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Tyner indb
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Bog'liq Judith A. Tyner. Principles of map design. New York. „The Guilford Press”, 2010geographic location
The geometry of geographic space is determined by reference to coordinates, either
longitude and latitude for conventional mapping, or some other form for cartograms
or other distorted maps.
leVels of clAssIfIcAtIon of geogRAPhIc dAtA
The data gathered for geographic phenomena may simply show location and nature,
that is, where and what the phenomena are; or they may rank phenomena, or they
may specify actual values. There are four levels of measurement for data, each of
which provides a certain degree of information. These are nominal, ordinal, interval,
and ratio classification. Nominal classification is also called qualitative and is the
lowest level of information. Ordinal, interval, and ratio classification come under the
general heading of quantitative information and represent successively higher levels
of information (Figure 7.4).
Nominal classification locates and names. Items are placed into mutually exclu-
sive categories and located on the map. The categories are qualitative, since no rank-
ing or value is attached, and no item can be placed in more than one category. Thus, a
vegetation map shows deciduous forests, coniferous forests, grasslands, and the like,
and a land-use map might identify urban and rural categories. Where there is some
mixture, such as a forest with 90% deciduous trees and 10% coniferous trees, the
category would be chosen as a part of the generalization process (see Chapter 5).
Ordinal classification “orders” or places data into ranked categories, but does
not give exact values within categories. Sometimes a series of numerical categories is
given, while at other times a descriptive ranking, such as small, medium, or large is
used. A city might be classified as 500,000–1,000,000 population, but no indication
of its exact place within the category is given.
Interval and ratio classification are similar, since actual values are given, as
well as function and rank. A uniform unit of measurement is used for both, such as
degrees, feet, meters, Euros, or dollars. They differ in that interval measurement has
an arbitrary starting point instead of an absolute zero point. The Celsius temperature
scale begins at an arbitrarily assigned zero, the temperature at which water freezes.
Population measure begins at an absolute zero. Therefore, comparisons such as twice
as hot or half as hot are not possible using interval measurement, but twice as many
people or half as many people is possible with ratio measurement. In cartographic
practice, the difference between these two levels of measurement has little or no effect
136 sYMBOLizaTiOn
on symbol selection. When symbols are attached to data that have been scaled by
interval or ratio measurement, value can be determined from the size or some other
characteristic of the symbol.
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