142 sYMBOLizaTiOn
must look the same on the map and in the legend. The symbol must be appropriate
for the purpose and audience of the map.
Frequently, several different symbol types will initially seem equally acceptable,
but upon consideration of all factors only a few will be truly suitable. Although GIS
can create a variety of symbols, the ultimate choice still resides with the cartogra-
pher.
Several factors are involved in making the symbol choice: the purpose and theme
of the map; the cartographer’s conception of the geographic phenomena; identifica-
tion of the basic spatial attributes of the phenomena; quality, nature, and measure-
ment level of the data; scale; abilities of the map user; cartographic conventions; and
compatibility of symbols.
Geographic phenomena, as we have seen, may be positional, linear, areal, or
volumetric, and they may also be continuous or discontinuous. Point symbols are not
limited to the representation of point data. Often it is desirable to symbolize areal
data at points. Figure 7.10 shows the number of farms in the western United States
by point symbols. Although the actual
data apply to an entire state, they may be
displayed as totals at point locations. In this example the points are at the centers of
the states. Volume phenomena may be symbolized by linear or areal symbols. Data
for continuous phenomena are often obtained only at certain points, for example,
weather stations for temperature or rainfall or control stations for elevations. In these
cases, although the data are determined at points, the actual distribution represents a
volume, and the distribution is shown with a linear symbol (Figure 7.11).
The
purpose and
theme of the map, as we have seen, must be considered at all
stages of the cartographic process but are especially important here. Several questions
might be asked: What are the general and specific purposes? The general purposes
of thematic maps are to provide qualitative or quantitative information for different
locations or to map the characteristics of the phenomena to reveal spatial patterns
NUMBER
OF
FARMS
Under 5,000
5,001-10,000
10,001-20,000
20,001-50,000
50,001-100,000
Over 100,000
