In case you aren’t subscribed to NASA’s Earth Observatory, I would like to draw your attention to their ongoing series called Subtleties of Color. Part One started on the 5th of August, and Part Two is up already as well. Written by Robert Simmon, they go into the history of color mapping at NASA, starting from some ‘paint-by-numbers’ of data from Mars in 1964.
One thing that Simmon writes, almost offhand, is how important color choice can be.
‘Careful use of color enhances clarity, aids storytelling, and draws a viewer into your dataset. Poor use of color can obscure data, or even mislead.’
This is putting it mildly, that color can obscure and mislead. How to Lie With Maps is already in it’s second edition and should be required reading for anybody who wants to create a map – however basic. Different ways of breaking the the data into classes can dramatically change how people interpret the data. Some things should be based on population density, some things based on geographical area, some would be better with points. Population per square mile could be shown as choropleth without any issues, but straight number of crimes commited in an area will likely only give you a different population map. Crimes are more likely to occur where there are people, it is as simple as that. That is where creating points that equal a certain number of crime per population would be better. This of course all requires a clear color scheme, and we’re right back to Simmon’s articles.
In the second post, he emphasizes a gradual change from colors when choosing a color palette.
A color palette that combines a continuous increase in lightness with a shift in hue is a good compromise that preserves both form and quantity. These three palettes show the smooth, even gradations that result from color scales calculated in perceptual color spaces. Color scales with varied hues and contrast are suitable for representing different datasets. (After Spence et al. (1999), chroma.js, and Color Brewer.)
In general, I highly recommend reading the articles and following along for the next four.