About the rainbow color map

This is a complementary ‘sub-page’ for the entry “Categories“.

“What’s wrong with this picture?”

In Rogowitz & Treinish’s (1996) own words (emphasis ours) :
“Would it surprise you to learn that the two pictures not only show the same data, but are represented by colormaps which are mathematically equivalent? In both cases, each value of the continuous variable, elevation in meters above and below sea level, has been mapped onto a unique value on a continuous pseudo-color scale. In the left-hand picture, elevation at each point has been mapped onto the most commonly-used colormap in visualization, the so-called “rainbow” colormap. In this hue-based colormap, show to the right of the visualization, the lowest value is mapped to blue, the highest value is mapped to red, and the intervening values are mapped continuously onto values interpolated between these two colors in red-green-blue space. In the right-hand picture, elevation has also been mapped onto a pseudo-color map. However, in this case, the colormap has been designed to take into account characteristics of the data and the human visual system. In particular, the colormap has been designed so that equal steps in the data variable will be perceived as equal steps in the representation. Since the data has a threshold value or boundary of interest to the user of the data (i.e., sea level), this characteristic of these interval data is also explicitly incorporated into the colormap. While comparing these two representations of the same data, the first thing you may notice is the importance of capturing zero-crossings or other thresholds in the data. In this case, preserving the zero-crossing at sea level allows the automatic demarcation of the coastline, which is obscured with the rainbow colormap. Another artifact produced by the rainbow colormap is the false impression created about the structure of the surface topography and the ocean bathymetry. Looking at the color bar, you clearly see that there is a blue region, followed by a cyan region, a green region, a yellow region and a red region. Perceptually, it appears as if there are only five discrete elevations, when in fact, the data set contains a sampling of a continuous function. The perceptual colormap on the right more accurately conveys the continuous nature of the data. The ocean depth decreases gradually over the continental shelf, then decreases steeply. Above sea level, the height of the coastal plain increases gradually in shades of green up into the maize of the low-lying mountains. In addition, artifacts due to the limited resolution and sampling inherent in this data set are visible in the perceptual colormap, but are hidden by the rainbow colormap.” [text copied from here: http://www.research.ibm.com/people/l/lloydt/color/color.HTM]