Geography Reference
In-Depth Information
processes affects the symbology. For some maps, geometric accuracy is less
important than linkages, adjacency and relative position. Among the more effective
highly generalised maps are linear cartograms portraying subway and rapid transit
systems, such as the London Underground 'map', where scale is relatively large in
the inner city and smaller towards the outer areas where development is less
dense. By sacrificing geometric accuracy, these schematic maps are more effect-
ive in addressing the Underground user's basic question of how to get from 'A' to
'B' on the system than a 'geographically true' map would be. As geometric general-
isation seeks graphic clarity by avoiding overlapping symbols, content or concep-
tual generalisation promotes clarity of purpose or meaning by filtering out details
irrelevant to the map's function or theme (Monmonier, 1996). Conceptual generali-
sation thus mainly affects attribute data, and requires knowledge of the map con-
tents. It has two essential elements: selection and classification. Selection, which
serves geometric generalisation by suppressing some information, promotes
content generalisation by choosing only relevant features. Classification, on the
other hand, makes the map informative as well as usable by recognising similarities
among the features chosen so that a single type of symbol can represent a group
of similar features. This is necessary because the graphic vocabulary of most maps
is limited to a small set of standard, contrasting symbols, thus requiring the
emphasis on similarities of map features. However, different classifications can
easily manipulate the message of a map and carry substantially different political
interpretations.
2
Monmonier (1996) has used the example of homes lacking tele-
phones in the 1960s to illustrate this. Depending on where map-makers chose to
put the class break, the resulting map could present most states either as poorly
connected or as well served (see Figure 3.3).
There is much use of standardised symbols in topographical maps, which are
effective because people have learned to read them. However, thematic maps, to
which planning maps belong, have other objectives than to describe the terrain and
its fixed assets. Because of the ever changing themes and ever changing aspects
of reality that are represented, information transfer is not primarily governed by con-
vention but by trying to use the innate characteristics of symbols (such as shape,
colour, size and texture). It was the French geo-cartographer Bertin (1983) who
placed the variations in graphical aspects in a logical structure, which are now
commonly known as the graphic variables (see Plate 1). The perceptual character-
istics of the differences in the graphic variables help map designers in selecting
those variables that provide a sensation which matches the characteristics of the
data or the communication objectives (Kraak and Ormeling, 1996). For example, a
series of five grey tones evenly spaced between light grey and black makes decod-
ing simple when darker means more and lighter means less for quantitative and
ordered data (Monmonier, 1996).
