5.6 Heat (isopleth) maps

Commonly known as heat maps, density maps or isopleth maps use semi-transparent overlays above a map or other image (such as a web page) to show the density (or frequency) of events happening at each point on the underlying map.

In contrast to a choropleth map, where values are plotted for different predefined regions, heat maps show colour-based contour lines that connect points of equal value.

For example, Figure 17, a house price heat map from mousePrice, shows house price inflation in the north-west of England between May 2007 and May 2008.

Figure 17 Heat map showing house price rises in the north-west of England from May 2007 to May 2008 (mousePrice, 2009)

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This very busy screen image shows a patchwork of red, pink and blue areas of varying shades overlaying a map of part of the north of England. At the top there are 4 buttons and the text ‘Select transparency level’ with a button for 0%, 30%, 60% and 100%. Under that are radio buttons with the text ‘Select data’ with a button each for the following choices:

• Average values
• New builds
• 1 yr growth
• 5 yr growth (selected)
• 10 yr growth
• Crime rate

On the map town names can be faintly seen. At the bottom a key shows the percentage house price growth relating to each colour. From the left, yellow indicates no data, dark blue minus 10% through various shades of blue to pale pink for 3% up to dark red for greater than 20%.

Figure 17 Heat map showing house price rises in the north-west of England from May 2007 to May 2008 (mousePrice, 2009)

The ‘hot’ colours (reddish) are naturally taken to mean areas where there was a high one-year growth in house prices and the ‘cooler’ (bluish) colours to mean a lower increase in house prices over the same period – or, indeed, a decrease.

Heat maps have come to be widely used for plotting the incidence of crime within city confines, particularly in the larger US cities. An initiative in 2008 required UK police forces to start publishing crime maps reporting on the level of criminal incidents within their own jurisdiction.

Heat maps and density mapping techniques are also widely used for displaying radio propagation data and satellite coverage data. For example, the SatBeams [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] website uses density mapping to plot geographical areas that are covered by particular communications satellites.

Heat maps elsewhere

As well as being used as overlays on geographical maps, heat maps are also widely used to provide reports about website usage. Information can be collected at a crude level based on the links that users click through on a web page to produce a click-density map, although it is possible to also track mouse cursor movements, or, in a laboratory setting, collect eye-tracking data.

Figure 18 shows the result of eye-tracking and mouse-clicking data collected and aggregated from multiple users of the Google website (Enquiro Search Solutions, Inc., 2005). The hot spots (red, orange and yellow colours) are the places on the page that the users were looking at most, and the purple crosses show where users clicked on the page.

Figure 18 Google golden triangle (Enquiro Search Solutions, Inc., 2005)

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In this screen image a Google search result is shown although the actual results cannot be distinguished. The page is coloured from bright red through yellow to blue with a number of pink crosses showing. The main dark red area is a triangle at the top left hand corner. A larger pale orange triangular border fans out across the page with an even larger yellow triangular border next. Blue areas cover the remainder of the text on the left-hand side. The centre of the page where there is no text is black and then there is a further blue area to the right over the sponsored links list. The sponsored links at the top of the list on the left of the screen fall into the yellow or even orange area.

Figure 18 Google golden triangle (Enquiro Search Solutions, Inc., 2005)

As well as supporting an understanding of user navigation behaviour on websites, eye-tracking heat maps can also be used to understand better how people read from the screen.

Activity 15 (exploratory)

Read F-Shaped Pattern For Reading Web Content, an article by Jakob Nielsen.

What do the eye-tracking results suggest about how people read web pages? How does the visualisation used in the Google ‘golden triangle’ screen make this sort of generalised pattern of behaviour apparent?

Activity 16 (exploratory)

Suggest two drawbacks of each of:

1. choropleth maps
2. heat or isopleth maps.

Comment

• a. 

  1. The fixed area boundaries in a choropleth maps suggest a discontinuity in the values portrayed either side of an area boundary, and different values of a measure within a geographical area are not shown.
  2. The use of a limited colour palette means that choropleth maps are unable to depict a wide range of actual values.

• b. 

  1. Heat or isopleth maps require a large number of data points in order to identify equal-value lines of a particular measure.
  2. Heat maps cannot be used in cases where the data is largely discrete or discontinuous – for example, the course codes of Open University courses being studied in households in a particular OU region.