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.