On the Ocean

Visualizing Oceanographic Data: Perceptually uniform

In some cases, oceanographers want to plot data spatially, rather than against time or along a transect, giving a two- or 3-dimensional snapshot of the marine environment at a given point in time. In these cases, such as visualizing topography, or sea surface height or temperature, line or scatter plots won’t work. To plot these types of data, oceanographers use colormaps.

A colormap links data values with different colors across a spectrum. The colors chosen are very important for clearly displaying oceanographic data. A common type of colormap is the rainbow, including all colors between red and violet. Rainbow colormaps are useful if an oceanographer wants to be able to find specific values, or a narrow range of values, on a figure very easily. Unfortunately, using the rainbow can be detrimental because the large number of shifts in color, coupled with the fact that several colors stand out more than others, may lead to the appearance of structure within the data that does not exist. Bright yellow or cyan, for example, stand out strongly and tend to draw the eye of the viewer.

Fig 1: Here, three colormaps are shown. In the black and white colormap on the left, there is a perceptually uniform shift between dark and light. In the middle rainbow colormap, there is no perceptual uniformity; different colors stand out, and there are rings which do not appear in the black and white colormap. In the final colormap, the shift from blue to yellow-green is perceptually uniform, as well.

Oceanographers prefer to use colormaps that are perceptually uniform. A perceptually uniform colormap is one in which equivalent shifts in data are accompanied by an equivalent perceived shift in the colormap across the range of data values. Scientists at Texas A&M University use a variety of perceptually uniform colormaps for different types of data, and have put effort into developing effective colormaps for different data. A perceptually uniform colormap ranging from a cool purple to warm orange is often used when plotting ocean temperature data. The darker color intuitively indicates colder water, and the orange warmer water to the viewer.

Fig 2: A global map of sea surface temperature using a perceptually uniform colormap. Figure from NOAA, , http://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v4.html

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links please go to ocean.tamu.edu and click “On the Ocean.”

Contributing Professor: Dr. Kristen Thyng, click here to visit her personal website.

Script Author: James M. Fiorendino