Popular Vote with Population

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Why We Love It

Sometimes election maps hide important factors, most crucially, population. We love how this map uses a technique called “value-by-alpha” to overlay a second layer that varies transparency by population density. It gives a better idea of the popular vote for the 2012 US presidential election. Areas with more voters pop with bright colours while areas with relatively low populations recede into muted tones.

Why It Works

Without the value-by-alpha layer this map would be a straight red/blue map showing counties that voted Democratic and those that voted Republican. It wouldn’t tell us much about the real voting patterns. In fact, it would suggest a Republican landslide. The value-by-alpha layer gives nuance to the map, immediately bringing highly populated counties into focus and diminishing the counties with fewer voters.

Important Steps

Value-by-alpha is achieved by using two layers in your table of contents in ArcGIS Pro. It can also be made directly inside ArcGIS Online map viewer; see instructions here.

The bottom layer is a standard choropleth, or in this case a red/blue map of counties showing the predominance of Republican or Democratic voting.

Add a second layer and choose population density as the variable to map.

Symbolize the layer using unique values and a color ramp that varies from black (for the lowest population value) to transparent (for the highest). This results in the counties with lower voting populations fading to black in the map.

The map uses a custom basemap projected in Albers. You can specify your own tiling scheme and publish to ArcGIS Online with a custom tiling specification from ArcMap, to use whatever projection you prefer. In this case an equal area projection better supports the thematic map.

Requirements

Data

This map needs two kinds of data: either numeric or categorical data for the colours (e.g., income, vote result), and a second numeric attribute to modify the transparency of the colours (e.g., population, life expectancy). Modifying the transparency of colours allows us to communicate two facts per location.

Analysis

Aim to reveal nuances. Working with the basemap, try different values for the transparency layer to verify how those values reveal the subject of interest: the more populous areas.

Time

It should take less than an hour to create the two map layers in your ArcGIS Pro project and then publish to ArcGIS Online. If working directly inside ArcGIS Online, it will take less than 5 minutes to create this map.

Transparency to Fade

Tips

Consider using transparency to “fade” the locations using measures of error, uncertainty, or other numeric data that assists visual interpretation of the main information.

Custom Legend

Tips

Use a custom legend with a simple image embedded to create something simpler and more suited to the map.

Map Author

Kenneth Field

Ken Field

@kennethfield | LinkedIn

Professional cartonerd, amateur drummer and snowboarder. Lifetime encourager of cartographic quality not quantity. Map with the times while building on the past.

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