Maps as Argument
History of Maps and Cartography
Course 1 · Chapter 8 · Thematic Maps and Maps as Argument
So far the maps in this course have shown where places are. From the eighteenth century, mapmakers began to use the map frame to show something else, such as disease, trade, population or a rail network, and to make an argument by the pattern. The same tools were used to persuade, to claim land and to draw borders. This chapter looks at the best-known examples and at how to read them with care. Names and dates are from general knowledge and flagged "check".
Maps That Show a Theme
Choropleth
Areas shaded by a value, such as population density by district. The boundaries chosen can change the picture (check).
Dot and symbol maps
Each dot or symbol stands for a count of something, as in a map of cases of an illness.
Flow maps
Lines whose width shows the size of a movement, such as trade or troops.
Famous Examples
| Map (check each) | What it did |
|---|---|
| Halley's chart of winds, 1686 | Edmond Halley published a map of trade winds, among the earliest thematic maps of a natural phenomenon. |
| John Snow's cholera map, 1854 | Showed deaths around the Broad Street pump in Soho, London. It supported his argument that cholera was spread by water, though the map was one part of an investigation that included interviews and statistics, and it did not by itself end the epidemic (check). |
| Charles Joseph Minard's map of Napoleon's Russian campaign, 1869 | A flow map showing the army's size, location and the temperature on the retreat, famous as an example of rich information drawn simply (check). |
| Harry Beck's Tube map, 1931 | A diagram that straightened the lines and equalised the spacing of stations, accepted by London Underground after some hesitation and revised many times (check). |
| Booth's poverty maps of London, 1889 | Charles Booth coloured streets by the social conditions of their residents, a pioneering social map (check). |
Maps That Persuade
- Projection as rhetoric. Maps centred on one country, or showing a threat by spreading a colour across Eurasia, send a message by design (check examples).
- Propaganda maps. In the twentieth century, governments used maps to claim territory or to alarm their citizens. Look at what is coloured, what is cropped and what is labelled (check).
- Empire. Maps of the British Empire often used red or pink for imperial possessions, which, combined with Mercator's inflated high latitudes, made the empire look larger (check).
- Colonial borders. Many borders in Africa and the Middle East were drawn by European powers, partly on maps with little knowledge of local people. The Berlin Conference of 1884–85 is often cited, though borders were set by many agreements over decades (check).
- Names. Choosing a name for a place on an official map can be a political act (check).
| Technique | Effect (check each) |
|---|---|
| Choice of colours | Red suggests danger or importance; pale colours suggest weakness |
| Choice of boundaries | Large districts hide local variation (the modifiable areal unit problem) |
| Scale and crop | A cropped map can leave out a neighbour that changes the meaning |
| Data classes | Where class breaks are set decides what looks "high" |
| Title and legend | The words around a map frame its message |
Exercises
Choose one thematic map and write down what it claims, how it shows the claim and what it leaves out.
Rewrite the common account of John Snow and the pump so that it matches the evidence more closely.
Take a set of numbers (such as ten districts' values) and show how changing the class breaks changes the story a shaded map tells.
Quick Reference
- Thematic maps: choropleth, dot, flow
- Examples: Snow, Minard, Booth, Beck
- Persuasion: colour, crop, boundaries, classes, titles
- Borders: often drawn by outside powers
- Habit: ask who made the map, and why