illustrate the spatial characteristics of a single characteristic or those of a closely
related set of phenomena. The purpose of the thematic map is to provide a spatial
representation of the distribution of such features and the relationship(s) between
them. A map showing the average per capita income of each state in the United
States for a given year would be an example of a thematic map. Such a map would
indicate the state boundaries like a general map, but the addition of the specific
attribute of per capita income, unique to each state, creates a specific theme for
the map. The possibilities for thematic mapping are infinite, as a limitless number
of spatial phenomena and data exist that might be mapped, and new data are con-
stantly being generated.
The process of creating a map is complex and requires great skill and care.
Frequently the cartographer must decide on a proper scheme of generalization
and symbolization of the data that are to be displayed. Information must be classi-
fied into categories, because mapping all the variation in the data points would
result in a map that would be overly complex and confusing. There are numerous
techniques for dividing the data into appropriate classes, and the way this is done
often depends on the nature of the data itself, as well as what facets of the informa-
tion the cartographer wishes to emphasize. One of the most common approaches
when working with statistical data is to produce achoroplethmap. On this type
of map the data is presented as continuous classes that highlight the magnitude
or intensity of the data within the defined boundaries on the map. For example, a
map showing the average income of residents in Florida by county would likely
have five or six classes ranging from the lowest average county to the highest.
Such a map will clearly show where the wealthiest residents live in the state, and
where incomes are relatively low. The counties within the classes can be indicated
using a distinctive pattern for each class, or by using different colors.
For thousands of years maps were drawn by hand using the same techniques
employed by artists producing sketches and pictures, and indeed many maps pro-
duced this way could be considered works of art in their own right. This meant that
map production, especially of maps showing a great deal of data or detail, was a
time-consuming process that required not only geographic knowledge but consid-
erable artistic skill. The revolution in technology that transpired at the conclusion
of the 20th century dramatically changed the way maps are generated. Computer
cartography, the ability to construct maps using computer software packages and
sophisticated graphics, has revolutionized map production. Maps can now be cre-
ated by anyone using mapping software, and may be updated instantaneously and
frequently without the need to redraw the entire map. Maps are the main output
generated byGeographic Information Systems (GIS),and the development of
GIS has meant that the analytical utility of maps has increased exponentially.
Using the multiple layers of data contained in GIS, a researcher can quickly218 Map