Choropleth maps Used to demonstrate trends in data

Choropleth maps A choropleth map provides an easy way to visualise how a measurement varies across a geographic area. Choropleth maps use graded differences in shading or colour in order to indicate the values of a property. The darkest colour shows the highest value, through to the lightest colour which shows the lowest value.

Choropleth map of urban population (%) The lightest colour shows the lowest % of the population urban The darkest colour shows the highest % of the population urban Various shades of the same colour are used to show value. A legend is used where data is grouped with regular intervals between each group Data sourced from: The World Bank: Urban population (% of total) http://data.worldbank.org/indicator/SP.URB.TOTL.IN.ZS/countries

Creating choropleth maps There are six steps involved in creating a choropleth map. Step 1 — look at the data Step 2 — rearrange the data Step 3 — determine breaks Step 4 — choose colour scale Step 5 — shade the map Step 6 — finish the map (BOLTSS)

Step 1 Look at the data table. What is the data we want to represent on the map? What is the unit of measurement? What are the highest and lowest values and what is the difference between them? Table 1: Average rainfall for Australian states and territories from 1961–1990 State/Territory Average rainfall 1961–1990 (mm) New South Wales and ACT 566 Northern Territory 548 Queensland 630 South Australia 236 Tasmania 1168 Victoria 654 Western Australia 352 932 Source: Australian Bureau of Meteorology

Step 2 Rearrange the data from the highest value to the lowest value. Table 1: Average rainfall for Australian states and territories from 1961–1990 State / Territory Average rainfall 1961–1990 (mm) Tasmania 1168 Victoria 654 Queensland 630 New South Wales and ACT 566 Northern Territory 548 Western Australia 352 South Australia 236 highest value lowest value Source: Australian Bureau of Meteorology

Step 3 Use your rearranged data to work out ‘logical’ breaks to create the required number of data sets (usually between four and six). There must be continuity in the values (that is, no gaps in the numbers). You may use < (less than) and > (greater than). Table 1: Average rainfall for Australian states and territories from 1961–1990 State/Territory Average rainfall 1961–1990 (mm) Tasmania 1168 Victoria 654 Queensland 630 New South Wales and ACT 566 Northern Territory 548 Western Australia 352 South Australia 236 900+ 600–899 300–599 <300 Source: Australian Bureau of Meteorology

Step 4 Choose a colour scale for your legend. For example: Table 1: Average rainfall for Australian states and territories from 1961–1990 State/Territory Average rainfall 1961–1990 (mm) Tasmania 1168 Victoria 654 Queensland 630 New South Wales and ACT 566 Northern Territory 548 Western Australia 352 South Australia 236 900+ 600–899 900+ 600–899 300–599 <300 300–599 <300 Source: Australian Bureau of Meteorology Lightest colour and lowest value is located lowest!

Step 5 Shade the map.

Step 6 Complete the map, ensuring all map components are included (BOLTSS). 900+ 600–899 300–599 <300 Average rainfall for Australian states and territories from 1961–1990 Average rainfall (mm) N Scale: Unknown Source: Australian Bureau of Meteorology

References Background slide: Modified from: CIA, http://commons.wikimedia.org/wiki/File:GINI_retouched_legend.gif Slides 5—8 Data sourced from: Bureau of Meteorology data cited in Rainfall distribution (National Water Commission) http://www.water.gov.au/WaterAvailability/Whatisourtotalwaterresource/Rainfalldistributio n/index.aspx?Menu=Level1_3_1_2