Designing and Choosing symbols 153
BAY AREA POPULATION
BY COUNTY
100,000-500,000
500,001-1,000,000
1,000,001-2,000,000
TOTAL POPULATION
fIgURe 8.8.
The circles here are arbitrarily chosen and are not proportional to the quanti-
ties.
Population
in Thousands
100-500
501-1,000
1,001-2,000
BAY AREA POPULATION
BY COUNTY
fIgURe 8.9.
Proportional squares can be used instead of circles.
154 sYMBOLizaTiOn
ating meridians. The squares cannot be oriented to the meridians without creating a
“tipsy” appearance; therefore they must be lined up with the borders of the map. The
same problems are encountered with triangles and other geometric figures.
Segmented proportional symbols or
pie charts are used in combination with pro-
portional or point symbols to show a breakdown of total value, such as independent
voters as a percent or proportion of total voters. These will be described in Chapter
9 on multivariate data.
Proportional spheres and Cubes
If the range of data is so great that the largest circle overwhelms the map and the
smallest figures are insignificant (Figure 8.10), a three-dimensional
figure such as
a sphere, a pyramid, or a cube can be used. For such symbols, the
volume of the
symbol, not the area, is proportional to the quantity represented. These figures can
result in very attractive maps, but readers have difficulty in interpreting some of these
symbols.
Proportional spheres are probably the most difficult to interpret. First, the same
problem arises as with circles, that is, estimating and comparing sizes. Second, because
the symbols only simulate the third dimension and are placed on a two-dimensional
map, readers have a tendency to compare or estimate areas rather than volumes. The
procedure of creating proportional spheres is similar to that for creating graduated
circles except that the cube root of the quantity is used instead of the square root
because the volume of a sphere
is represented by the formula V =
π
R
3
.
Columns or piles of blocks whose volumes are proportional to the amount repre-
sented are also used. While these are striking, some users have difficulty visualizing
