What is scale?
Scale is the ratio between distance on a map and the actual distance on the ground. It lets us show a large area on a small sheet of paper. There are three kinds of scale: a statement scale (e.g. 1 cm represents 2 km), a representative fraction or RF (e.g. 1:100 000), and a linear scale (a marked bar).
An RF such as 1:100 000 means that 1 unit on the map represents 100 000 of the same units on the ground. The number after the ratio sign is called the scale denominator.
Key idea
Actual distance = map distance × scale denominator. Convert the answer to km by dividing by 100 000 (1 km = 100 000 cm).
Steps to calculate distance
Measure the distance on the map with a ruler (for a winding road, lay a piece of string along it and then straighten it). Multiply that measurement by the scale denominator to get the actual distance in centimetres, then convert to kilometres.
Example
Two towns are 4 cm apart on a map with a scale of 1:100 000. Actual distance = 4 × 100 000 = 400 000 cm = 4 km.
Remember
A linear scale is the most useful because it stays accurate even when the map is enlarged or reduced during photocopying.
Large scale and small scale
A large-scale map such as 1:10 000 has a small denominator, covers a small area but shows a lot of detail such as roads and buildings. A small-scale map such as 1:1 000 000 has a large denominator, covers a wide area such as a whole state but shows less detail. Being able to read a scale is important for planning journeys, measuring the distance between towns, and understanding the real size of an area on a map.