172 sYMBOLizaTiOn
the isarithms. Evenly spaced lines represent an even rate, closely spaced lines repre-
sent
a steep gradient, and widely spaced lines represent a gentle gradient.
The population map in Figure 8.35, while similar in appearance to the rainfall
map, is an isoplethic map. The isopleth does not permit the estimation of population
at any point because the control points represent values for areas, not actual points.
That is, the values describe population per square mile, not numbers of people at spe-
cific points. This map does, however, permit the reader to note patterns and general
gradients. In the same way that rate of change can be determined from an isometric
map, areas of rapid or gradual change can be found on isoplethic maps.
300
200
100
0
0
100
200
300
fIgURe 8.33.
Formation of isarithms.
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
60
35
30
22
24
40
15
12
8
9
8
8
9
7
30
14
23
5
7
8
13
19 14
9
16
14
17
22
