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I completely disagree with the first example - rainbow map vs. custom color map for height data on a geographic map. The custom color map is designed to show a

by maho 13y ago

I completely disagree with the first example - rainbow map vs. custom color map for height data on a geographic map. The custom color map is designed to show a step at 0, which highlights the coastline. Depending on what you want show, this is very misleading! Besides the water, there is nothing special about the coastline. The rainbow map, on the other hand, shows how shallow the slope of the coast is, and how sensitive the coastline is to changes of the ocean level. I also don't share the author's concern that rainbow color maps can introduce "bands" in continuous data. Maybe it's because I had to read so many of them over time, but in all except one of the examples, the rainbow color maps gave me at least as much information as the alternative encodings.

sillysaurus2 13y ago
I think both, side-by-side, convey more valuable information than either alone.
jacobolus 13y ago
If the goal is to merely map elevation to color in a way that best reveals slope, then both colorings are pretty terrible. What you’d want instead is to have uniform change in lightness [lightness differences are the most salient perceptually]. The big problem with “rainbow” gradients is that they typically have some sections with steep change in lightness, and other sections of nearly uniform lightness, mixed together with nonuniform (in terms of human perception) hue and chroma changes from one part of the gradient to another. It’s very difficult to compare the slopes of a rainbow-gradient-mapped function across different parts of the gradient, and in some sections of the gradient very difficult to visually distinguish salient details. It would be much better to have uniform changes in lightness. The simplest such map just goes from black to white in a perceptually uniform way. But if more distinctions are needed, then another possible method is to make the lightness attribute sort of sawtooth (or can potentially use more of a triangle wave, in some cases), while keeping chroma relatively constant and letting hue change close to uniformly. This makes a few places where the perceived lightness has either a change of slope or a discontinuity, but since the rate of lightness change is otherwise constant, it is still fairly easy to make comparisons. [I really should make a proper article/essay about this at some point, with some demonstrative images / interactive widgets: it’s much easier to show visually than to describe in text] In the case of the chart under discussion, if the goal is to show the slope across the shoreline boundary and compare it to the slope elsewhere, I would recommend having uniform lightness change from dark [low elevation] to light [high elevation], combined with a hue discontinuity at the shore boundary. Because lightness is the most perceptually salient color attribute for noticing object boundaries, textures, fine details, motion, etc., it would still be possible to have a good idea of the slope across the shoreline boundary, while being able to perceive the boundary when closely examining the image. [Note: such schemes will also generally be helpful for colorblind users]
hcarvalhoalves 13y ago
I think both maps are useful, it just depends on what information you are concerned about. The first map only shows the earth crust, so it's useful if you're only concerned with that. The second map, with the step at 0, raises the perception of a "hidden" feature, which is the ocean level.

by maho 13y ago

sillysaurus2 13y ago

I think both, side-by-side, convey more valuable information than either alone.

jacobolus 13y ago

If the goal is to merely map elevation to color in a way that best reveals slope, then both colorings are pretty terrible. What you’d want instead is to have uniform change in lightness [lightness differences are the most salient perceptually]. The big problem with “rainbow” gradients is that they typically have some sections with steep change in lightness, and other sections of nearly uniform lightness, mixed together with nonuniform (in terms of human perception) hue and chroma changes from one part of the gradient to another. It’s very difficult to compare the slopes of a rainbow-gradient-mapped function across different parts of the gradient, and in some sections of the gradient very difficult to visually distinguish salient details. It would be much better to have uniform changes in lightness. The simplest such map just goes from black to white in a perceptually uniform way. But if more distinctions are needed, then another possible method is to make the lightness attribute sort of sawtooth (or can potentially use more of a triangle wave, in some cases), while keeping chroma relatively constant and letting hue change close to uniformly. This makes a few places where the perceived lightness has either a change of slope or a discontinuity, but since the rate of lightness change is otherwise constant, it is still fairly easy to make comparisons. [I really should make a proper article/essay about this at some point, with some demonstrative images / interactive widgets: it’s much easier to show visually than to describe in text] In the case of the chart under discussion, if the goal is to show the slope across the shoreline boundary and compare it to the slope elsewhere, I would recommend having uniform lightness change from dark [low elevation] to light [high elevation], combined with a hue discontinuity at the shore boundary. Because lightness is the most perceptually salient color attribute for noticing object boundaries, textures, fine details, motion, etc., it would still be possible to have a good idea of the slope across the shoreline boundary, while being able to perceive the boundary when closely examining the image. [Note: such schemes will also generally be helpful for colorblind users]

hcarvalhoalves 13y ago

I think both maps are useful, it just depends on what information you are concerned about. The first map only shows the earth crust, so it's useful if you're only concerned with that. The second map, with the step at 0, raises the perception of a "hidden" feature, which is the ocean level.