Module 6: Proportional Symbol and Bivariate Choropleth Mapping

The objectives of this week's lab was to explore proportional symbol and bivariate choropleth mapping techniques, as well as customizing legends for each technique.

One of the data sets for this lab included positive and negative values. The final map utilizes proportional symbols to show both positive and negative values. This map is essentially two proportional symbol maps overlaid on each other, one for positive job growth and another for negative job growth, with the symbols for both variables sized the same. Utilizing proportional symbols to show both positive and negative values is an effective way to visualize areas that show the greatest and lowest values. These areas are easy to see quickly on the map and give the map reader a simple way to visualize the variable being mapped.

Fig 1. Proportional symbol map using positive and negative values. 
Another data set for this lab examined contained two data variables: percent of the US population in each state that is obese and physically inactive. The goal was to create a bivariate choropleth map showing the relationship between the two variables. In order to do this, each variable had to be reclassified into 3 classes using quantile classification. A quantile classification was used because each each class contains nearly an equal number of features per class. The variables were reclassified based on the class breaks. Once each variable was reclassified, each class was assigned a color from a 3-class complementary color scheme. I chose to use a color scheme very similar to one in the lab on Page 13, but also found in the Stevens (2015) article. I created a customized legend by converting the legend to graphics and moving the elements around to create a 3x3 table and labels. The final map effectively shows a strong relationship between obesity and physical inactivity.

Fig 2. Bivariate choropleth map showing the relationship between obesity and physical inactivity in the US.

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