A cartogram resizes places by a variable such as population; its areas no longer represent land area.
A cartogram makes the size of a place proportional to a chosen quantity. A small but densely populated district may become visually large. This shifts attention from the extent of land to the distribution of people or another total.
Check the title and key before comparing sizes. In the example, districts A, B and C have equal land areas but populations of 100, 200 and 300. The population cartogram gives C three times A's display area. It does not mean C has more land.
Distortion can make familiar places difficult to recognise and alter apparent distances, shapes or adjacency. Some designs preserve neighbouring relationships while others use separated shapes. Use a geographic map alongside a cartogram if location or route distance matters.
For a needs study, a population cartogram can highlight where many people need services. It cannot alone show unmet need: a large population may have good access. Combine the quantity with a suitable rate or service measure.
Step by step
Identify the scaling variable
Read what the displayed area represents.
Compare areas
Use the key or labelled quantities, not familiar geographic size.
Recognise distortion
Return to a conventional map for geographic distance or precise location.
Worked example: Choose the right map for the task
Use the cartogram to compare population totals. Use a route map to work out how far a resident must travel to a clinic. The distorted cartogram cannot supply that distance.
Watch out for this
A larger cartogram area always means higher population density.
If area represents total population, density still needs the real land area as a denominator.
Check your understanding
A district doubles in area on a population cartogram between two years with the same scale. What can that indicate?
- Its geographic land area doubled.
- Its population doubled.
- Its average journey distance doubled.