How to graphically process data on a map?
Note to readers:
This article is part of a set of methodological resources related to the map exercise required in the Thinking IR globally course at Sciences Po. The « map » is essentially identical to the « geographic sketch » used in high school. Most of the points developed in these resources apply to secondary education.
See and not read
A map is a picture to be seen. It’s always the result of a series of choices: what blank map, what framing, what data were used, what solutions were adopted to answer…? The data used undergoes a graphic processing process that results in an image to be seen.
This processing from the readable to the visible is based on a graphic system of signs (colors, shapes, sizes, etc.) formalized, notably in France, by Jacques Bertin. It divides data into two main categories: numbers and texts.
Illustration 1. Readable map (left) VS seeable map (right): people employed for UN peace missions, 2024
Graphic processing of numbers
Numbers allow to evaluate and compare quantities or ratios of magnitudes, but also to order elements by indicating a rank.
Evaluate or compare volumes, stocks, or numbers (CO2 emissions in the World or populations living in a slum in Italy): the simple proportional relationship between the data is shown.
To do this, we choose a simple geometric object (a bar, a disc, or even a square) whose size or height (depending on the object chosen) is modified according to the values in the table.
Illustration 2. Graphic processing of numbers
Ratios of magnitudes: a tool can display as many shades of the same color as there are different values in a data series (illustration 3, left). However, the human eye is unable to distinguish all these shades. We therefore proceed to the step of reducing the original continuous data of the series.
The map then shows the data aggregated into classes whose hues follow the order of the series, from the lightest (low values) to the darkest (high values) (illustration 3, right).
Examples: proportion of Christians in the World, Covid-19 mortality rate in New York...
Illustration 3. Graphic processing of ratios
Ordered rankings: we proceed in the same way as for showing ratios of sizes.
Examples: HDI in the World, RSF ranking…
Graphic processing of texts (names)
In this case, data can be unique texts (never twice the same occurrence in the data series) or words or noun phrases that constitute categories.
Unique texts: These are names of geographical objects or their associated standard codes (ISO or french INSEE codes for example). This information is written, or not, as needed, taking care not to unnecessarily overload the map.
Differentiate categories: We graphically process different data using flat areas of different colors, or using different pictograms. The goal: to distinguish the different categories on the map but also to facilitate the visual association of similar occurrences.
Illustration 4. Legend of an historical map (colonies)