The choropleth mapping technique should only be applied to appropriate data types-

Appropriate data:

Enumeration data; such as the statistical units used by the Australian Bureau of Statistics and administrative and political subdivisions; including cities, counties, districts etc. For example, we might choose to find population density within Perth, W.A, using selected postal codes. Note- the mapped variable is directly related to the postal code (which is the enumeration district, zone).

Inappropriate data:

Geographical phenomena where the data is continuous in nature, eg. Average annual temperature, rainfall data, height data etc. Mapping these data would be suited better to other methods such as some form of isoline map eg. a rainfall map, using isohyets; a weather map, using isobars; or a topographic map, using isohypses (contours) or hypsometric tints.

Enumeration data

Total Values eg. Number of persons living in a census tract are not suitable. But why?

Derived Values eg. Average annual income is suitable. But why?

Assumptions in Choropleth Mapping

Choropleth mapping is a valuable cartographic method to convey to the map reader geographic trends and values. But not all data are suitable for choropleth maps. More info

Fig 1. A section through the statistical surface