Choropleth maps — Geography, 11–13
How shaded areas can reveal patterns in data, and why the numbers must be comparable. Geography, 11–13 years.
Colouring data on a map
A choropleth map shades whole areas according to a number, such as unemployment rate or rainfall. Light and dark colours let you see a pattern across space quickly. The colour belongs to the area’s data value, not to every person or object inside that area.
Why use one?
A table can list hundreds of places, but it hides where nearby values form a pattern. A choropleth map was developed to connect statistics with location at a glance. It only works fairly when areas are compared using rates, percentages or another value that allows for different population sizes.
Worked example: bus use
Town A has 2,000 residents and 400 use the bus: 400 ÷ 2,000 × 100 = 20%. Town B has 5,000 residents and 700 use it: 700 ÷ 5,000 × 100 = 14%. A map of totals would shade B more strongly, but a map of percentages correctly shows that bus use is higher in A.
The biggest-area trap
It is tempting to assume that the darkest or largest-looking region has the most people or the worst problem. Large areas catch your eye, while small areas can contain far more residents. Read the title, units and legend first, and check whether the map shows totals or a rate.
Where it is used
News organisations use choropleth maps to show election results, health rates or income patterns. Emergency planners can spot areas with greater need, while environmental agencies can map pollution or rainfall. The map helps you ask a better question: is the pattern real, or does the choice of areas and colours create it?
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