Traditional maps of election results tend to paint the US as red or blue which hides a number of important factors, most crucially the distribution of the population. They often create a visually skewed impression of reality and leave you wondering how come the Democrats won in 2012 when the map is mostly red! This map uses a technique called value-by-alpha to overlay a second layer which varies transparency by population density. It corrects the map’s message by bringing the areas with more voters into focus and so we see not only how the vote went but where and to what magnitude.
Map type
The purpose of a value-by-alpha thematic map is to show how areas differ in type by using an equalizing variable to account for the problems of visual interpretation of areas of different sizes. The technique is explained in this paper by Robert Roth et al. In this example of the 2012 Presidential election, the map is designed to show the percentage vote by colour with strong blue representing increasing Democrat vote share, strong red representing increasing Republican share and colours tending towards purple for an equal share. Total votes are used as an equalizing value so counties with a low overall number of voters tend to dissolve into the background and disappear from the map (becoming increasingly white). The map is an alternative to the choropleth (which requires an equalizing variable to be used to normalize the data) or a cartogram (which modifies the shape of the enumeration areas to account for differences)
Data
For the theme being mapped, the data should be categorical (qualitative) and represent differences between features on a nominal or an interval scale of measurement. Here, ‘Republican’ or ‘Democrat’ share of vote is measured on the interval scale. Additionally, the equalizing variable is numerical (quantitative) and measured on an interval or ratio scale.
Symbols
Symbols should be designed so that different categories of data can be easily distinguished from one another such that once the equalizing variable is added the symbols can still be differentiated. If the different categories are equivalent in importance, then the symbols should reflect that. In this example, the two categories are symbolized with red and blue hues that diverge towards purple to match the political affiliations but if the theme has no specific colour associations then the symbol should be neutral and not value-laden. Hue can be successfully used as a differentiating visual variable and the value and lightness are kept relatively constant so that neither appears more important than the other. The alpha channel of each county is symbolized along a colour ramp from white (for low votes) to transparent (for high votes). The creation of this sort of map is a simple process in ArcGIS Pro with two layers - the first representing the coloured share of vote; and the second representing the total vote.
For a little added interest we've added city locations as polygons to give the map a little more structure and also symbolized the outlines of each county as blue or red to emphasize the predominant electoral party.
Marginalia
The mapped categories should appear in a legend overprinted with alpha channel detail so we can establish the different relationships between the differentiating variables and the total votes. The map should be finished with a succinct title, source details and relevant credits. Popups can usefully be used to 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 differences between the categories through the different levels of translucency that the alpha channel defines. The important areas (of high numbers of total votes) shine as spotlights across the map. This is where the eye focuses attention. On this map, the result is to shine a light on the main populous areas and not the large swathes in middle America that would dominate the map without the value-by-alpha effect. The multiscale characteristic of a web map means we can design the map to reveal more detail at larger scales as the reader zooms in. This not only increases the interest in your map but enables you to take advantage of the different scales.
