Geographic Data Science

Choropleths
Dani Arribas-Bel

Choropleths

Choropleths

Thematic map in which values of a variable are encoded using a color gradient of some sort
  • Counterpart of the histogram
  • Values are classified into specific colors: value –> bin
  • Information loss as a trade off for simplicity

Classification choices

  • N. of bins
  • How to bin?
  • Colors

How many bins?

  • Trade-off: detail Vs cognitive load
  • Exact number depends on purpose of the map
  • Usually not more than 12

How to bin?

Unique values

  • Categorical data
  • No gradient (reflect it with the color scheme!!!)
  • Examples: Religion, country of origin…

Equal interval

  • Take the value span of the data to represent and split it equally
  • Splitting happens based on the numerical value
  • Gives more weight to outliers if the distribution is skewed

Quantiles

  • Regardless of numerical values, split the distribution keeping the same amount of values in each bin
  • Splitting based on the rank of the value
  • If distribution is skewed, it can put very different values in the same bin

Other

  • Fisher-Jenks
  • Natural breaks
  • Outlier maps: box maps, std. maps…

Color schemes

Align with your purpose

  • Categories, non-ordered Qualitative
  • Graduated, sequential Sequential
  • Graduated, divergent Divergent

TIP: check ColorBrewer for guidance

Tips

  • Think of the purpose of the map
  • Explore by trying different classification alternatives
  • Combine (Geo)visualisation with other statistical devices

Creative Commons License
A course on Geographic Data Science by Dani Arribas-Bel is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.