In this article we will learn various concepts related to coordinate reference system in GIS. Coordinate reference system is very important in preparing a map. The location of a place or object is represented using the coordinates in map. Coordinate system used for preparing the map plays a key role in identifying the coordinates for a particular location.
Here in this article, we will discuss about the following important concepts.
- what is a coordinate system
- why is it required
- what are different types of coordinate systems used in GIS
- What are characteristics of different types of coordinate systems used in GIS
A coordinate reference system can be defined as a reference frame/space/system where it is possible to denote or locate the objects using their positions.
Coordinate systems normally use reference axis for measuring the distance to objects in order to identify the position.
Some of the examples of coordinate systems include Cartesian system, Geographic coordinate system etc.
Types of Coordinate Reference Systems Used in GIS?
There exist many types of coordinate systems. However, in the context of Geographic Information System we can classify all coordinate reference systems into 2 types.
- Geographic Coordinate Reference System
- Projected Coordinate Reference System
Geographic Coordinate Reference System:
This coordinate system makes use of 3-Dimensional model of earth surface for measuring objects location. This is also called as spherical coordinate system. It is prepared based on a spheroid model, where a spheroid is used to represent the shape of earth. It makes use of latitude, longitude which are angular measurements, for representing location.
The reference lines i.e., prime meridian and equator consider for measuring other locations are curved lines.
Projected Coordinate Reference System:
Projected coordinate system makes use of 2-Dimensional or flat plane surface representing earth surface objects. It is normally used in preparing maps and making linear measurements. Hence, these are also called as map projection systems.
A projected coordinate reference system is always depends on the Geographic coordinate system. It uses the spheroid used in geographic system for converting the angular coordinates into plane coordinates using mathematical transformations. It uses linear units for representing locations.
To Read more about geographic and projected systems: Visit
Important Terms Related to Coordinate Reference Systems:
Following are some of the important terms related to coordinate system. The details given below helps in understanding coordinate reference system in a best way.
A geoid can be defined as an equi-potential surface of the earths gravitational field. It is a surface formed by Mean Sea Level (MSL) at all locations on earth surface. Since, earth gravity is not uniform at all locations, MSL changes from one location to another. As a result Geoid represents earth as an irregular and non uniform surface.
Spheroid is simplification of Geoid. It is the shape formed by rotating an ellipse around one of its axis. The major axis of axis is considered as the radius towards the equator, and minor axis is radius towards pole for forming the spheroid.
Since, earth is not uniform at equator, there exist multiple spheroids representing the earth as a 3-Dimensional model. These spheroids vary in major and minor axis lengths.Spheroid is also considered as a mathematical surface often as it is formed from the ellipse.
Examples: WGS 1984, Clarke 1866 etc are different spheroids.
All spheroids are not suitable for all locations. They approximate or represent close to Geoid at only few places. Based on the study area spheroid chosen accordingly. But still, these spheroids dont reflect the reality so well at many places. The reason is spheroids are smooth ellipsoidal surfaces. To over come this, datums are built above the spheroids for representing local variations. Hence, there exist several types of datums based on a single spheroid. The coordinates of a same location can change if we change the datum. Hence, it is important to select coordinate reference system, with relevant datum for all data sets of study area when we are working on a project.
Latitude is the angle made by the horizontal line drawn parallel to equator at the center of earth with respect to equator. Latitude is measured from equator towards north and south poles. It is measured in degrees and the value ranges from 0 to 90.
Longitude is the angle measured from the prime meridian to the vertical line drawn from one pole to another pole of earth at the earth’s center. It is measured from east to west from prime meridian. It is also measured in degrees and the value ranges from 0 to 180.
6. Prime Meridian:
It is an imaginary line passing from one pole to another pole of earth. It divides earth into 2 equal halves. It is considered as 0 degree longitude. As it passes over Greenwich, London it is also called as Greenwich meridian often.
To know more about prime meridian one can refer to this link.
Similar to prime meridian, it also divides earth into 2 equal halves i.e., northern and southern hemispheres. It is 0 degree latitude. The diameter of earth is considered to highest about the equator.
How Coordinate System Defines Objects Position?
Different coordinate systems work differently for locating the objects.
Cartesian coordinate system is widely used in the field of mathematics. It uses a horizontal and vertical axes with measurement scales. These are also called as X and Y axis, and are used for measuring objects location from the origin. Origin is the intersection point of X and Y axis.
Working of Geographic Coordinate Reference System:
Geographic coordinate system is used for locating the objects on earth surface. It uses latitudes and longitudes values at a given point for representing the location. The location details are represented in angular units i.e., degrees, minutes and seconds. The details of latitude and longitude are observed based on the spheroid and local datum used in the selected coordinate system.
Working of Projected Coordinate Reference System:
Projected coordinate reference system makes use of 2-Dimensional plane surface for representing the objects location. A projected system always relies on geographic system and the values representing the location are derived using mathematical calculations.
Mathematical calculations convert or transform the geographic coordinates into projected coordinates. In this process, depending on the mathematical functions used for transformation, the output may vary accordingly. Some projection systems preserve the shape of objects (i.e., boundary), while others may preserve length, area etc.
The projection system to be selected is always based on the coordinate system under use for the data and the features to be preserved or displayed.
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