3.3.5 Coordinates, coordinate systems and projections
Locating and collecting geographic data requires an understanding of how it is managed in a Geographic Information System.
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The geographic coordinates are expressed as degrees of angle with respect to a reference meridian and parallel, namely :
- The Greenwich meridian or origin meridian: meridian serving as an international reference of longitude;
- The Equator: an imaginary line perpendicular to the earth’s axis of rotation at equal distance from the two poles (north and south).
These angle values are called longitude and latitude, and it’s important to have a good grasp of these two terms because an inversion can take you far from the desired location. It’s up to you to find your mnemonic.
The longitude (X) corresponds to the east/west position of a point on the Earth with respect to the Greenwich Meridian. It can take values between -180° (west) and +180° (east).
The latitude (Y) corresponds to the north/south position of a point on the Earth with respect to the Equator. It can take values between -90° (south) and +90° (north).
Latitude and Longitude
By convention, coordinates are expressed by stating the longitude first and then the latitude. Generally, geographic coordinates are expressed in :
- Degree Minutes, Seconds (DMS): 36° 23’ 35” N, 24° 54’ 45” E
- Decimal Degrees (DD): 36.39305555, 24.91249999
Note that it is possible to convert these coordinates from one format to another, via many online tools.
A coordinate system is a reference system that allows the exact location of a point on the Earth’s surface. Without a reference system, coordinates have no meaning, they must be associated with it 1.
The most common system is the WGS 84, associated with the GPS positioning system. Other systems exist, such as RGF93 in France, but WGS84 remains the most widely used.
Projection systems were developed in order to be able to represent information from a spherical shape on a two-dimensional map. Do the exercise by peeling an orange and trying to lay the peel flat. The principle is the same for the Earth, to represent it on a flat surface, it will be necessary to cut it out, deform it. The logic that we will follow to deform the Earth is governed by the projection system chosen.
From a round surface to a flat surface
Below are images showing examples of projections used for map representation. Depending on the type of projection used, more or less important deformations can be observed.
Let’s take the example of the Mercator projection which is of the cylindrical type and the most used. This projection has distortions at the poles. Thus, the regions located at the poles appear extremely large compared to the other regions. Africa appears to have the same size as Greenland, which is not the case in reality. The following figure illustrates the deformation of the surfaces due to this projection.
Surface deformation from “The True Size Of”
From all these examples, we can retain that:
- All projections are “false” and distort reality in different ways.
- Each projection has its advantages and disadvantages 2.
- The choice of projection has influences on the map produced and its expression.