THE QUESTION OF SCALE

As summer approaches, a map can often be the only navigational tool you bother using – but which mapping scale is best? In PART 10 of our navigation series, Lyle Brotherton explains how to interpret them.

In Great Britain, we are fortunate to have the best maps for walking in the world, and all due to the creation of our national mapping agency, Ordnance Survey (OS), on 21 June 1791. Originally all Ordnance Survey mapping used Imperial measurements: miles, yards, feet and inches. Today they all use the much simpler metric system (kilometres, metres and centimetres) and we also have access to other excellent maps available from Harvey Maps.

The tricky part: map scales!

The scale of a map shows how much you would have to enlarge your map to get the actual size of the piece of land you are looking at. It is expressed as a ratio and is always printed on the map.

If your map has a scale of 1:25,000, this means that every 1 unit on the map represents 25,000 of those same units of measurement on the ground. These maps use metric measurements and we navigate using metres and kilometres, so using the aforementioned ratio 1cm on the map equals 25,000cm, or 250m, on the land. This means that every 4cm on a 1:25,000 map = 1km in real life. To make life easy the blue grid lines (grey on Harvey Maps) are exactly 4cm apart, so every square is 1km by 1km. The first number (map distance) is always 1. The second number (ground distance) is different for each scale – the larger the second number is, the smaller the scale of the map. This sounds confusing, but in fact it is easy to understand. Large scale maps show small features on the land, such as an individual house. Small scale maps show large features, such as an entire city. So, a 1:50,000 map has large area (and therefore less detail) on one sheet, whereas a 1:12,500 map has a small area (and therefore more detail) on the same space.