ABSTRACT. Cartographers have long discouraged the use of spectral, or rainbow, color schemes on thematic maps of quantitative geographic data, though such color use is common in GIS and scientific visualization. Recent research, however, has shown that spectral schemes are preferred and are interpreted accurately when used as multi-hue renditions of diverging schemes. Both spectral and diverging schemes can emphasize a critical point within a data range with light colors and emphasize both high and low extremes of the data with dark colors. Although spectral schemes include multiple saturated hues, they can be designed to accommodate map reading by people with red-green impaired color vision by skipping over the yellow-greens in the spectral sequence. Cartographers should encourage use of spectral color schemes for depicting diverging quantitative data, rather than insisting that these schemes should not be used.
KEYWORDS: cartography, map design, spectral color schemes, color vision
Spectral color schemes are common on rainbow weather maps in the daily news, in scientific visualizations, and in GIS-based mapping. Yet, cartographers continue to discourage their use for representing quantitative geographic data on thematic maps. Spectral schemes are preferred and interpreted accurately when used as multi-hue renditions of diverging schemes (Brewer et al. 1997), and careful variation in both hue and lightness within spectral schemes aids map reading. Like a diverging scheme, spectral schemes can be designed with light colors emphasizing a critical point within a data range and dark colors emphasizing both high and low extremes of the data. The multiple saturated hues of spectral schemes may confuse people with red-green color-vision impairments but, again, careful design permits spectral schemes to accommodate these map readers. Cartographers should encourage use of well designed spectral color schemes for thematic maps of diverging data, rather than generally discouraging use because of inappropriate applications of spectral schemes to sequential data.
This paper begins with a critique of the assumption that spectral schemes are illogical. Examples of diverging data and spectral color use are presented from mapping in cartography, geography, and scientific journals. Results from two research projects in which spectral schemes led to good map reading performance by people with both normal and impaired color vision are described. Finally, the suggestion that spectral schemes do not accommodate people with impaired color vision is addressed with a recommended adjustment to the spectral sequence.
Bertin's writings (1981; 1983) are influential in shaping our discipline's approach to data representation. His opinions about the use of color hue are cast in a strident tone: "I am indeed against color when it masks incompetence; ... when people believe it capable of representing ordered data" (Bertin 1981, p. 222). The following sample of recent quotes about spectral schemes by cartographers and other authors echo and elaborate Bertin's opinions. Rarely do we find this degree of consensus or emotion expressed about a subjective challenge of symbolization. We have been recommending that spectral schemes should not be used to represent ordered data. These claims have established a conventional wisdom that spectral schemes are illogical and inappropriate for the representation of quantitative data. My own earlier writings on the issue are no exception, as can be seen in the following list of statements:
Do not use the saturated spectrum as a sequential scheme. [Brewer 1994a, p.
138] There appears to be general agreement among cartographers that the
color dimension of value [lightness] be used to symbolize an ordered array
of data magnitudes ... Hue differences alone should not be used, but if
they are, part-spectral schemes are preferred over the full-spectral ones. …