164 sYMBOLizaTiOn
One reason that the dasymetric choropleth is used less than the simple choropleth
is that the technique requires more information than what appears for an enumeration
unit in the census. Local knowledge of the area is necessary, which may be obtained
through fieldwork or imagery, to see where the subject data are concentrated.
Unclassed Choropleth
The
unclassed or
classless choropleth is a symbol suggested by Waldo Tobler in 1973.
Although not difficult in concept, this technique was not feasible before the era of
computer-assisted cartography because of the great difficulty
in creating the areal
symbols for enumeration areas. Cross-hatch patterns that vary in density were first
used to represent each enumeration area value, that is, each value on the map has
a separate pattern density. Now a continuous range of grays is used to create such
maps. Essentially no generalization is done for the reader. The unclassed choropleth
is the most exact representation of the data model possible, but it does not permit the
cartographer to stress specific characteristics of the distribution or present a region-
alization (Figure 8.24). It is useful in seeing the overall picture as a stage in classifica-
tion for simple choropleths.
design considerations for choropleth maps
Because choropleth maps use area symbols
and represent quantities,
the variables
most appropriate are color lightness, color saturation, and pattern combined with
texture. Pattern isn’t the first choice, but simple line or cross-hatch patterns combined
with varying textures were the only option for early computer-drawn maps. Color
lightness alone only permits a limited number of steps (also called
ramping). The
limitation is owing to the number of steps that the human eye can distinguish. The
range can be extended by adding saturation (see Chapter 4).
Lake
Swamp
Rural
Suburban
Urban
Rural
Suburban
Urban
0
1-50
51-200 Over 200
People per Sq. Mile
Uninhabited
Rural
Suburban
Urban
