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  The graphic codes that represent similarities and differences among features and data values on a map. In addition to making visible the otherwise inconspicuous scaled-down versions of geographic features, map symbols help viewers identify patterns and relationships not readily apparent in written accounts or numerical tables (Tufte, 1997, pp. 29-35).

Map symbols usually reflect the dimensionality of the features portrayed. For example, radio towers and accident sites are commonly represented as points, railways and political boundaries as lines, and counties and countries as areas. Map scale and context play an important role, though, in determining whether a city or an airport is generalized as a point or an area.

Feature and symbol can differ in dimension. Because solid three-dimensional models are expensive to construct and reproduce, mapmakers usually portray terrain and other surfaces either with contours (isolines) or as perspective block diagrams consisting largely of line symbols. Moreover, because area symbols varying in intensity or pattern are less suited to showing magnitudes than point symbols varying in size, map-makers often portray the populations of countries and provinces with graduated circles centred within their borders. Cartographic animation, which adds time as an active dimension, affords symbols adept at portraying the order, duration and rate of events (DiBiase et al., 1992).

Jacques Bertin (1983) identified six retinal variables that affect the efficiency with which viewers decode map symbols. Because size is most suited for showing variation in magnitude, proportional point symbols are ideal for mapping population size and other count data. And because graytone value is most appropriate for showing variation in intensity, choropleth maps are effective for mapping growth rates, median income, population density and other intensity measures. Similarly, symbols that vary in hue, shape or pattern, all of which connote difference in kind rather than quantity, are most suited to mapping qualitative differences, whereas orientation symbols like contours, arrows and flow lines provide concise descriptions of directional trends.

Colour is common on topographic maps, which exploit cultural associations of blue with water and green with vegetation. Pictorial symbols rich in shape cues are especially helpful for general users, most of whom readily associate tiny picnic benches with picnic areas. On orienteering maps, weather maps and other cartographic genres too complex for readily decoded pictorial symbols, efficient map reading benefits from standardized symbols familiar to frequent users (MacEachren, 1995, pp. 290-2).

Typographic labels are another important element of cartographic symbolization. As a link between graphic marks and written language, words on maps help users locate specific features and obviate a litany of highly abstract symbols. Because typography has its own visual variables, map authors use the size and style of labels to describe or reinforce similarities or differences among features (Monmonier, 1993, pp. 106-9).

Large or bold labels thus point out a map\'s more important features, whereas italic type underscores the geographic kinship of rivers, lakes and other hydrographic phenomena. (MM)

References Bertin, J. 1983: Semiology of graphics: diagrams, networks, maps, trans. W.J. Berg. Madison: University of Wisconsin Press. DiBiase, D. et al. 1992: Animation and the role of map design in scientific visualization. Cartography and Geographic Information Systems 19: 201-14, 265-6. MacEachren, A.M. 1995: How maps work: representation, visualization, and design. New York: Guilford. Monmonier, M. 1993: Mapping it out: expository cartography for the humanities and social sciences. Chicago: University of Chicago Press. Tufte, E.R. 1997: Visual explanations. Cheshire, Connecticut: Graphics Press.



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