Scale and Distance
ICSE Class 7 · Geography · Chapter 2
What this chapter covers
- Why a map needs a scale
- Types of scale
- Units you need for converting scales
- Converting between scales
- Measuring straight-line distances
- Measuring curved distances
- Using scale in everyday life
Key definitions
Scale is the ratio between a distance on a map and the same distance on the ground.
Key points
- Scale is the ratio between a distance on a map and the same distance on the ground.
- A large-scale map shows a small area in great detail; a small-scale map shows a large area with little detail.
- A statement scale uses words, such as "1 cm represents 2 km".
- A representative fraction, or R.F., is written as 1 : 50,000 and has no units.
- A linear scale is a divided line; when it has secondary divisions, its zero sits after the first primary division, with the secondary divisions to its left.
- The linear scale stays correct when a map is enlarged or reduced.
- 1 km equals 100,000 cm, which is the key to every scale conversion.
- Straight-line distances are measured with a ruler, a divider or the edge of a paper strip.
- Curved distances are measured with a thread, a paper strip turned at each bend, or a divider stepped along the line.
- Curved distances measured on a map are slightly shorter than the true distance, so they are given as approximate.
Questions answered
What does the scale of a map tell you?
The scale tells you the ratio between a distance on the map and the same distance on the ground, so that you can find real distances by measuring on the map.
Name the three types of scale and give one example of each.
Statement scale, for example "1 cm represents 4 km"; representative fraction, for example 1 : 100,000; linear scale, a divided line marked 0, 1, 2, 3 km.
Why does a representative fraction have no units?
Both the map distance and the ground distance are in the same unit, so the units cancel. The scale works for centimetres, inches or any other unit.
Which is the larger scale, 1 : 25,000 or 1 : 250,000? Give a reason.
1 : 25,000 is the larger scale. Its denominator is smaller, so the land has been reduced less and more detail can be shown.
Convert "1 cm represents 4 km" into an R.F.
4 km = 400000 cm, so the R.F. is 1 : 400,000.
Convert the R.F. 1 : 20,000 into a statement scale.
1 cm represents 20,000 cm, which is 200 m. The statement is "1 cm represents 200 m".
On a linear scale with secondary divisions, why is the zero placed at the end of the first primary division?
The first division is split into secondary divisions to the left of zero. A single measurement can then give whole kilometres on the right and the remaining metres on the left.
Why is a linear scale still correct when a map is photocopied larger?
The linear scale is enlarged by the same amount as the rest of the map, so its divisions still match the map distances.