Before we jump into the differences between Geographic and Projected Coordinate Systems, let’s learn about coordinate systems.
In general term, A coordinate system is a method for identifying the location of a point on the earth. The coordinate systems use two numbers— (x and y) or (latitude and longitude) commonly known as a coordinate. These coordinates give the distance between the point and some fixed reference point, called the origin.
Now, depending upon the representation of the surface, the geographic and projected coordinate system serve in two different ways. If you consider the earth as a 3-Dimensional plane, then geographic coordinate system comes into the play whereas if you consider the earth as 2-Dimensional surface, then projected coordinate system comes into play.
Geographic Coordinate System
Consider earth as a 3-Dimensional Spherical Surface (ellipsoid), any location on the earth surface is defined by an angular unit of measure like degrees, prime meridian, and a datum—and this coordinate system is Geographic Coordinate System (GCS). These points are referenced by their longitude (measured east-west) and latitudes (measured north-south). A common choice of coordinates is latitude and longitude. Learn more about Geographic Coordinate System here.
An example is: WGS84 coordinate system with unique EPSG code 4326
Projected Coordinate System
Now convert/project this 3-D ellipsoid into a plane surface, the coordinate system used to represent the earth into 2-D (map) is Projected Coordinate System.
In a projected coordinate system (PCS) you project the geographic coordinate that you have measured, to, for example, a cylinder which you roll out easily on two-dimensional surface (the map). There exist many different projections and we’ll not go in further detail about that here. Learn more about Projected Coordinate System here.
|Geographic Coordinate System||Projected Coordinate System|
|– Deals with earth in 3-D||– Deals with earth in 2-D|
|– Large (area) coverage||– Small (area) Coverage|
|– Easy to identify location in a globe||– Easier to calculate spatial locations and relationships|
|– Less distorted but more difficult to work with||– Easier to work with but quantities like distances and angles are often distorted due to map projection.|
|– Although longitude and latitude can locate exact positions on the surface of the globe, they are not uniform units of measure.||– Projected coordinate system has constant lengths, angles, and areas across the two dimensions|