Designing and Choosing symbols 161
simple Choropleth
Conceptually, the simple choropleth is a relatively uncomplicated type of symbol.
Values are determined for each enumeration area and
then placed into categories
or classes; shading patterns or hue lightness steps are chosen for each category and
applied to the enumeration areas. The number of classes is limited by the number that
the human eye can recognize. Very little information is needed other than boundaries
and values (Figure 8.19).
LiMiTaTiOns OF siMPLe CHOROPLeTH
Because shading or patterns cover each enumeration area uniformly, it is not possible
to show variation within enumeration areas. This is a limitation of the choropleth
symbol. Thus, a choropleth map should not be used when the purpose is to compare
enumeration areas. Figure 8.20 illustrates this problem. In the county highlighted,
the population is concentrated in one corner, but the uniform shading can give an
inexperienced reader the impression that population is evenly spread throughout the
county.
The boundary lines on choropleth maps have no values attached to them; they
are simply the lines outlining the enumeration area polygon: county or state boundar-
ies, census tract borders, or Zip code boundaries, for example. A change in pattern
or color does not represent a change in value along the boundary line. It only tells the
reader that the two adjoining
areas have different values. This is a second limitation
of choropleth maps: exact values cannot be determined.
United
States Population Density
Population Per
Sq. Mi.
Over 1000
501-1000
101-500
11-100
1-10
