Designing and Choosing symbols 167
determined by examining the data. With computerized mapping, the categories can
be experimented with easily.
Category Types
In general there are several category types that may be used for choropleth maps:
equal steps,
standard deviations, quantiles,
arithmetic progression,
geometric pro-
gression, and natural breaks. These are described here briefly, but since a large part
of cartography and GIS involves spatial analysis, it is strongly recommended that GIS
and cartography practitioners acquire a background in statistics. An advantage of
computer methods is that one can experiment with different categories to visualize
the distribution. As can be seen from the following discussion and the accompany-
ing figures, the resulting maps vary in appearance and information depending on the
method of categorization. The maps have been constructed from the data in Table
8.2.
Equal steps are constant intervals. They are found by determining the range of
the data; the range is then divided by the number of classes, which gives the common
difference. The class limits are established by starting at the lowest value and adding
the common difference to get the upper limit of the first class, adding the common
difference to this to get the limit of the second class, and so on, until the upper limit
of the data is reached (Figure 8.28).
Standard deviations are a second type of constant interval. This method is most
effective if the distribution approximates a normal distribution (bell-shaped curve),
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