTM (Universal Transverse Mercator coordinates (X, Y) (meters) and
- Lambert Conformal Conic (meters)
Geographic coordinate system
It is a spherical coordinate system composed of parallels of latitude and meridians of longitude. Both divide the circumference of the earth into 360 degrees which are further subdivided into minutes and seconds.
In the spherical system, horizontal lines, or east-west lines, are lines of equal latitude, or parallels. Vertical lines, or north-south lines, are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule. The origin of the graticule (0,0) is defined by the earth’s centre where the equator and prime meridian
Marcator Co-ordinate System(UTM)
Mercator is an international rectangular coordinate system which extends around the world from 84 degrees north to 80 degrees south. The world is divided into 60 zones, each covering six 6 degrees longitude. Each zone extends three (3) degrees eastwards and three degrees westwards from its central meridian. Zones are numbered west to east from the 180 degree meridian. Because of the small area covered by each zone, a high degree of accuracy is possible. For example, Nepal falls under zones 44 and 45.