184 sYMBOLizaTiOn
.36
.35
.30
.10
.06
.02
.62
.68
.31
.34
.27
.46
Precipitation (inches)
30
40
50
60
70
80
90
100
110
110
100
90
80
70
60
50
40
30
Jan
Feb Mar Apr May Jun
Jul
Aug Sep Oct
Nov Dec
Temperature ( F)
o
Temperature ( F)
o
Average Temperatures and Precipitation in Joshua Tree N.P.
Highs
Lows
fIgURe 9.8.
Climographs can be used as point symbols on
a map or they can be placed
outside the subject area.
High
Medium
Low
Low
Medium
High
fIgURe 9.9.
Black-and-white bivariate choropleth chart.
Multivariate Mapping 185
The color choices are described more fully in Chapter 4, but the basic concept
is shown in Figure 9.9 and Plate 9.2. In general, the color schemes used are pairs of
sequential schemes, pairs of diverging schemes, or paired sequential and diverging
schemes. In Plate 9.2 the simplest form is shown, a paired sequential scheme, using
only two hues that vary in lightness. The three subtractive primaries of cyan, yellow,
and magenta are used to create nine different combinations. Figure 9.10 shows the
percentage of each primary used in order to obtain the nine colors. This scheme is
easy to understand and create. More sophisticated schemes can be made by varying
saturation as well as lightness.
Most commonly, the legend on bivariate choropleth maps takes the form of a
rectangular grid on which positive and negative diagonals progress from low to high
or high to low.
Judy Olson has provided guidelines for creating bivariate choropleths that were
summarized by J. Ronald Eyton (1984):
1. The colors should be distinguishable and the transitions should be smooth
and visually logical.
2. One should be able to distinguish individual categories, and one should be
able to differentiate the two distributions.
3. The arrangement of colors in the legend should correspond to the arrange-
ment of a scatter diagram of the distribution.
4. The colors should progress in lightness from light to dark with high data val-
ues represented by dark tones and low data values by light tones.
5.
To convey a relationship, the positive and negative diagonals should have
visual coherence.
6. The bivariate map should be constructed as a direct combination of the spe-
cific sets of colors assigned to the two individual maps.
7. The combination of colors on the two individual maps should look like com-
binations of the specific colors involved.
8. The number of categories should not be so large
that the reader is over-
whelmed.
100 C
100 C
100 C
0 M
50 M
100 M
100 Y
50 Y
0 Y
50 C
0 M
100 Y
50 C
50 M
50 Y
50 C
100 M
0 Y
0 C
0 M
100 Y
0 C
50 M
50 Y
0 C
100 M
0 Y
