Geography Reference
In-Depth Information
Figure 11.2
A good example of legend selection for geospatial data, carefully selecting both colour
and grey-scale values with respect to the human visual system [data source from Peters
et al.
(2007),
colour choice adapted from Stone (2002) courtesy of Blackwell Publishing]
We list here four key features that can be observed directly within the brain through
visualizations, which then specify some of the limiting features of the human-visual
system.
Edges and change
The intermediate cells convert absolute numbers for colour and grey-scale values into relative
values over different sized areas. This means we effectively see only edges and local changes in
value over neighbourhoods and not precise values. It is, therefore, impossible to recognize
a specific intensity value except in relation to neighbouring intensity values. This has a
significant impact in the choice of values, for example, in legend colours (see an example in
Figure 11.2).
A simple illusion, based upon this principle and described by Ernst Mach, is illustrated
in Figure 11.3. It is impossible for a viewer to correctly estimate that the central small
squares are in fact the same intensity values. The exact reasons for these illusions are, as
always, contended by different researchers, but the effects in human response are statistically
significant and agreed upon.
Figure 11.3
Two versions of the famous Mach illusion (Adapted from Kaiser, 2007) www.yorku.ca
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