A choropleth map may be used when...

• Portraying a geographical theme whose data occur within well-defined enumeration units
• Only if the data can be represented as ratios or proportions. Derived values not total values.
• Not to show precise values within enumeration units.

Considerations:

• Geographical Phenomena (What is mapped?) - understand the variables that you map.
• Map Scale
  • Areal units large enough to see, and differentiate the areal patterns.
  • Available space
• Number of kind of areal units
• Data Processing - Data can be in ratio or rate form.

Number and kind of areal units:

• Spatial detail added as the number of enumeration units increases.
  As the number of units increases, their size decreases, making difficult symbol differentiation.
  • How many units? This depends on time, cost, map purpose, map size, scale and symbolisation.

These kind of units are generally adopted from local, state or federal census sources