Designing and Choosing symbols 173
Creating isarithms
Before data collection begins, a researcher selects control points for which the data
are gathered. For isometric maps, these points may already exist, such as weather
stations, but for isoplethic maps the points are conceptual and must be chosen. There
are many options, and the choice of control point makes a difference in the look of
the map. Three examples are centroid of an enumeration area, center of the data dis-
tribution, and some known place, such as a capital city or county seat. As can be seen
from Figures 8.36a, 8.36b, and 8.36c, three different population maps of Arizona
result from these three choices. For some kinds of studies, especially biogeographic
studies, the points may be simply regularly spaced. In this case, you should avoid a
rectangular pattern because it results in ambiguous situations (see Figure 8.37). Tri-
angular or hexagonal arrangements work best.
A second decision is the isarithmic interval. For isometric maps, ideally, the inter-
val chosen is even—for example, 10, 20, 30 or 20, 40, 60. This makes the map easier
to interpret since the spacing of the lines tells the reader the nature of the surface.
If an uneven interval is chosen, the line spacing can be misleading (see Figure 8.38).
For isoplethic maps the data often have sharp differences and wide ranges, making it
difficult to choose an even interval. In this case, the uneven interval must be clearly
shown in the legend. The techniques for choosing categories for choropleth maps may
be used for isopleth maps.
Design
Considerations
The line widths for isarithmic maps are uniform; thus, the major design consideration
is whether to shade between the lines and what scheme to use for shading. If other
information will be shown on the map in addition to the isarithms, as on topographic
maps, the lines are simply numbered and there is no other variable; however, it the
data represented by the lines are the primary subject of the map, shading called
layer
2
2
2
2
2
2
2
2
2
10
10
10
10
10
10
50
50
50
50
50
100
200
100
200
100
200
100
50
100
Population Per Square Mile
fIgURe 8.35.
Isopleth map showing population density for Australia.
