80 THe geOgRaPHiC anD
CaRTOgRaPHiC FRaMeWORK
not available, then one must use original surveys, remotely sensed imagery, or even
construct a base from field surveys. Other sources are digital imagery and aerial pho-
tographs. Sheet maps and imagery can be scanned or digitized if they are not already
available in digital form.
Much information is now available through the Internet in digital form. The
USGS provides maps and outline files (sometimes called “shape files”) online and
many libraries
have digitized map collections, especially of historic maps. Many
websites sell digitized outlines. Software packages often include outline maps. When
using any base map make sure to check the outline for accuracy against other maps.
The author has found digitized outlines that place coastal cities 50 miles inland or
that omit major islands and bodies of water. Such mistakes, the result of sloppy digi-
tizing, are unfortunately common.
Primary Data
The primary or thematic data are the heart of the map. What is included here depends
on the objective of the map. The term
thematic is used in two different ways. In GIS,
various layers are sometimes called “themes,” so that one theme might be roads,
another vegetation, and a third boundaries.
For thematic maps, the
theme is the
subject of the map. Keep in mind that a thematic map should not normally illustrate
more than one theme, although the theme may include more than one variable, such
as education attainment related to income. It is better to make two or three maps
than to overload a single map by trying to show too much information.
sOURCes OF PRiMaRY DaTa
It is not feasible to list the sources of all possible kinds of data here, but several cat-
egories may be examined:
Field
studies
•
•
Imagery, including air photos,
satellite imagery, radar, and the like
•
•
Statistical sources
•
•
Published maps
•
•
Other
printed materials
•
•
Data on the Internet or on disk
•
•
Interviews
•
•
Field studies include those published by various agencies, companies, and indi-
viduals as well as those that might be undertaken by the cartographer him- or herself.
Frequently, maps created for journal articles, theses, dissertations, and the like are
completely original, and their thematic information was compiled in the field by the
author (who might also be the cartographer). Many books are available on fieldwork
in geography.
Remotely sensed imagery ranges from black-and-white photographs taken from
conventional aircraft to digital imagery relayed from satellites. This imagery is an
important resource and should not be ignored. The subject is far too large to treat
briefly or even as an entire chapter here. Anyone now planning a career in the map-
scale, Compilation, and generalization 81
ping sciences should have a strong background in remote sensing, and even the casual
cartographer, who
makes maps only occasionally, is strongly advised to become
familiar with the basics of the subject, either through course work or reading. (Sev-
eral sources are listed in the Bibliography.)
Statistical data are commonly presented on thematic maps. The primary statisti-
cal sources for social data are the various kinds of censuses. The first regular national
census was taken in Sweden in 1749. The United States produced its first census of
population in 1790 and has taken a census every 10 years since. The early censuses
recorded the names and ages of every member of the household, but this is no longer
done. However, many more kinds of information are now obtained from the census of
population and housing. In addition to the 10-year census of population and housing,
censuses of agriculture and censuses of economics are also taken every 5 years. This
information is online as well as in hard copy. The website
www.census.gov is main-
tained by the U.S. Census Bureau. It is not always necessary to go to the full census
publications. The
City and County Data Book provides a summary of many kinds
of information. If data are not needed on a block or tract basis, these summaries are
helpful. For information on an individual state basis, the
Statistical Abstract of the
United States is invaluable. This annual publication provides reference sources for
each table so that more detailed information can be found. The
Statistical Abstract is
also available online at the Census Bureau website. See Appendix B for a list of URLs
for census information.
The Internet, of course, provides a wealth of statistical data. In addition to gov-
ernment censuses, there are also some private or commercial censuses or surveys. The
latter are usually undertaken by special-interest groups, such as the Dairy Council or
the California Wine Institute.
Other printed materials prove useful for some map topics. A helpful reference
librarian can save hours of research time. Do not neglect books or articles on the map
subject itself. Other maps may be available on the topic. Be aware that the laws of
copyright apply: one must not copy or adapt, even by tracing, another map without
permission and without giving credit. If no printed sources are available, interviews
with experts, travelers, and others might be necessary.
evaluating sources
One must be concerned with the accuracy and reliability of sources. Since a map is
normally compiled from a variety of sources, it becomes necessary to evaluate the
credibility of each. If there is a difference in the accuracy of the sources used, the
resulting map will be less reliable in some respects or in some areas than in others.
One should, of course, use caution when using online sources; many websites created
by individuals or even organizations have biases and don’t provide complete informa-
tion.
Know your sources.
