1.8. Geographic and Projected Coordinate Systems

 

Locations on the Earth's surface are measured and represented in terms of coordinates. A coordinate is a set of two or more numbers that specifies the position of a point, line, or other geometric figure in relation to some reference system. The purpose of coordinate systems is to provide a frame of reference to a particular place or area on the earth's surface. There are two types of coordinate systems—geographic and projected.

Geographic Coordinate System or Spherical Coordinates System

            A geographic coordinate system uses a three-dimensional spherical surface to define locations on the earth. It provides a mechanism for describing the geographic location of an object on the earth's surface using angular measures of latitude and longitude. Longitude and latitude are angles measured from the earth's centre to a point on the earth's surface. The angles are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). A geographic coordinate system is also known as spherical coordinates system. It includes an angular unit of measure, a prime meridian, and a datum.

            In the geographic or spherical coordinate system, horizontal lines, or east–west lines, are lines of parallels. Vertical lines, or north–south lines, are lines of meridians. These lines encompass the globe and form a gridded network called a graticule. The line of latitude midway between the poles is called the equator. It defines the line of zero latitude. The line of zero longitude is called the prime meridian. The prime meridian is the longitude that passes through Greenwich, England.  The origin of the graticule (0,0) is defined by the intersection of equator and prime meridian. The sphere is then divided into four geographical quadrants that are based on compass bearings from the origin. North and south are above and below the equator, and west and east are to the left and right of the prime meridian.

        For many applications that do not require the highest accuracy, the sphere is an adequate representation of the Earth. Performing GIS analysis and mapping applications requires a more accurate coordinate framework, which is provided by projected coordinate systems. 

Projected coordinate system

            A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. In a projected coordinate system, locations are identified by x, y coordinates on a grid, with the origin at the centre of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position. A projected coordinate system consists of a geographic coordinate system, a map projection, a set of projection parameters that customize the map projection for a particular location, and a linear unit of measure. A projected coordinate system is always based on a geographic coordinate system.


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