scale, Compilation, and generalization 89
Reader’s abilities
In all cartographic activities the reader must be considered. Although the cartogra-
pher does not always know the specific skills of the audience, some perceptual abili-
ties are considered universal. For example, the normal human eye cannot distinguish
more than six shades of gray between black and white on a map. Therefore, it is
pointless to use 12 different shades of gray to illustrate 12 different categories on a
statistical map. Either a different method of symbolization must be found or the data
must be grouped to form fewer categories.
The eye cannot perceive very fine lines or read lettering smaller than about 0.04
inch; therefore, to use such fine lines or small letters defeats the communication pur-
pose of the map.
Environmental factors must also be considered. As discussed in Chapter 2,
the requirements for a map that is to be viewed in the
low-light conditions of an
airplane cockpit are different from those for maps that will be viewed in a brightly
lighted room; a map designed for an automobile GPS screen has different require-
ments than those of a paper road map. These factors affect the degree of simplifica-
tion, the number of categories, or the type of symbolism. Sometimes it is necessary
to perform tests with focus groups to determine the amount and kind of generaliza-
tion needed.
Cartographer’s equipment
and skills
Although most professional maps and many amateur maps are produced with the
assistance of computers, the kind of equipment used and the skills of the mapmaker
are still important. Screen, printer, and scanner resolution all affect the amount of
detail that can be shown or printed. Software capabilities are an important factor.
Programs vary widely in their design and generalization usefulness. Much software
is updated on a nearly annual schedule, so the mapmaker needs to be aware of its
current status.
While drafting skills aren’t
important for working with GIS,
illustration, or
CADD programs, skill at using the programs is. It takes time to master any software
program. While one might produce a simple map without having complete mastery of
a program, one requires more advanced skills to create more sophisticated maps.
Quality
of Data
The quality and nature of the data must also be taken into account. Imprecise or
poor data do not permit one to make a precise map. The data may not be proper
for the kinds of symbols the cartographer wants to use. For example, a quantitative
map cannot be made from qualitative data. If the data form is inappropriate, it is
necessary either to find suitable data or to change the map type. One must also be
aware of bias. Unless he or she is making a map designed to persuade, the cartogra-
pher should strive to be objective. One must realize that data from special-interest
organizations might be highly subjective. Bias can also enter the map through selec-
tion of the phenomena to be mapped, the data used from a set, or the control points
used.
