GIS 6005 Lab 9: Bivariate Choropleth Mapping

Bivariate choropleth maps provide an efficient way to display two variables and how they are related in a manner that is easy to interpret by the viewer. Like univariate choropleth maps, a bivariate map uses color progression to symbolize classes of data, the difference being its use of two color ramps for two related variables and the combination of those ramps to display the correlation of those variables.

Creating a bivariate choropleth map requires preparation of the data. We first must ensure the data for the two variables are normalized. In the case of the map above, the data for obesity and physical inactivity were already normalized by population. Next, we will create a new field in the attribute table for each variable and populate them with codes for each variable class. To keep the map understandable for the viewer, each variable is generally limited to three classes; this is because the number of classes displayed in the final map is exponential based on the number of classes for the two base variables. Thus, three classes for the two variables equals nine classes in the final map. Four classes for the variables would create a confusing map of 16 classes.

A third field is added to the attribute table and is populated with the concatenation of the two fields created above. This field will be used for the symbology of the final map and represents the combination of the two chosen variables (i.e., obesity and physical inactivity rates). The creation of the color symbology in the above map required some experimentation. The foundation of color choice for bivariate choropleth maps lies in choosing complementary colors for the two variables. Hue, saturation, and value options can then be adjusted for the overlapping classes. Once the symbology is finalized, the legend must be manually adjusted to create a suitable bivariate choropleth legend. This is accomplished by converting the legend to graphics, ungrouping the elements twice, and manually placing the color squares in their final placement. Text elements may then be added to label the legend as is appropriate.