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Cartograms are used for thematic mapping. They are a particular class of map type where some aspect of the geometry of the map is modified to accommodate the problem caused by perceptually different geographies. Standard thematic maps, such as the choropleth, have inherent biases simply due to the fact that areas will likely be very different in size from one another. The tendency to see larger areas as more important, regardless of the variable being mapped, can cause confusion. Cartograms tackle this by modifying the geography, effectively normalizing it to create a map where each area takes on a new shape and/or size based on the variable being mapped. Cartograms therefore depict geographical space diagrammatically as they lose their relationship with true coordinate system geometry. There are four main types of cartogram which each represent the mapped variable differently – non-contiguous, contiguous, graphical and gridded.
cartogram: a diagrammatic map type that represents the mapped area by distorting the geometry of the feature itself
non-contiguous cartogram: adjacencies are compromised as areas shrink or grow; individual area shapes are preserved but they become detached from the overall map
contiguous cartogram: adjacencies are maintained but shape is distorted to accommodate the mapped variable
graphical cartogram: maintains neither shape, topology or location; instead using non-overlapping geometric shapes (e.g. circles or squares) to represent the mapped variable
gridded cartogram: uses repeating shapes of the same or different size to create a tessellated representation of the mapped variable
topology: non-metric spatial relationships that are preserved under continuous transformation e.g. adjacency
2.1 The case for cartograms
It might sound paradoxical but geography can get in the way of making a good thematic map. The basis for most thematic maps is providing the reader with a map from which comparisons are clear and discernible. This might be achieved in many ways such as through normalizing absolute values by a common denominator for a choropleth map, to take account of the inherent differences in the size of each area. Instead of changing the data and maintaining the geography, an alternative is to retain the data values but modify the geography. Consider the United States electoral system in which states with larger populations get more electoral votes. Yet, the more populous states are not necessarily the same as the largest states in area and so a map that shows the predominant result as either red or blue in the geographical sense inevitably skews our perception. We end up with an accurate, though somewhat misleading, map because densely populated states are relatively small and vice versa. People simply are not good at seeing through the mediated view of the data that the map technique delivers. While it’s possible to accommodate such issues using different design approaches for symbology, the alternative is to morph the geography itself. This is the basis for a cartogram – a diagrammatic form of map that distorts the geography to overcome some of the problems of heterogenous reality.
One of the disadvantages of the cartogram is that it inevitably changes the visual representation of geography. This has consequences as the map attempts to balance statistical accuracy, geographical accuracy and topological accuracy. In some respects any cartogram could be thought of as a unique map projection – one that is modified by the data variable being mapped. As with any map projection that tries to warp 3D space onto a 2D surface, some distortion is inevitable. Cartograms therefore contain inherent distortions as they strive to minimize the natural distortions caused by our perception of real geography.
- Describe the different purposes that cartograms serve in relation to other thematic mapping techniques.
- Compare and contrast the advantages and limitations of non-contiguous, contiguous, graphical/Demers/Dorling, and mosaic cartograms.
- Critique examples of different types of cartograms by their relative success at communicating information.
- Design a range of cartograms to suit particular needs
- Given a series of different cartograms (and other thematic maps) that map the same data variable, assess the difference between their ability to communicate the data. This might be in terms of the overall message or the way in which different techniques mask certain characteristics of the data. Additionally, attention should be focused on the perceptual and cognitive dimensions of map use, seeing and interpretation.
- In lab ___, create a series of cartograms to reflect the data appropriately. For each of the data variables, choose a variant that you feel depicts the essential characteristics and compare the outcome to a more typical thematic map (e.g. choropleth or proportional symbol).
- Perform a user study to evaluate how people perceive and interpret different cartograms. For this exercise you will need to create a range of cartograms of the same data variable as well as more typical thematic map types. Your evaluation might be through the use of simple questionnaire surveys to ask how people are interpreting the maps. Alternatively, you may used observation techniques and assess metrics such as the time it takes people to respond to particular questions about each map.