Choropleth Maps

What is a Choropleth Map?

• A map which shows relative density per area

• This includes percentages, ratios and per capita information (e.g. average income)

• Different shading densities or colours are used to represent ranges of values as outlined on a key

Considerations for key

• Ensure there are equal number of values in each group

• Make sure all groups increase in size evenly

• Should have enough groups that it is useful – not too many as it becomes too complicated to interpret.

• When shading in the colour it works best when the darkest colour shows the highest density or value as it makes it stand out.

Advantages

• They quickly give an impression of the distribution of a variable, showing the contrasts between well and poorly represented areas clearly.

• Shading gives very quick impression without having to refer to a colour key

• The smaller the scale the more accurate the choropleth map.

• Uses countries, regions or boroughs

• Groupings can be flexible to accommodate the spread of value

Disadvantages

• The distribution pattern shown is subjective and the impression given can vary significantly depending on the way in which the data has been divided into ranges and the areal units used

• A false impression of abrupt change in the value of a variable may be given at the boundaries of adjacent areas. In reality the change would be more graduated. (See diagram below)

• It treats the area as a whole so hides variations within areas. This can be misleading and mask anomalous data.

• Finding a suitable scale is essential

• Choropleths are often not suitable for showing total values. Proportional symbols overlays  are one solution to this problem.

• It can be difficult to distinguish between different shades.

• Areas that are left white/blank may be confused with areas of “no data”