Multivariate Mapping 179
fIgURe 9.1.
Two variables can be shown by two maps.
(AWAII
5-10
10.1-15
15.1-20
20.1-25
% of Population
in Poverty
Poverty and Crime by State
2005
Data Source:
U.S. Statistical
Abstract 2005
Violent Crime Per
100,000 People
under 150
151-300
301-450
451-600
601-800
fIgURe 9.2.
The data for the maps on Figure 9.1 are combined on a single map using chorop-
leth for poverty and proportional circles for crime.
180 sYMBOLizaTiOn
open versus solid circles, or the shape of the dots can be varied, such as a circle and
a square or a triangle. However, with small symbols it can be difficult to distinguish
between solid
circles and solid squares, so multiple dots work best with color maps.
Other dot methods have been suggested for showing multiple variables, such as
the
pointillism technique put forward in the 1950s by George Jenks that employs dif-
ferent colored dots to represent different crops or products in areas of overlap. Jenks
reasoned that map readers would see the dots blending and visualize areas of transi-
tion. In the 1990s, Alan MacEachren and
David DiBiase suggested the chorodot,
which used small shaded squares within enumeration areas. The shading represents
quantities, as on choropleth maps, but the squares themselves do not represent enu-
meration areas. Neither of these techniques has been widely used, but both might
bear revisiting with increased use of animation.
