Three ways to show scale

A scale can be shown in three ways, each with strengths and limits:

1. Statement of scale: written in words, such as '1 cm represents 10 km' or '1 inch represents 10 miles'.

• It is the simplest method.
• But a reader who knows only the other system may not understand it.
• If the map is reduced or enlarged, the statement becomes wrong, and a new scale must be worked out.

2. Graphical (bar) scale: a line divided into primary and secondary divisions, each marked with ground distances (kilometres and metres, or miles and furlongs).

• Like the statement, it is readable only by those who know its units.
• Its unique advantage: it stays valid when the map is reduced or enlarged, because the bar shrinks or grows with the map.

3. Representative fraction (R.F.): such as 1 : 24,000, meaning one unit on the map represents 24,000 of the same units on the ground: 1 mm to 24,000 mm, 1 cm to 24,000 cm, or 1 inch to 24,000 inches.

• Because it uses units, not a named system, anyone can read it in their own system. That makes it the universal method, and the most versatile.
• By convention, it is turned into a statement using centimetres (metric) or inches (English).
• In an R.F., the numerator is the map distance and the denominator is the ground distance.

Example (new): a map printed with '1 cm represents 2 km' is photocopied at double size. Now 1 cm on the copy is only 1 km on the ground, so the printed statement is wrong; a bar scale on the same map would have doubled in length and still be right.