Other charts and maps Choropleth maps

Choropleth maps use shading to represent data values for different geographic areas, such as local authorities.

They are the standard way to we show data values geographically.

An example of a choropleth map

When to use a map

Consider a map if you have data values for geographic areas and the pattern in the data you want to show is primarily geographic, such as north versus south or urban versus rural.

Having geographic data does not automatically mean a map is the best choice, if there is little geographic pattern in the data, the map may appear noisy and fail to help users interpret the data.

Avoid using a map if you only have data for a few geographic areas, such as the four nations of the UK or the nine regions of England. In these cases, a bar chart or another chart type could allow for easier comparison of values.

An example showing comparisons between values can be more easily made on a bar chart than a map

Use standardised rates

Choropleth maps should display data as standardised rates, not absolute values.

For instance, use the unemployment rate (percentage of the population unemployed in each area) rather than the total number of unemployed people in each area. Using absolute values can be misleading because they are distorted by the size of the area.

In most cases, data should be standardised by population. In some cases, data can be standardised by area. For example, the rate per square kilometre can be used for land use data.

Showing change or differences

To visualise changes between two time periods, consider calculating the rate of change and display it on a single map. This is generally easier to interpret than using two separate maps for different time periods.

This is an example of a derived variable. See the What data to include guidance (opens in a new tab)  for more information.

An example of a choropleth map showing change between two time periods

Geographic level

Generally, map data should be represented at the most detailed geographic level available, unless this level poses risks of disclosing sensitive information or has data quality issues.

If your analysis focuses on a broader geographic level, such as local authorities, aggregate the more detailed data accordingly.

Colour palettes

Choropleth maps should use colour scales to represent data values.

Depending on the data, use either a sequential palette (ranging from low to high values) or a diverging palette (with two contrasting colours diverging from a neutral midpoint).

Number of colour bands

Typically, use a scale with five discrete colour bands for choropleth maps.

A diverging colour scale can include up to six colours.

Avoid using a fully continuous colour scale or too many bands as this can make it difficult for users to differentiate and interpret values.

Setting breaks

Breaks determine the range of values represented by each colour band. To enhance readability, you can round the break intervals to the nearest whole number or round number.

The standard method we use for setting breaks is the Jenks natural breaks (opens in a new tab) , which creates an effective visualisation with most datasets.

In specific situations, you can set breaks using different methods or define specific values, such as to differentiate areas above or below zero.

Alternatives

If a map does not clearly show patterns in the data, consider alternative chart types.

For example, use a scatter plot to show a relationship or a bee swarm chart to illustrate distributions.