WHAT ARE CARTOGRAMS ?
A Cartogram is a map or diagram showing geographical statistical information (Dorling 1995). In this context the term cartogram should be taken to mean equal population cartogram. This distiction needs to be made because ordinary maps are in fact a form of cartogram based on equal land area. Cartograms differ from traditional maps as they use a variable other than area to derive the size of areal units on the map.
WHY USE CARTOGRAMS ?
A major draw back of using traditional map based cartographic representations for portraying human based socio-economic information is that areas with high populations and high population densities, e.g. cities are displayed very small on maps. Traditional maps therefore tend to highlight patterns in the least important areas, i.e. where few people live.
In contrast cartograms represent areas in relation to their population size. As a result patterns are displayed in relation to the number of people involved instead of the size of the area involved. Example 1 below, clearly shows how using a cartogram can give a vastly different impression of overall trends.
From the traditional map it is easy to get the impression that the majority of areas have approxiamatly 56% (yellow and green) of the male population of working age. The cartogram gives a different overall trend, suggesting that the majority of areas have a male population of working age of over 60% (red and purple on the map).
ADDING A TEMPORAL ELEMENT TO CARTOGRAMS
For a wider review of the benefits of cartograms and further examples of cartograms I suggest looking at either Dorling D. (1994) Cartograms for visualizing human geography in 'Hearnshaw H.M. & Unwin D.J. 'Visualization in GIS' or Dorling D (1995) 'A new social atlas of Britain'.
We are particularly interested in developing animated cartograms to show changing trends through the 19th and 20th centuries. Although much work remains to be undertaken, example 2 shows how animated cartograms can be used to display a twin pattern of overall rising numbers of males of working age and the growth of population in London and Lancashire / West Yorkshire.