Map Scales

Map scale is the relationship between a unit of length on a map and the corresponding length over the ground_*. Map users will come across three different ways of describing scale.

A typical verbal scale might be One inch to one mile. Such a description is clear and concise, easily understood but perhaps difficult to apply accurately.

Many maps carry a graphic scale such as this bar scale.

Graphic scales provide a direct measure of the distance over the ground represented by the corresponding distance on the map. Graphic scales are simple to use and can make estimating distances a very straightforward task. Graphic scales also have a unique benefit they remain accurate even if the map image is enlarged or reduced by (say) a photocopy machine.

The third type of scale is the Representative Fraction (RF). This very flexible way of describing scale is probably the least understood.

A representative fraction (RF) shows the relationship between one of any unit on the map and the same units on the ground. RFs may be shown as an actual fraction (e.g. 1/25,000). They are more usually written like a mathematical proportion with a colon (as in 1:25,000). In this example, one unit of any length (one mm, one cm, one inch, etc.) on the map represents 25,000 of the same units on the ground. The RF is versatile because it is not tied to any specific unit of measure. You may work in any unit you choose - metric, US or any other.

What do we mean by Small Scale and Large Scale?

Cartographers use these terms differently to normal English usage. Cartographers are referring to the relative size of the representative fraction. A large scale map is where the RF is relatively large. An RF of 1:25,000 (1/25,000) is larger than an RF of 1:1,000,000 (1/1,000,000). A large scale map uses more map area to describe a given area on the ground than a small scale map. Many people are initially confused by this terminology as the small scale map shown a much greater area of the ground than a large scale map of the same size.

Map scales help us measure distance and area of and between the real world objects represented on the map.

* True distance along the ground may be influenced by the topology of the ground in question. Map scale is the relative difference between a unit length on the map and the corresponding distance as the crow flies (even if the crow has to fly very high and level).