A place in history: a guide to using GIS in historical research

CHAPTER 6: VISUALISATION FROM GIS

6.6 Other forms of mapping

Although choropleth maps are a highly effective way of communicating information from polygon-based data they do have certain drawbacks. One of the largest is that with many administrative geographies the largest administrative units tend to be sparsely populated rural units, while the smallest tend to be densely populated urban areas. This means that the map can distort the pattern as it emphasises the areas in which few people live while almost obscuring the areas containing most of the population. One way around this is to distort the map pattern to make the polygon sizes proportional to the size of their population rather than their surface area. A map of this sort is called an area cartogram. An automated method of doing this while retaining perfect connectivity between each polygon and its neighbours has not yet been devised. Dorling presents a method where each polygon is converted into a circle whose size is proportional to its population (or any other variable) (Dorling 1994; 1996). The position of the circles is then moved to prevent circle overlap while attempting to keep a circle as close as possible to its neighbours. Dorling (1995) presents an atlas of modern Britain based almost entirely on cartograms but that is otherwise very similar to atlases such as Kennedy et al. (1999), Spence (2000b) and Woods and Shelton (1997).

Figure 6.1: Choropleth and cartogram representations of infant mortality in England and Wales, 1890s
Both maps show the same pattern. The choropleth makes it appear that the highest rates of infant mortality are rare. The cartogram shows that they are in fact common in urban areas where the majority of the population live. In fact, the five classes contain approximately equal numbers of people. Source: Gregory et al. 2000, 140.

Cartograms can be criticised as they are an unfamiliar representation of space and it can be difficult to establish exactly where a place is on the map. Gregory et al. use choropleths and cartograms together in their study of changing patterns of poverty (Gregory et al. 2000). Figure 6.1 gives an example of this showing registration district-level infant mortality in the 1890s. From the choropleth map it appears that the highest rates of infant mortality are relatively rare. The cartogram gives a very different impression by showing that because high rates of infant mortality were largely an urban phenomenon, a large proportion of the population were in fact affected by these rates. In actual fact, the shading scheme used by these maps puts approximately the same number of people into each of the five classes.