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Graphical Skills » Choropleth, Isoline and Flow Line Maps

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.4.2

Complete and read choropleth maps, dot maps and proportional symbol maps
Complete and read isoline maps, including gradient, contour and value
Complete and read desire line and flow line maps
Choose the right map for the data

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Maps that show data

Choropleth maps use shading to show how data varies by area

Last lesson was about graphs. Maps can show data too, and they show something a graph cannot: where things happen. In the exam you may be asked to complete part of one of these maps, or to read one and describe what it shows.

Every map in this lesson comes with a key. Read the key first, every time. It tells you what the shading, dots, circles or lines mean.

Key terms:

Choropleth map: a map where areas are shaded in different tones of one colour to show amounts.
Dot map: a map where each dot stands for a set number of things.
Proportional symbol map: a map where the size of each symbol shows the amount.
Isoline: a line on a map joining places with the same value.
Desire line: a straight line joining a start point and an end point to show movement.
Flow line: a line whose width shows how many people or things move along a route.

Choropleth maps

A choropleth map shades whole areas, such as counties, countries or wards. The darker the shade, the higher the value. The key shows the groups, for example 0-9%, 10-19% and 20-29%.

✎ Completing one

Find the value for the area. Find which group in the key it falls into. Shade the area with that exact tone. Keep the shading even and stay inside the boundary.

🔍 Reading one

Say which areas are darkest and which are lightest. Then describe the pattern, for example higher values in the north and lower values in the south.

Use light tones for low values and dark tones for high values. Never use random colours. A reader should be able to see the order straight away.

Worked example

The key for a made-up map of unemployment says: light = under 4%, medium = 4-7%, dark = over 7%. An area has 5.5% unemployment. 5.5% is in the 4-7% group, so you shade it medium.

Dot maps

On a dot map, one dot stands for a set number, for example 1 dot = 500 people. Where dots are close together, there are lots of people or things. Where there are few dots, there are few.

To complete one, divide the value by the dot value. A village of 2,000 people on a map with 1 dot = 500 people needs 4 dots.
To read one, describe where the dots are clustered, where they are scattered and where there are gaps.

A dot map shows a pattern well, but it does not give exact values for one area unless you count the dots and multiply.

Proportional symbols

Proportional symbols are usually circles or squares. The bigger the symbol, the bigger the value. A key shows what each size means, for example a small circle = 10,000 and a large circle = 100,000.

To complete one, pick the size from the key that matches the value, then draw it on the right place. To read one, compare sizes and say which place is biggest and which is smallest.

Common mistakes

Drawing a symbol that is the wrong size for the key. Always match the size to the key rather than guessing, and do not use the same size for two different values.

Isoline maps

Isoline maps join places with the same value, like temperature

Isolines join places with the same value. You have already met one type: a contour line joins places of equal height. Other isolines join places with the same temperature or the same air pressure.

Three ideas matter on an isoline map:

Value

Each line is labelled with its value. A point on a line has exactly that value.

Contour

On a height map the isolines are contours. They show the shape of the land.

Gradient

Lines close together mean the value changes quickly. Lines far apart mean it changes slowly.

Finding a value between two lines

A point between two lines has a value between the two. A point halfway between the 10 and 20 isolines is about 15. A point very close to the 20 line is just under 20.

Drawing an isoline

Mark the values at each point on the map.
Choose the value you are drawing, for example 12.
Find the points either side of the line, one higher and one lower, and mark where 12 would fall between them.
Join the marks with a smooth line. Do not use straight sections or sharp corners.
Label the line with its value. An isoline never crosses another isoline.

Worked example

Two towns have temperatures of 8 °C and 12 °C. The 10 °C isoline passes halfway between them. If the towns are 40 km apart, the temperature changes by 4 °C over 40 km. Where isolines are packed closely, the temperature changes over a shorter distance, so the gradient is steeper.

Desire lines

A desire line is a straight line from where people start to where they end up. It does not follow roads. A desire line map might show where shoppers live and which town centre they travel to. Many lines meeting at one place show that it attracts lots of people.

To complete one, use a ruler to draw a straight line from the start point to the end point.
To read one, find where the lines meet or bunch together (the destination) and where most of them start.

Desire lines are usually drawn the same width, so they show where people go, not how many.

Flow lines

A flow line map is similar, but the width of the line shows the amount. A thick line means lots of people or goods. A thin line means few. Flow lines often follow the real route, such as a road, a rail line or a shipping route.

To complete one, use the key to match the width of the line to the value, then draw it along the route.
To read one, find the thickest line first, then compare with the others. State the direction too, such as more people travelling into the city than out.

Worked example

A key says 1 mm width = 1,000 vehicles a day. A road carrying 4,500 vehicles is drawn 4.5 mm wide. A road drawn 2 mm wide carries about 2,000.

Choosing the right map

Pick the map that fits the data:

Amounts or rates for areas: choropleth.
Where individual things are clustered: dot map.
Different sized amounts at places: proportional symbols.
Values that change smoothly across space: isolines.
Where movement goes: desire lines.
How much movement there is: flow lines.

Common mistakes

Shading a choropleth with a tone from the wrong group. Writing only "it is dark" instead of saying what the dark shade means. Drawing an isoline with straight edges. Forgetting to label isolines with their value.

Exam-style question

A map uses isolines to show average July temperature. The 14 °C line is close to the 16 °C line in the north west of the map, but the 16 °C and 18 °C lines are far apart in the south east.

(a) What is the value of a point halfway between the 14 °C and 16 °C isolines? [1 mark]

(b) Describe what the spacing of the isolines shows about how temperature changes across the map. [2 marks]

Model answer

(a) 15 °C (1)

(b) In the north west the lines are close together, so the temperature changes quickly over a short distance (1). In the south east the lines are far apart, so the temperature changes slowly (1).

Exam tip

In part (b), name both areas and say what the spacing means. Close lines mean a steep change, far apart lines mean a gentle change.

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