Sunday, March 26, 2017

Bi-variate Choropleth Mapping

For this week, our task was to create a Bi-Variate Choropleth map. This map was to utilize two different variables that were shown to have at least some correlation between one another. Here is my product for this task:

Preparing a Bi-Variate Choropleth can be quite tricky, as there are not built-in functions in ArcGIS for such a task. This requires manipulating the data so that it can be manually used to create the values needed to display the two variables together.

The first step was to rank the two variables. This was done by classify the data using quantiles and then assigning the data a rank based on which class it was in. Essentially, if the data was in class one, it was assigned the rank of "1" in a new field. For the second variable, the ranks were assigned the same way but instead of a number, it was given a letter (A, B, C, etc.). The two newly created rank fields were then combined to create an overall ranking system of A1, B1, C2, B3, etc.

The way this ranking worked is simple. Ranks of A1 were low on both variable scales. A3 values were high on the first variable scale and C1 values were high on the second. C3 values were high on both scales and indicated a strong correlation between both variables.

The values were then assigned a Bi-Variate color scale. This was created by taking two 3-class color scales and blending them to create a bi-variate scale with 9 overall color values. When these colors were shown on the map with their individual values, it created the map as shown above.

Overall, creating a Bi-Variate Color map is a manual process. Hopefully one day ArcGIS will have a program or toolset integrated into it that will allow for quicker and more efficient creation of Bi-Variate Choropleth maps.

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