Continuous scale or groups?

When a continuous numerical quantity is displayed on a map, a choice must be made about how to use colour to represent it.

Continuous colour
The quantity can be represented with a continuum of shades between two colours (e.g. red and blue). However it is often difficult to assess where any particular shade lies in this continuum. Adding a third contrasting colour to the continuum helps.
Grouped colour
Alternatively, the values can be grouped into discrete classes with distinct colours for each. If there are more than two or three classes (and colours), care should be taken with their choice. If the colours do not change in a predictable way, there is a danger that the map will appear to be a random mosaic of colours.

Choose the colours for a map carefully.

In black-and-white publications, it is usually best to use group the values and use a small number of grey shades.

African life expectancy in 2005

The map below shows life expectancy varied in Africa in 2005.

Use the pop-up menu to investigate the use of different colours. Observe that:

The two choices Continuous yellow -> red -> blue and Five groups, smooth are probably the best colour representation.