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sensitivity curve (Welsch and Liebmann
2012
). Affected people therefore perceive
colors slightly differently. Additionally, CVDs appear more often during screen
handling for a small field of vision (FOV) for up to 2
even though persons
concerned do not have any CVD with a FOV around 10
(Schumann and M¨ ller
2000
). Results of an experiment showed that 23 % percent of participants were not
able to identify numbers on two Ishihara plates while only 13 % of the participants
indicated that they in fact had some type of CVD (Weninger
2013
). We explain this
as a result of
situational
CVD caused by different devices and thus various patterns
of use, screen contrast, visual angles and FOV. The issue of
situational
color vision
deficiencies and its consequences on map design, according to our best knowledge,
has not been studied in cartography, but suggests important insights to support web
mapping for inclusion.
Scheme Types
Brewer (
1994
) identifies two kinds of schemes besides qualitative schemes: sequen-
tial and diverging. Diverging schemes have a positive and a negative end which are
divided by a neutral point. Sequential schemes can represent ordinal or numeric
data in a perceptual order and consist of one or more hues in different lightness
steps. If multiple hues are used in combination it has to be assured, preferably by
means of a change in lightness
and
saturation, that they can be brought in a
perceptual order. Multi-hue schemes with at least one hue-transition are especially
reasonable for more than five to seven classes.
As mentioned before, color is highly associative. Therefore the specification of
different kinds of scheme types is important to find a suitable scheme for a use case.
We therefore suggest a further subdivision of sequential schemes on the basis of
some of Imhof
s scheme types (Imhof
1965
). This characterization is an important
basis to decide on the level of abstraction that is described as one decision criteria
(see section “Criteria for Decision Support”). His research into relief presentation
involved a comprehensive analysis of color schemes as well. Relief presentation as
the representation of classified heights is comparable to the presentation of other
thematic contents, like physical distributions and phenomena that are usually
represented with a low level of abstraction. Amongst other types Imhof defined
the following ones:
'
Contrasting schemes: Contrasting colors are supposed to enhance distinguish-
ability. The amount of contrast of single-hue schemes is defined by the lightness
of the first and darkness of the last class in connection with the number of
classes. Many single-hue schemes therefore do not contrast much. In addition,
the amount of contrast of multi-hue schemes is determined by factors described
in section “Color Contrast”. It should be adjusted in line with the color design
guidelines and decision criteria, that are illustrated in the subsequent section.
