Projections

The earth is a three-dimensional object whereas most maps are two-dimensional. As such, a mathematic approach is required to present the curved surface of the Earth on a flat surface such as a paper map, a computer screen or a mobile phone screen – this process is known as a map projection.

It is however impossible to represent a curved surface on a flat plane without distorting one or more characteristics such as distances, directions, shape, or area. Try it: peel an orange and try to squash it flat without tearing or creasing it …. spoiler - you can’t.

How do they work?

So how do they work? Imagine a light source was placed inside the centre of the Earth and then a piece of paper held up to the Earth or wrapped around it (without making a fold or crumpling it). If the landmasses were opaque, they would form shadows on the paper when the light is turned on – this is your projection. Unfortunately though, the shadows will be distorted, either in size or shape. Here-in lies the problem – projections result in distortion.

Projections can be split into 3 main types; Cylindrical, Conic and Planar (Azimuthal), which define the type of developable surface used – a cylinder, a cone or a flat plane respectively. Each of these surfaces can be moved, rotated and positioned so that they just touch the Earth in one place, or along a single line (tangential), or it can intersect the Earth (secant). Along the lines where the paper intersects or touches the Earth’s surface, scale is true i.e. a distance measured on the map is true to its distance on the ground.

A classic cylindrical projection is best suited for use in the equatorial regions where the country has a broadly east-west extent, or in its transverse form (rotated 90⁰), a country with a broadly north-south extent. Conic projections are best suited for countries in the mid latitudes with a greater East-West extent than North-South extent. Finally, azimuthal projections are commonly used for countries which are broadly circular in their extent and are often used for mapping in the high latitudes. There are equidistant (preserves distances), conformal (preserves angles and shapes) and equal area versions of cylindrical, conic and azimuthal projections – these are discussed below.

Different projection methods preserved different characteristics. This means that the best projection to use will depend on the purpose of the map (plus the shape of the area to be mapped and its location on the globe). The choice of map projection becomes very important for maps greater than the size of a small country due to the distortions created across large areas. A poor choice of projection can result in the misrepresentation of what you are trying to show. For maps of small areas, the distortion is minimal and therefore the choice of map projection matters less.

As noted above, different map projections preserve different characteristics. Those which keep areas correct are called β€˜equal area’ projections, those which preserve distance are known as β€˜equidistant’ projections, and those which preserve angles are known as β€˜conformal’ projections. In addition, there are a number of projections which are a compromise between these. Think carefully about what your map is showing and what needs to be preserved before choosing your projection.

Projection Types

Conformal projections preserve angles. This means that the shape of features appears correct and means that a straight line drawn on a map between two locations represents a line of constant bearing in reality i.e. measure the angle on the map, set your compass and off you go. Conformal projections do however distort areas and distances. The projection often used for maps of the world (rightly or wrongly – you decide) is the Mercator projection, which is a conformal, cylindrical projection, created by Gerhard Mercator in 1569. This projection was designed to represent sailing courses of constant bearing (rhumb lines) as straight lines. It is still used to this day for maritime and air charts. As you can see on the map below, distances and areas are distorted, with the distortion becoming more pronounced towards the poles. On a Mercator projection, Greenland and Africa appear to be a similar size, however Africa is approximately 14 times larger than Greenland. Not a great projection to use if you want to represent the relative areas of countries correctly.

It is important to note that distances vary across this projection so, if using this sort of projection at a continental or global scale, you should not include a standard scale bar as it will only be valid for distances measured along the equator.

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