Choropleth

Introduction

A choropleth map is likely to be the most common kind of map visualisation that AURIN users will make with the data that they access. A choropleth (from Greek χώρο (“area/region”) + πλήθος (“multitude”)) is a thematic map in which areas are shaded or patterned in proportion to the measurement of a variable being displayed on the map, such as population density or average income, or as shown below, percentage of the population involved in volunteering.

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Inputs

Choropleth maps can be created quickly and easily in the AURIN Portal, and provide a useful first pass at detecting and visualising interesting spatial patterns in your data.

To show how to do this, we will create the map above, but with a different colour palette. In order to do this:

  • Select New South Wales as your study area
  • Select LGA11 Community Strength as your dataset, selecting the following variables:
    •  Local Government Area Code
    •  Local Government Area Name
    •  Voluntary Work – Rate per 100 Population

Once you have done this, open the Choropleth tool (Maps, Charts and Graphs → Map Visualisations → Choropleth) and enter the parameters as shown in the image below. These will be explained below the image

To create a choropleth map, select Map Visualisations and then Choropleth in the Visualisations pop up window (shown below). This will bring up a range of fields that need to be populated.

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  • Select a dataset: Here you can choose which of the datasets you would like to display as a map. In this exercise we use LGA11 Community Strength
  • Select an attribute: This is the field that you want to map. If you want your map to make sense, and actually display the variable you are interested in, it is important to make sure you have selected the right attribute to map together with right classifier. For this example we choose Voluntary Work – Rate per 100 Population
  • Select a classifier: Here we define how we break up our range of values int he attribute. For an attribute that is numerical in format (either an integer or a decimal),  the default setting for this field is Jenks (Natural Breaks), which breaks your data up into intuitive groups based on the shape of distribution of values. You can select Quantiles or Equal Intervals. If your attribute is categorical – that is, if it is a description or a word (such as a land use zone, or a name, or any kind of “string”) then the parameter will automatically set to Pre-classified. For this example we choose Quantile.
  • Number of Classes: This slider allows you to define the number of breaks in your data (minimum of 3, maximum of 12). The number that you choose should depend on the distribution of your values, the number of data points (areas) and the information that you are trying to portray with your data. For this example we choose 5
  • Select a palette type: Here you can choose the type of colour scheme for your data – Sequential, which shifts from a shade of one colour to another;  Qualitative, where the colours are unique along the palette (used for Pre-classified) ; and Diverging, where colours shift to two colours from a central point  along a natural spectrum. For this instance we choose Sequential.
  • Palette: This allows you to choose the actual colours of your palette (you can switch the ends of the palette around by clicking the Reverse Palette box at the bottom of the box. AURIN uses colours generated by Colour Brewer. For this example we select RdPu
  • Default Opacity: This slider allows you to define how opaque your map is over the base map. 0.00 indicates completely transparent, 1.00 indicates completely opaque. Here we select 0.85
  • Hover Opacity: This slider allows you to define how opaque you want specific areas to be when you hover over them with a mouse, with the same values as for Default Opacity. Here we select 1.0
  • Name: The default for this field is “Choropleth-X”. It’s a good idea to change the name of this to something that reflects the data, particularly if you plan on having multiple choropleth maps from different datasets. The name that you choose here will also be displayed in the legend automatically generated for your map. Here we use the name: Map of Volunteering Rates across New South Wales LGAs (2011)

Once you have selected your parameters click Add and Display.

Outputs

Your map should automatically appear on the screen, and should look something like the image below. It will also appear under your Visualise pane on the right. You can turn the map on and off by clicking on the little map icon to the left of the name of it in that pane.

[Click to Enlarge]

[Click to Enlarge]

You can edit aspects of the choropleth, such as the parameters you’ve chosen for its visualisation, or changing its name, by clicking on the spanner symbol to the right of the map name in the Visualise pane