Viametris backpacks and road scanners capture GNSS data, allowing users to easily geo-reference any scan. Having georeferenced data opens up a world of possibilities for what can be done with it, but how to know which projection is the right one for the data and applications?
The map of the world that is shown in school to learn Geography, as an accurate representation of the Earth, is only one way of representing the planet (usually in the Mercator projection).
The Earth is not a flat surface. Its shape can be defined as an ellipsoid that approximates a sphere. This is why the most accurate way of representing it is by using a globe. However, making and carrying a globe is not practical and poses limitations. For this reason, people invented maps and geographic projections.
What are geographic projections?
Projections are a way of representing the spherical Earth as a flat surface. This might sound like a simple task but it poses many challenges and there are many approaches to this problem. Let’s imagine a fully inflated beach ball representing the Earth. If the ball is cut in an effort to have its whole surface lying flat on a table, one ends up with some parts of it full of wrinkles and other parts stretched. This is the reason why it is impossible to represent the Earth’s surface on a flat map.
This is the problem projections are trying to solve. Let’s imagine a translucent globe with a light source inside of it. The whole surface of the earth would be projected outside of the globe. By placing a structure (cone, cylinder, plane) around or tangent to the globe, it is possible to capture this projected image into a flat surface, as can be seen in the image below.
Types of geographic projection
There are different ways of classifying map projections. In this article we will talk about two of them:
Classification based on the tangent surface
As mentioned in the previous section, a projection can be obtained by placing a surface tangent to the earth’s ellipsoid. Based on the way this is done, there are three main categories:
These projections are also known as Azimuthal or Zenithal projections and they project map data into a flat surface that is tangent to the globe at only one point. In most cases, it is used to map polar regions, so this contact point corresponds to the North or South pole.
Conic projections map data into a cone placed over a globe. The cone is tangent to the globe in a single latitude line, called the standard parallel. Distortions increase north and south of this line, so they are usually applied to only portions of a hemisphere.
Cylindrical projections place a cylinder around the globe to project the data onto it. The cylinder can touch the globe along a parallel (latitude) or a meridian (longitude).
Classification based on preservation of a metric property
All projections introduce some type of distortions because they are trying to represent the globe in two dimensions. There are four metric properties that can be affected by projections: area, shape, direction, and distance. These properties are handled differently by each projection and at least one of them is preserved but not all can be preserved at the same time. Based on this, four main categories can be defined:
Equal-area projections present any area on the map in true proportions to its real area on Earth. Generally, this leads to distortions in shapes.
Conformal projections preserve angles and shapes. With these projections, all angles measured from a point are correct. This comes at the cost of distortions in area measurements.
In equidistant projections, the length of a particular line in the map must correspond exactly to the length of the lines on the curved surface (the ellipsoid).
Azimuthal projections ensure that all directions from a single point to any other point on the map are true. This point is where the plane is tangent to the ellipsoid.