Every GIS requires at least one coordinate system. There are two types of coordinate systems used in a GIS: geographic coordinate systems and projected coordinate systems.
Geographic coordinate systems
A geographic coordinate system (GCS) uses a three-dimensional spherical model to identify points or areas on the surface of the earth. The pairs of coordinate values that identify a feature on a map are relative to its geographic coordinate system. Each coordinate system is commonly illustrated with a network of intersecting lines of latitude (parallels) and longitude (meridians) called the graticule.
Latitude lines are horizontal and run parallel to the equator. Longitude lines are vertical lines that converge at the earth’s poles. Combining the latitude and longitude lines creates a graticule.
Latitude lines are parallel, run east and west around the earth’s surface, and measure distances north and south of the equator. Longitude lines run north and south around the earth’s surface, intersect at the poles, and measure distances east and west of the prime meridian.
Components of a geographic coordinate system
Each geographic coordinate system in a GIS is composed of four primary components.
angular unit of measurements : The graticule of latitude and longitude lines is an angular measurement system. All features on the earth’s surface are located using measurements that are relative to the center of the earth.
Prime Meridian: A prime meridian is the zero value for longitude. While the official prime meridian passes through Greenwich in southeast London, United Kingdom, the choice of prime meridian for a given coordinate system is somewhat arbitrary. It is not necessary to define a zero value for latitude because it is always the equator.
Spheroid: A spheroid is the mathematical model that estimates the size and shape of the earth. Because the earth’s surface is not perfectly symmetrical, the semimajor and semiminor axes that fit one geographical region do not necessarily fit another one, which is one reason why there are multiple spheroids.
Datum: A datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines. While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the center of the earth. The underlying datum and spheroid to which coordinates for a dataset are referenced can change the coordinate values.
Projected coordinate systems
A projected coordinate system is based on a geographic coordinate system. Projected coordinate systems are used to convert feature locations from the spherical earth to a flat map. To do so, latitude and longitude coordinates from geographic coordinate systems are projected to planar coordinates.