What is a choropleth map?
A choropleth map is a type of thematic map. It couples a way to visualize data with mapping, which itself is a way of looking at the world. So choropleth maps are meant to help visualize the relationship between data and the world.
That's still pretty general though. To be more precise, choropleth maps use colors or patterns on geographic boundaries to represent units of statistical variability in an area such as unemployment rate by county or GDP per capita by country.
Choropleth maps help you visualize variance in a measurement in relation to discrete boundaries on a map. They are useful for making sense of how the measurement varies within a region. Looking at a choropleth map might, for example, quickly give you a general idea of which parts of the United States are primarily Republicans or Democrats and which countries in the world are rich or poor.
In this post, we'll take a look at several different types of choropleth maps and which type might be better for certain measurements.
Types of Classificatons
- Equal Interval
- Natural Breaks
- Standard Deviation
- Pretty Breaks
Equal interval is pretty straightforward. You just divide your measurement classes into groups of, well, equal intervals. So if your data range is 0-30, you can divide your measurements into groups of 3 (0-10, 10-20, 20-30) or groups of 5 (0-6, 6-12, 12-18, 18-24, 24-30) and so on.
Quantile classification distributes a set of values into classes that contain an equal number of values. This way, no classes are empty nor do any classes have too many or too few values.
Natural Breaks (Jenks)
Natural breaks or Jenks, named after its inventor George Frederick Jenks, uses an algorithm to minimize variation in each class so that colors look more distributed on a map.
Standard Deviation Classification
Standard deviation is a way to measure how spread out a dataset is compared to its mean. So classes are group into a standard deviations from the mean.
The second simplest choropleth type after Equal Intervals. You simply make classes based on rounded numbers such as 1000, 2000, 3000... etc.