Thematic cartography offers powerful tools for analysing statistical tables, through space and time. It highlights territorial contrasts that would not be apparent simply by reading rows of figures. Thematic maps help establish correlations between different phenomenons. Readers can relate to the information displayed thanks to their prior knowledge of the territory, through recollections and visual associations.

The display modes used in statistical cartography are quite standardised. The basic visual variables are colour, symbols, and the size and direction of these symbols. Legibility, a concept held dear by Jacques Bertin, must always prevail over complex representations (see Sémiologie graphique, by J. Bertin, the leading reference book in the field).

The art of simplicity
Here, you will see no 10-bracket discretisations, histograms or pie charts on each geographic unit, or any other sign of counter-productive, headache-inducing madness. It is hard to keep it simple. Statisticians must strive to make sense of a vast collection of data, by bringing out the main points. They must not flex their computer programming muscles and display 10,000 pieces of information any old how.

Each presentation must be chosen according to the type of variable studied. A rate (ratio, density, etc.) cannot be displayed like an absolute value. The distribution of a population must be illustrated using proportional symbols, not by colour gradations on geographic units. This is a common mistake, even on many sites designed by "GIS experts."

Vocabulary
On the other hand, ratios and densities should be represented using "choropleth" maps. This lovely word (you can always drop it into a conversation and impress your friends) comes from Greek: khôra means country, space, territory, while plêthos means a large quantity and plêthein, whole, full. In these maps, territories are therefore filled - with colour!