Basics of symbolization 143
and emphasis. The specific purpose of the map must also be known. What exactly
will the map show? What is its theme? What will it be used for?
Another vital concern is the
nature of the data. What is their level of measure-
ment? Are they sequential or bipolar (diverging from a central value). Lower levels
of information cannot be portrayed by higher levels of symbolism. That is, data that
are in ranked form cannot be symbolized on an interval scale. If it is desirable for the
reader to be able to make estimates of value, the data must be collected with this end
in mind. The quality of the data is reflected in the quality of the map. A good map
cannot be made with poor data.
In addition, the visual variables do not work equally well for all types of data.
As we have seen, form works best with qualitative data and lightness works best with
quantitative data. Table 8.4 shows visual variables and the data types with which
they work best.
Map users vary widely in their map-reading abilities.
Map interpretation is a
skill that must be learned; it is not inborn, although some people seem to grasp the
concepts more quickly than others.
Since maps are, at least implicitly,
aimed at a
specific type of user, the symbolism chosen should suit the presumed abilities of that
user. Usually, the cartographer does not have a personal acquaintance with the pro-
posed user or group of users, so knowledge of these abilities is inferred from place of
publication. Even among users with roughly equivalent abilities, perceptions must be
considered. A major area of cartographic research has been the perceptual abilities
of users. Many studies were carried out in the 1970s and 1980s on how readers per-
ceive symbols and how well symbols communicate information. Maps made for the
visually impaired have different requirements than those prepared for readers with
normal sight. Since the advent of the computer, animations and interactive maps have
been studied.
20
20
20
20
20
40
80
80
80
80
60
60
60
40
40
40
40
40
40
ANNUAL RAINFALL
in inches
