Designing and Choosing symbols 151
circle would be 1.4 times larger than the smaller circle. One advantage of creating
proportional symbols with computer software is eliminating the drudgery of calcu-
lating radii.
Graduated and proportional symbols are used for the following:
1.
When the range of data is too great to be represented by dots. If the dots in
the most dense area stack on top of one another so that the enumeration area
appears
solid black, a proportional symbol might be a better choice.
2.
To symbolize totals over an area, such as total water power for a region.
3.
To symbolize totals at a point, such as populations of cities. Sometimes these
symbols
are combined with dots, with dots showing rural population and
circles showing urban population.
4.
To compare one subdivision with another, for example,
counties within a
state or states or provinces within a country.
For proportional symbols, a different size can be calculated for each enumera-
tion area; thus for county populations in California, fifty-eight circles would be used.
Figure 8.6 is a proportional circle map for the San Francisco Bay Area. It and the
variations in Figures 8.7, 8.8, and 8.9 were created from the data in Table 8.1.
A problem with truly proportional circles is that readers are not likely to cal-
culate the value for each of the circles; indeed, this information can be shown more
effectively in a table. Thus, the range of data can be divided into several categories
and those categories are represented in the legend. The figures are drawn propor-
tional to the midpoint of the range for each category. These are called
range-graded
figures (Figure 8.7).
A third way of representing data with figures of different sizes is simply choosing
BAY AREA POPULATION
BY COUNTY
Santa Cruz
1,820,176
San Francisco
817,537
Napa
135,554
fIgURe 8.6.
The circles on this map are truly proportional to the quantities they represent.
152 sYMBOLizaTiOn
a series of arbitrary sizes (graduated symbols) that are not proportional. If the pur-
pose is merely to show quantities, not relate specific values to one another, this can
be a simple method. This also works for data that are ranked or ordered, but have
no values attached, such as small, medium, large or village, town, city, metropolis
(Figure 8.8). It is important that these circles be shown clearly in the legend.
Other proportional figures such as squares, triangles, and even human figures
can be used, but not as much research has been undertaken on these as on circles. The
principle is the same as for graduated circles—that is, the area of the figure is propor-
tional to the quantity represented. The size of squares may be easier to estimate for
readers, and it may be easier to compare areas of squares than areas of circles (Figure
8.9). There are, however, some design problems that arise for these symbols. It is
more difficult to orient squares than circles on the map if the projection used has radi-
