WHAT
Visualize how a measurement varies across a geographic area.
WHY
Choropleth maps provide an easy way to visualize how a measurement varies across a geographic area or show the level of variability within a region.
WHEN
Choropleth maps are important to support analysis across pre-defined geographic area, e.g. an election map divided by electoral regions or per-capita income by county. Choropleth maps are a great way to deal with the problem of showing too many individual points like incidents or occurrences by aggregating them into an at-a-glance view at smaller scales. In that respect Heatmaps are similar but do not use geographic boundaries.
HOW
Choropleth maps are based on statistical data that is aggregated over a region (e.g. state, county, or zip). Each geography is shaded or patterned in proportion to the measurement of the statistical variable being displayed.
It’s important to normalize the raw data values to show density (variable divided by total area) instead of magnitude because larger geographic areas would otherwise be overrepresented either by count or visual appearance. Other forms of normalization include calculating change (variable divided by previous year) or other statistics like mean, median or standard deviation.
Different cartographic choices exist, with the single-color hue progression that fades from a dark shade of a color to a very light shade of the same color or white, being the most prominent for mapping magnitude with higher values represented with darker hues. Another widely used color progression is the bi-polar scale showing changes in value from negative to positive to depict ‘hot’ (red) versus ‘cold’ (blue) or change over time in ‘increase’ or ‘decrease’.
Any choropleth map demands the display of a Legend to understand the correlation between the color code and its associated values. The app developer should therefore provide a legend that is open by default and explains the values or ranges depicted on the map.
The most important design decision is the right choice of color, consider that
- color choices have direct impact on the interpretation by the user, i.e. darker colors are perceived as being higher in magnitude.
- It’s also advisable to limit the range of categories as represented through hues of a color to seven or less which is about how many the human eye can easily distinguish.
- Also consider providing alternative color scales that assist people with color blindness.
Representing large regions of the same color can be misleading if users are not familiar with their underlying boundaries but the choice of regions will ultimately depend on the map’s intended audience and purpose. It’s advisable to give users the explicit choice to select the geography of interest (e.g. state vs county) which will also make it clear what is currently selected and displayed.
EXAMPLES
