scale, Compilation, and generalization 83
goals of generalization
The goals of generalization are to preserve geographic patterns and to stress thematic
information. The cartographer strives for objectivity
and uniformity of treatment
in generalization. To accomplish these goals, the cartographer must be aware of the
concepts of truth and accuracy as applied to maps.
Accuracy is usually taken to mean
locational or positional accuracy. Strict positional accuracy, however, is not always
possible for thematic maps. Often, thematic maps are drawn at very small scales, so
features cannot be drawn exactly to scale or located with precision.
For example, even at a comparatively large scale (for a thematic map) of
1:1,000,000, a major road 50-feet-wide would,
if drawn to scale, be only 0.0006
inch or 0.0015 millimeter wide. Thus, to show both major and minor roads, the size
must be exaggerated even though strictly speaking this is not accurate. Often it is
desirable to show roads, rivers, and railroads. If these parallel one another through
a narrow valley, which is a common occurrence, it becomes necessary to widen the
valley. Otherwise, all three features cannot be shown. This also is not accurate. The
USGS has established accuracy standards to solve such problems in generalization for
topographic maps; this information is available at the USGS website listed in Appen-
dix B. Similar standards can be devised for thematic maps.
It is possible to be truthful, however. If one accepts the idea that the map itself is
a symbol and that the lines and other marks on it are symbols, not exact representa-
tions, then
truth means showing the essence of patterns and relationships. Because
the line representing a road in our first example is a symbol, it need not be a scale
representation of the road. Because the relative position of the linear features in our
second example is important, not the width of the valley, the fact that the valley is
represented some feet wider that it really is presents no difficulty on a thematic map.
