GEOGRAPHIC SKILLS: CHOROPLETH MAPS
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A choropleth map is a thematic map that displays statistical variables, such as population density, through shaded areas, with the term originating from Greek words for 'area' and 'multitude'. The first known choropleth map was created in 1826, and effective use requires normalization to accurately represent data independent of polygon size. Key principles for interpretation include recognizing geographic features, checking values against the legend, and using limited color categories for clarity.
GEOGRAPHIC SKILLS: CHOROPLETH MAPS
- 2. WHAT IS A CHOROPLETH MAP? • A choropleth map is a thematic map in which areas are shaded or patterned in proportion to the measurement of the statistical variable being displayed on the map, such as population density or per-capita income. • The term comes from the Greek for "area/region“ and "multitude“.
- 3. A choropleth map that visualizes the fraction of Australians that identified as Anglican at the 2011 census.
- 4. FIRST CHOROPLETH MAPS • The earliest known choropleth map was created in 1826 by Baron Pierre Charles Dupin. • They were first called "cartes teintées" (coloured map in French). • The term "choroplethe map" was introduced in 1938 by the geographer John Kirtland Wright.
- 5. Normalisation: the map on the left uses total population to determine colour. This causes larger polygons to appear to be more urbanized than the smaller dense urban areas of Boston, Massachusetts. The map on the right uses population density. A properly normalized map will show variables independent of the size of the polygons.
- 6. USABILITY When using CHOROPLETH MAPS, there are two important principles: • first is that darker colours are perceived as being higher in magnitude; • second is that, while there are millions of colour variations, the human eye is limited as to how many colours it can easily distinguish. Generally, five to seven colour categories are recommended. The map user should be able to easily identify the implied magnitude of the hue and to match it with the legend.
- 7. HOW TO INTERPRET A CHOROPLETH MAP 1. Identify the geographic feature or phenomena being mapped. 2. Verify the value of each shade used on the map. This can be done by reading the map’s legend. 3. Identify the scale of the administrative regions shows on the map (does the map show suburbs, census areas, states or countries). 4. Using the key as a guide, identify the areas of the map that share the same colour shading and the same quantity or volume of the feature being mapped. 5. Describe the density or concentration of the feature both within and between different areas of the map.
- 8. STEPS TO CONSTRUCT A CHOROPLETH MAP 1. Obtain a base map of the area or administrative regions you wish to show. 2. Examine the data to be presented and determine the range of data. This can be calculated by subtracting the lowest value in the data from the highest. 3. Divide the range by the numbers of categories you plan to use. 4. Assign a shade for each category. The darkest shade will be assigned to the highest value category. 5. Complete a category key on the map and sort the data into categories. 6. Shade in the administrative regions on your map according to your key.