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chapter 8
symbolizing geographic Data
Excellence in statistical graphics
consists of complex ideas
communicated with clarity, precision, and efficiency.
—E
dward
R. T
ufte
,
The Visual
Display of Quantitative
Information (1983)
As
we saw in the previous chapter, visual variables are
used to determine symbol
types to represent geographic phenomena and to vary their design. However, a num-
ber of kinds of symbols are used on maps. I will discuss symbolizing data at points,
along lines,
over areas, and as volumes.
symBolIzIng dAtA At PoInts
Two kinds of data are symbolized at points: those that actually occur at points, such
as locations of places on small-scale maps, and data that are aggregated at a point,
such as totals for counties or other enumeration areas. In one sense, this is a function
of scale. Cities, for example, actually cover area, but even a sprawling city like Los
Angeles is merely a point on a small-scale world map.
Both qualitative and quantitative data may be shown by point symbols.
Qualita-
tive symbols may be pictorial, associative, or geometric and most often use the vari-
ables of shape and hue. Qualitative maps tend to be fairly straightforward.
Quantita-
tive point symbols may also be pictorial, associative, or geometric and most often use
the variables of size and location (Figure 8.1).
