Basics of symbolization 139
guish between different nominal categories. If a circle is used to represent cities, the
size of the circle can be varied according to the population of the city. Linear symbols
may vary in width and be either single or double lines. The gauge of a line (line width)
indicates relative importance or actual amounts. The size of area symbols, like shape,
is governed by the extent of the area symbolized and therefore this variable does not
apply in the symbolic sense.
Each of the three dimensions of color, hue, value, and intensity (see Chapter 4)
can be used as a visual variable.
Hue is a very strong distinguishing characteristic for
those with normal color vision and is effective when similar symbol forms or sizes
must be used, such as red lines for roads, black for railroads, blue for rivers, or differ-
ent colored dots to represent different agricultural products. Normally, hues should
be used to represent differences in kind, that is, different categories, rather than dif-
ferences in amount. Temperature maps commonly show cool temperatures in shades
of blue and warm temperatures in shades of red; here the difference between cold and
hot can be conceived of as different categories.
Differences in
color value or
lightness refer to the range of shades from light to
dark within a hue, such as light gray to black on a black-and-white map or pink to red
on a color map There are limits to the number of value steps that can be used because
the human eye can only distinguish six to eight steps.
Although lightness can be used to represent differences in kind within a category
such as vegetation (e.g., all woodlands in lightness steps of green), it is not a good
practice to use lightness to distinguish between categories. The common convention
is for differences in color value or lightness to symbolize
differences in degree or
amount. Normally light tones are used to represent smaller amounts and dark tones
are used to represent larger amounts. Differences in
saturation combined with light-
ness can be used to show subcategories within a larger group, for example, vegetation
types.
Symbols have not only hue or tone but also pattern or texture.
Pattern is the
spatial arrangement of smaller elements within the symbol.
Texture is made up of
an aggregate of smaller elements, such as dots that produce an overall impression
of coarseness or smoothness. Pattern is less evident in
fine-textured symbols than
in coarse ones. Texture and pattern can only be applied to symbols that cover some
area, although they are not limited to areal symbols; they can be used on circles, for
example.
Orientation refers to the direction of the symbol mark or of the patterns used
within the symbol. Orientation of the symbol may show the direction of the feature
symbolized, such as road direction, or it may be used with pattern to represent differ-
ences in kind. If the direction of symbols has no significance to the distribution and is
not a part of an areal pattern, then the symbols should be oriented or aligned with a
common reference line, such as the map margins; otherwise, the reader assumes some
significance for the orientation (Figure 7.7). Different orientation of patterns can be
used for differences in kind, although it is not the best choice, but should not be used
for quantitative maps.
Some authors consider
location to be a visual variable. This is certainly the case
when drawing roads or rivers, when point symbols are placed in their actual geo-
graphic positions, or when areas are colored according to land use. However, sym-
bols are not always placed where the phenomena occur; sometimes this is a result of
140 sYMBOLizaTiOn
the nature of the symbol and sometimes because of the data. A line showing average
traffic volume on a freeway may approximate the actual route; a circle that symbol-
izes the total number of cattle for a state may be placed in the center of the state, not
where the majority of cattle are found.
Boundaries, especially of areas, may not be clearly defined or there are transi-
tions between one category and another. These “fuzzy boundaries” may be areas of
uncertainty, or may indicate a transition; placing a solid line could mislead the reader
into thinking there is a sharp boundary where there is none. In this case the variable
of
crispness can be applied. A line that appears blurry or less crisp can be used to
outline the area. The variable of
transparency can be used to indicate areas of overlap
(Figure 7.8).
$30,000-34,999
$35,000-39,999
$40,000-44,999
Per Capita Income
2007
