In this article we will learn about, what is a map projection, how map projections are classified and what are different types of projection systems used in GIS for preparing maps etc. Following are the key questions one can address easily after reading this article.
What is a Map projection
Classification projection systems/ Projected Coordinate systems
How to select a Projected coordinate reference system for a given area or project
Earth is a 3-Dimensional body. Maps are 2-Dimensional representation of a part or whole of the earth surface. Map projection is the process, where a function (mathematical) is used for converting coordinates from 3-Dimensional earth surface onto a 2-Dimensional plane surface.
Alternatively, one can simple define map projection as a mathematical function which is used for preparing a plane map from 3-Dimensional earth model.
The properties of projected coordinate reference system are based on the projection method along with other things such as reference spheroid, datum chosen etc. Different types of projected coordinate systems can be prepared by using different types of projection systems.
Following are different types of Projection Systems used in preparing the maps.
Classifying Different Types of Projection Systems:
The projection of map can be done in different ways. Based on how we project and what we criteria are used for projection, the properties of a map change accordingly. Some of the important factors used for classification of projected coordinate systems are shown below.
1. Method of construction
2. Based on shape and arrangement of development surface used for Projection
3. Based on details preserved in the projected map or projected map properties
4. Based on location of light source used for projection.
Types Map Projection Based on Method of Construction:
Based on method of construction projections can be classified into perspective projection and orthogonal projection system.
All projected rays coming from different objects from the earth surface pass through a single point or have a common origin (or intersection) point before hitting the plane of projection. Such type of projection methods are called perspective projection.
For example, if we place a light inside the globe and map the shadow of objects forming on a plane it can be called as a perspective projection.
In case of orthogonal projection all projected rays are parallel and are drawn from the object location onto the plane of projection. Traditional maps prepared using surveying techniques are prepared using orthogonal projection.
Other than, these 2 types if a projection system is simply based on a mathematical function it is categorizes as non-perspective projection system.
Based on Shape and Arrangement of Development Surface Used for Projection:
Map projection process involves using a plane surface. Objects from the earth surface are projected onto that plane surface. The 2-Dimensional plane surface used for projection is spread without any shearing or tearing to develop a map. Hence, the plane surface used here is called as development surface. Depending on the shape of development surface and orientation or arrangement of it with respect to the spheroid/globe the projections are classified into several types. Some of the important types of such map projections and their properties are listed below.
The development surface is similar to a cylinder which is wrapped around the sphere/globe.
The objects and grid system present on the globe or spheroid are projected on to the cylinder.
After the projection, depending on the cylindrical surface arrangement, it is cut along a latitude or longitude to form the complete map.
Depending on the orientation or arrangement of cylinder they are further classified into 3 types.
Normal Cylindrical Projection:
Here, cylinder is placed vertically i.e., equator acts as line of tangent with the cylinder. The developed map contains latitudes and longitudes as straight lines.
Transverse Cylindrical Projection:
In this case, meridian acts as a line of tangent with the cylinder. Here in the developed map, latitudes and longitudes or not straight lines. They appear as curved lines.
Oblique Cylindrical Projection:
Here, cylinder is placed at certain inclination with respect to the globe. Hence, neither latitude and longitude acts as a line of tangent.
The projection involves a projection plane which is in the shape of cone.
The cone is placed over the globe or spheroid for projection to take place.
Once the projection of objects and graticules is over, the plane surface is developed by cutting along a longitude to form a complete map.
In this type of projection it is possible to cover only one half of the earth surface at a single time. That means either southern or northern hemisphere are shown in a single map.
This map represents all latitudes as arcs and longitudes as straight lines converging towards either north or south pole.
Depending on the arrangement of cone, conical projections are also classified into 2 types.
Normal Conical Projection:
In this case, there exist only one line of tangent between the sphere/globe and cone.
Secant Conical Projection:
In this projection, cone is arranged in a way it cuts through the earth/globe. Hence, there exists minimum of 2 line of tangents in secant map projection system.
Azimuthal Map Projection:
This map projection system is also called as zenithal map projection.
In this method a simple plane of paper/surface is kept tangential with the globe or earth surface.
The graticules and objects on the globe are then simply projected onto the plane.
Depending on the orientation of the plane, different types of azimuth map projections are as follows.
Plane is kept tangential with a point on the equator.
In this projection, Plane is tangential to one of the poles on the globe surface. In the map, all latitudes on the globe appear as concentric circles, and longitudes as straight lines coming out from the centre.
Here, plane is kept parallel with any point other than lying on equator and poles.
Based on details preserved in the projected map or projected map properties:
Based on this criteria, different types of projection systems classified are as follows.
Equal Area Projections:
These projection systems preserve the property of area. That is for a given map prepared using equal area projection system, the area measurements will be precise.
True Shape Projections:
As the name says, this type of projected coordinate systems preserve the shape of a given area in the map.
Different types of projection system classified under equidistant preserve the scale and distance properties in the prepared map. However, it is not possible for any projection system to store the correct scale and distance properties all over the map. Hence, these equidistant projections give reliable measurements only along one of the meridian or parallel.
Based on Location of Light Source Used for Projection:
Following are different types of map projections based on location of light source used for projecting the rays from the globe.
In this projection system, light source is placed exactly at the center of the globe and all rays projected from the center through objects of the globe on to the plane.
Here, light source is placed on a point above the surface of globe. The point lies exactly opposite to the point where the projection plane is kept on the globe. In other words, they are placed on 2 opposite ends of diameter of the globe while projecting the map.
In this projection, Light source is assumed to be placed at an infinite distance from the globe. Hence, all projected rays appear as parallel and straight lines.
The above methods are various types of projection systems used in map preparation. Various types of projected coordinate systems are developed using the above methods and are used in producing different types of maps. A specific map projection and coordinate system is chosen based on the location of study area and application of map.