Wednesday, 20 July 2016

Taste the rainbow - third helpings

If I'm going to have a carto-argument I may as well have it with the President of the United States.

Today, President Obama used this tweet to encourage us to be aware of rising temperatures and the need to stay safe and watch out for others. I have no qualms with the intent but as you'll appreciate, the cartonerd in me got quite irked by the leading statement.

"This map says it all". Well despite rainbow colour schemes being used to death to map anything and despite people preferring their own facts and telling me everyone understands the scheme I resolutely beg to differ.

I wrote about this problem several times before herehere and here. And so have others here, here and here. Rainbow colour schemes do not work well for numerical data where the analytical task is to determine what is higher and what is lower across the map.

Put simply, our brains do not process colour (strictly speaking hue) in a way that tells us a quantitative difference between them. Different hues (yellow, red, blue etc) work very well for showing qualitative differences on a map. Single hues, that vary from light to dark, or even a dichromatic scheme that blends a couple of colours do a much better job at encoding a quantitative difference.

In the map above, there's no logical consistency in the brightness of the colours. Some stand out more than others. Magenta is arguably the colour we see as the brightest yet there's a sort of light cream colour in there that actually means 'more'...and that's before we get to yellow. Yellow is not 40 degrees hotter than cyan on the spectrum either. Rainbow schemes are not perceptually uniform, they modify meaning beyond what the data supports and create visually false boundaries. You could also easily reverse the scheme and it'd make just as much sense.

The argument that people understand it is borne out of the simple fact they see them every day so they believe they understand them. They're conditioned to them. It's called brainwashing. I get it, We're all liable to a bit of brainwashing from time to time and just because you relate hot with red and cold with blue then it's all good OK?

Well no...because on this map yellow, magenta, a light pink and burgundy is hotter than red which happens to be in the middle of the spectrum. And how hot is green exactly? I've never been green-hot. I would still argue vehemently that without a legend these sort of maps are harder to interpret than they need to be.

Given in every other aspect of life people seem to want the easiest, laziest path of least resistance to something, why they force their brains to interpret rainbows baffles me. More so, the arrogance of constantly using your own facts to contradict cognitive science just shows a complete unwillingness to accept a better way. Hell, rainbow palettes aren't even 508c compliant!

End rainbows! #endtherainbow

1 comment:

  1. Ha, I just tweeted a similar thing. It's not just a rainbow, it's an angry rainbow of oversaturated colors. For some reason NOAA still frequently uses this scheme, including the comical spilling-out to fluorescent pink. Make it stop!

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