Geography Reference
In-Depth Information
that consist of hues that have a perceivable order, like yellow to dark-red, yellow
to purple, or yellow to dark-blue. The addition “harmonical” refers to the
necessity to balance the scheme considering the saturation and object size
(cf. section “Color Contrast”).
2. Match values or class distances and color distances: The distance of elements
or represented classes is next to ordering (see above) and continuity type one
data characteristic (Andrienko and Andrienko
2006
). Class ranges can be equi-
distant, i.e., all classes have the same size, or they can be different according to
quantiles or natural breaks. Perceptual uniformity is often described as an aim
within cartographic science. In color systems like Munsell (
1905
) or CIE L*a*b
the Euclidean distance between any two colors fits with the perceptual distance.
For equidistant classes this approach is appropriate, but not for the representa-
tion of classes with different ranges or logarithmic scales. Logarithmic data is
special, it appears to be equidistant although classes of the antilogarithms would
not be. Equidistant schemes could give a wrong impression and classes with
higher values would be underrepresented. This has to be considered in the
representation. At this point, however, we cannot give any straightforward
recommendations about color distances.
3. Consistency of colors on a variety of output devices is a major aim. It is the aim
to use colors that can be recognized regardless of object size, adjacent colors, or
device in use and enable a correct assignment of the object colors to the color
patches in the legend. However, we have described many effects that counteract
this objective (cf. section “Color Perception”), therefore we recommend the use
of different hues in a harmonical hierarchy and fewer steps in lightness per
hue—in contrast to single-hue palettes with a high number of classes—to
support the recognition of colors.
4. Avoid colors that are not suitable for (situational) color vision deficiencies.
To consider (situational) CVDs is crucial to facilitate the accessibility of digital
maps (cf. section “(Situational) Color Vision Deficiencies”). Already small
improvements of the scheme, without completely changing it, turned out to
have positive effects (Kr¨ger et al.
2013
). Also a reduction of the amount of
classes and the use of colors with strong contrast can work against CVD.
Criteria for Decision Support
Each use case is different and therefore a consideration of the specific objective,
data characteristics and context is needed. Consequently we propose criteria that
support the decision for or development of a color scheme and help to set priorities.
Within the development process several criteria can be considered, but there need to
be priorities. Although the underlying color design guidelines are the same for each
use case, the aim to achieve a high distinguishability leads to other recommended
color schemes than the aim to highlight patterns. At this point we cannot present a
complete list, but an initial concept.
