When measurements are made from different geographical regions, we are often interested in whether adjacent regions have similar values. Colouring the regions of a map to represent the values is an effective way to display the geographical spread of the measurement.

Circles

A major problem with colouring regions on a map is that large regions are very prominent, even if they are sparsely populated. A solution is to superimpose coloured circles on a standard map whose areas are proportional to a better measure of their sizes (e.g. population or GDP).

Pie and bar charts

More complex displays such as pie charts and bar charts can be draw over each region of a map to show the geographical distribution of other information. However these displays can easily become too cluttered with information if there are many regions or complex displays for each region.

Distorted maps

Another way to represent 'size' information on a map is to distort the boundaries of the regions to make their areas proportional to the values. These distorted maps are used to represent populations, but can also be used for other 'quantity' values such as GDP or rainfall. If the distortion is too great, the regions can be difficult to identify.