186 sYMBOLizaTiOn
Because of the complexity of the map and legend, a three-by-three legend is easier
to comprehend than a four-by-four or five-by-five. A five-by-five legend requires the
reader to compare 25 different colors and understand their meanings.
Plate 9.3 is a simple bivariate map that combines the information on Figures 9.1a
and 9.1b.
While three-variable (
trivariate)
choropleth maps can be made, they are less
common because they are more difficult for the reader to understand. Plate 9.4 is an
example of a trivariate color chart for a map on which the hues are the three subtrac-
tive primaries.
Because multivariate maps are more complex than single-variable maps, the deci-
sion to create such maps should take into account the probable map-reading sophisti-
cation of the reader and whether the purpose of the map is to provide detailed infor-
mation or general patterns. Specifically, will the patterns and relationships presented
be shown effectively
with a multivariate map, or will a map series be more useful?
sUggestIons foR fURtheR ReAdIng
Brewer, Cynthia A. (1994). Color Use Guidelines for Mapping and Visualization. In Alan M.
MacEachren and D. R. Fraser Taylor (Eds.),
Visualization in Modern Cartography (pp.
123–147). New York: Pergamon.
Eyton, J. Ronald. (1984). Map Supplement:
Complementary-Color, Two-Variable Maps.
Annals of the Association of American Geographers,
74, 477–490.
Harris, Robert L. (1999).
Information Graphics: A Comprehensive Illustrated Reference.
New York: Oxford University Press.
Olson, Judy M. (1981). Spectrally Encoded Two-Variable Maps.
Annals of the Association of
American Geographers,
71, 259–276.
Slocum, Terry A., et al. (2005).
Thematic Cartography and Geographic Visualization (2nd
ed.).
Upper Saddle River, NJ: Pearson Prentice Hall.
Tufte, Edward R. (1983).
The Visual Display of Quantitative Information. Cheshire, CT:
Graphics Press.
Tufte, Edward R. (1990).
Envisioning Information. Cheshire, CT: Graphics Press.
