Coordinate Systems

Coordinate of an entity is the most important feature of data in GIS and remote sensing. This only can make possible the exact location of each feature expressed by the data. In a simple Cartesian coordinate system there are x axis and y axis intersecting at right angle where the intersecting point is origin. The location of a point in Cartesian surface is determined by its relative position from origin in x axis and y axis. In similar way, on surface of the earth the equatorial plane and central meridian plane forms x axis and y axis respectively. The location here is given in terms of longitude and latitude, which are an angular measurements made by the intersecting planes.

Longitude and Latitude

Longitude is the angle is made by intersection between a central meridian plane and any other meridian plane whereas latitude is the angle made by the intersection between equatorial plane and an imaginary line coming from the point of interest. The intersection at both case should pass from the central point of the earth. This can be further clarified from following figure.

Here in above figure O is the center of the earth and WPEQ is the equatorial plane, whereas NPSQ is the central meridian plane, and N, S, W, and E are four directions. Now as we move east from point P the angle will increase and reach 90^o at E and 180^o at Q and it also same if we move west. These angle at a particular location gives a measure of longitude. Longitude ranges from +180 (or 180° E) to -180° (or 180° W). Similarly, if we move north from point P the angle will increase and reach 90^o at N and is same while one moves south. These angles at particular location moving north or south from equator gives a measure of latitude. Latitude ranges from +90° (or 90° N) at the North Pole to -90° (or 90° S) at the South Pole.

General types of coordinate systems

Longitude and latitude makes the coordinates of earth surface and co-ordinate of a point depends on its datum and projection. A datum is frame of reference for measuring locations on the surface of earth. So far different ellipsoids have been developed closely fitting to the surface of earth either or a part of it and they are called datum. When datum is closely fitting to the entire surface of earth, they are global datum and when they are closely fitting to a particular portion of the surface of earth they are called local datum. Projection is the method of converting the coordinate system from the spherical surface of the earth to the planar surface. In projection a three dimensional surface information is transformed into a two dimensional information. Co-ordinate of the same point differs when either its datum is changed or its projection is changed or parameters of projection are changed. Thus, based on various datum and projections methods there are numbers of coordinate systems.

The following are two common types of coordinate systems used in GIS:

• Global coordinate system (GCS): In GCS an earth is considered as a spheroid or ellipsoid, where longitude and latitude are a measuring unit of the coordinate system. Some of such coordinate system are developed to fit the particular part of the earth surface, so it can provide faulty readings on area which it is not fitting properly. As an example Everest 1830 is a GCS that have good fit for countries in South and Southeast Asia, Airy 1830 is used in Great Britain, NAD83 epoch 2010.00 for North America.  These ellipsoid not necessarily perfectly fits the whole earth. There are many such local GCS that may not fits properly to neighboring nations. To avoid this there is development of CGS that fits the whole globe and this was achieved with numerous satellite observations and datasets from global Navigation Satellite System (GLNASS).  International Terrestrial Reference Frame (ITRF), Geodetic Reference System 1980 (GRS80) and World Geodetic System 1984 (WGS84) are some such examples that have global coverage. The WGS84 GCS is the most commonly used references.

• Projected coordinate System (PCS): Projection simply means estimation of something from an existing data. In case of coordinate system projection refers to translation of surface coordinates of earth into a map or screen. Thus the coordinate system used in mapping for a particular area is projected coordinate system whose reference is based on GCS. In PCS a position on earth’s surface is measured in the terms of x co-ordinate and y co-ordinate or easting and northing using linear unit of measurement from an assumed origin. Maps are flat, but the surfaces they represent are curved so there are unavoidable distortions in at least one of the properties of a map:  either shape, area, distance, or direction. There are numbers of methods for projection. There are different projections methods like conic projection, cylindrical projection, planar projection, and so on. Some examples of PCS are universal transverse Mercator (UTM), Albers Equal Area, or Robinson. UTM is most widely used cylindrical projection method.