Points of View: The functions of maps

Background

Map Scales

Atlas and Ordnance Survey maps obviously have to be drawn to scale if they are to get a large area of the Earth's surface onto a manageable piece of paper. On large-scale maps, small areas can be shown in some detail: for example, individual houses can be drawn in. To cover a greater area, such as a continent, the scale has to be reduced, and small-scale maps are used. On a world map a common scale is 1 to 70 million. That means that one centimetre on the map represents 70 million centimetres, or 700 kilometres, on the Earth's surface. United Kingdom maps in atlases use a larger scale, for instance 1 to 3 million, so that one centimetre is equivalent to 3 million centimetres or 30 kilometres. For a city and its surrounding area (such as Exeter), the most widely used map would be the Ordnance Survey's 1:50,000-scale map, followed by the 1:25,000-scale map (which is the map Julia uses in the balloon), and finally, the 1:10,000-scale map - one centimetre on this map represents just 0.1 kilometres or 100 metres on the ground. The scale of this map is large enough to include street names. Atlas Maps - Projections

There is no 'correct' flat map of the world: all maps have to compromise somehow because the Earth is a sphere and simply cannot be flattened out. Imagine cutting open a football and trying to flatten it out. This is what a map has to achieve: it has to attempt the impossible, to represent a three-dimensional world in two dimensions.

To understand one of the simplest ways of making a map of the world, imagine a powerful light at the centre of the Earth. This light projects the shape of the land masses onto a big screen that circles the Earth. The screen is then cut down through the middle of the Pacific Ocean and unwrapped. This is called the Mercator projection, and it is particularly useful for navigating, because regardless of where you are in the world, north is always at the top of the map, and south is always at the bottom.

Another way of drawing a map of the world is again to imagine a powerful light in the centre of the Earth. This time, the light projects the land masses on a big circular screen that actually touches the surface of the planet at a particular point. For example, if it touches at the North Pole, then this becomes the centre of the map. This is an illustration of the azimuthal projection, and is good for plotting air routes, because a straight line drawn from the centre to any point on the map shows the shortest route between these two points.

Traditional methods of map drawing show countries and continents in a way that often bears little relation to their real relative size. For example, in comparison with the rest of the world, the Mercator projection makes South America and Africa look smaller than they actually are. However, one technique mathematically adjusts the geometry of the Earth's features, so that the countries, continents amd land masses are shown in proportion to their true relative sizes. This is called the Peters projection, and on it South America and Africa look much larger than they do in the Mercator projection. Its main drawback is that it makes the land masses in the tropics look more stretched than they are in reality.

So, choice of map projection all depends on how you want to use the map.