Map type
The Demers Cartogram is a technique for representing data for areas that eschews geography in preference for a geometric shape that represents the unit areas. It is ostensibly the same as a Dorling cartogram except it uses squares instead of circles to reduce the gaps between mapped objects.
A Demers cartogram maintains neither shape, topology or object centroids and is an abstract representation of the spatial pattern of the phenomena being mapped. However, whereas the Dorling cartogram attempts to limit the distance of the eventual position of each object from its original position, the Demers sacrifices distance to maintain contiguity. The positions of areas are moved to deal with the problem of overlapping objects caused by some non contiguous cartograms.
A cartogram can be a powerful approach to mapping population data since it provides a strong visual and does not require data to be normalised. The drawback is that the distortions and abstract portrayal can be a hurdle for some in their understanding. In this example of the 2012 Presidential election, the map is designed to show the size of the overall vote at State level with the addition of colour to represent the winning candidate.
Data
For the theme being mapped, the data should be numerical (quantitative) and represent differences between features on an interval or ratio scale of measurement. The map type normally requires data to be absolute, as totals, but it can be used with relative measures such as ratios of percentages. Here, the vote totals are augmented by symbols that define a second categorical characteristic of the data, namely ‘Republican’ or ‘Democrat’.
Symbols
Symbols should be designed so that different magnitudes of data can be easily distinguished from one another through variation in the size of the symbol, used as an ordering visual variable. Symbols should be scaled so that the smallest are visible and the largest do not overly smother the map with the largest symbols representing the largest magnitudes. Some overlaps are inevitable but using techniques like an outline allow different symbols to be seen.
In a Demers cartogram the symbols should be squares so that size estimation, based on differences in the area of the symbol, remain possible amongst symbols. In this example, the categorical data are symbolized with red and blue hues to match the political affiliations.
Marginalia
The numerical data may be represented using a legend that equates to the different sizes though the interactivity a web map supports allows popups to reveal actual values. The overall pattern is easily seen without a legend in this context. The categorical data is represented in the legend to differentiate the two political parties and the map should be finished with a succinct title, source details and relevant credits. Popups give readers access to data values for a more detailed exploration of each area.
Map Use
When viewed, the map reader should be able to efficiently see the relative differences between symbols and be able to determine a pattern across the map. When size is used as an ordering visual variable we are ascribing more importance to the larger magnitudes of data. We visually interpret the symbols as differently sized so we perceive larger symbols as meaning ‘more’. The multiscale characteristic of a web map means we can design the map to reveal more detail at larger scales (e.g. labels) as the reader zooms in. This not only increases the interest in your map but enables you to take advantage of the different scales
