Rainbow colour scales in hydrologic maps and charts

I’ve seen lots of discussion lately about the dangers of using rainbow colour scales (or Jet colour scales) in maps and graphs.  See #endrainbow on Twitter and Ed Hawkins articles:

and this article by Light and Bartlein from 2004:

The issue is that rainbow scales can ‘distort, mislead and confuse’ by creating false boundaries between values.  For design guides, the use of rainbow colour schemes is likely to increase error as users do not get an intuitive feeling if a value is high or low.  Rainbow colour scales come up in some hydrologic publications and many of the issues identified by Ed Hawkins, and others, are relevant.

There is some good examples in Australian Rainfall and Runoff.   The first figure below is a map of Baseflow Peak Factor using a rainbow scale.  It is not visually obvious that the reds are low values and the blues are high.  The second figure shows similar data using a sequential pallette.  High values are dark, low values are light.  This figure is much easier to interpret.

BaseflowPeakFactor-Rainbow

Figure 5.4.5 from Australian Rainfall and Runoff (Book 5).  The rainbow colour scale makes interpretation difficult.  Without looking at the legend would you think the red areas had high or low values?

BaseflowPeakFactor-sequential

Figure 5.4.3 from Australian Rainfall and Runoff (Book 5).  The sequential colour scheme leads to the obvious interpretation.  Light blues are low values, dark blues are high values.

There is some good advice on picking a suitable colour scale for maps and graphs (here).  In summary if you wish to show order in the data, then choose an ordered palette.  People expect that darker means more.  For departure from a reference value i.e. values can be either positive or negative (or above or below a value), a diverging palette may be suitable.

sequential

 

diverging

This issue has been around a long time but should be easy to fix, let’s start by improving the figures in Australian Rainfall and Runoff while it is still a draft.

 

One thought on “Rainbow colour scales in hydrologic maps and charts

  1. Nick Marsh

    Great post Tony. We have just been having this conversation about as we design our ‘default’ mapping colour schema.

    Reply

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