Simplification reduces the amount of geometric detail for individual lines or
areas. This can be done by arbitrarily removing points that describe the
shape of a line or areas, or can be done with several algorithms, or can be
done by hand. In each case, varying degrees of changes to the positions
result, leading to possible distortion.
Illustration of simplification operation.
The characteristics of things and events stored as GI, or associated with the
corresponding GI, need to be clearly organized for communication.
Humans can only understand a certain amount of information at once. The
rule of thumb for the cartographic representation is to use three to seven
categories. Of course, this “rule” is violated plenty of times. It really serves as
a guideline that is helpful for thinking about cartographic design. For car
navigation or mobile phones, three categories is probably the ideal starting
point. A land-use map or study of soils will require many more categories,
which does make such maps harder to read, but only then can the map meet
the professional or scientific needs for which it was created.
Classification is used for quantitative data, including counts, measure-
ments, and calculations. Ordinal, interval, and ratio data can all be classified.
Nominal data can also be classified, but only as individual categories or by
clusters—for example, land-use types by generic land use categories, such as
urban, mixed, and rural.
Classification is important for cartographic representation. Some people
may even claim that its main use in creating choropleth maps makes it at
least one of the most important cartographic representation techniques.
A choropleth map uses the boundaries of geographic units (e.g., counties,
countries, or states) to determine the area represented with a particular shade
or color. Since the geographic units are distinct, the cartographic representa-
tion makes it easy to compare the characteristics of each unit (see Plate 7).
Choropleth maps and visualizations start with defined geographic units,
for which data is either collected, or, if the data comes from other data sets,
is associated with the same geographic units. These units may be counties,