By Kory Korir,
Map projections always reminds me of my former senior lecturer at the University of Nairobi, Mrs. Tabitha Njoroge, commonly known as TMN. Her strictness made us grasp what map projection was all about. For a professional in the geospatial profession, Map Projection is the foundation of every little task one is bound to undertake. So even if you are an enthusiast with no background in spatial sciences, the first lesson after knowing the definition of GIS is to learn about map projections.
What is map projection?
Map projections can be basically defined as mathematical transformation of 3D positions or locations to a 2D space or map.
The earth is spherical, and one mathematician by the name Carl Friedrich Gauss once said it would be close to impossible to project the earth on a planar surface while conserving all the metric properties of a map which include
When one is choosing a model for the shape of the earth, one has to decide whether to approximate the earth to an ellipsoid or to a sphere. The reason for the aforementioned is that the earth is not entirely a sphere but rather an ellipsoid with a bulge at the poles.
For small scale maps where accuracy isn’t the focus, spherical models are more desirable. For large scale maps, ellipsoidal models are more accurate since they fit more to a specific part of the earth. Such models are considerable for topographic maps.
Since the earth cannot be projected perfectly, there are compromises that have to be taken. These are what we term as distortions. It always depends on what one wants to use the map for. With the different objectives of whichever map one wants, the following are different types of projections;
- Projections that preserve areas.
This is when all areas on the map are proportional to the true areas on the surface of the earth. When one is working with quantitative data, the most desired maps are the ones that are able to preserve the area. This is due to the fact that density map work well with equal area projections.
- Projections that preserve shape.
These are also called conformal maps and are applicable for general mapping. The reason behind it is because such maps tend to preserve the form of places and familiarity.
- Projections that preserve directions
This means that direction from a point in map are the same in all directions. Maps that preserve directions are most suitable for navigation purposes.
- Projections that preserve distances.
This means that the distances are portrayed correctly in all directions from a point in the map. These maps are most suitable when one wants to depict the true distance between places.
Types of map projections
Cylindrical map projections.
This is a projection where the earth’s spherical surface is rolled out on a cylindrical surface. The poles tend to be overly distorted while along and around the equator most of the metric properties are conserved. It is a kind of projection that has its meridians and horizontal parallels crossing each other at right angles. Some of the examples of cylindrical map projections include; Cassini, Gauss Kruger and Mercator projection.
Conical map projections
The genesis of the name of the type of projection was from the way the projection is carried out. The spherical earth model is rolled out into a conical surface.
The meridians of the projection are of equal distance and is not suitable for visualization of the entire earth.
Examples of such projections include Albers Conic and the Lambert conformal conic.
Azimuthal map projections
This is a type of map projection in which the earth is projected onto a flat plane. In this projection, the plane is always placed on top of the north or South Pole then the model unwrapped unto the model.
The longitudes and the latitudes intersect at right angles and can conserve different properties depending on the perspective of the projection.
Stereographic projection are conformal
Orthographic and gnomonic are neither conformal nor orthographic.
Non perspective projections like Postel Azimuthal Equidistant Project preserve distance just as the name suggests while Lambert azimuthal Equal-area projection preserve area.
Geokov. Map Projections: Types and Distortion Patterns. 2014. Web access 22 April 2017. http://geokov.com/education/map-projection.aspx
Geolounge. Map Projections: Types of map projections. 2015. Web access 22 April 2017.