FIGURE 8.1 Six graphic variables that can be used to symbolize geographic phenomena. SOURCE: Bernhardsen, 1999, p. 275. From Geographic Information Systems: An Introduction; Copyright 1999 T. Bernhardsen. This material is used by permission of John Wiley & Sons, Inc.

  • the mapping tools used to assign values in the data to these visual variables (e.g., line thickness, symbol size, color hue).

The cartographic capabilities of most GIS are quite advanced, providing users with perceptually valid color spaces and symbology. Yet GIS are not yet as graphically advanced as information visualization systems (e.g., Advanced Visualization System, Data Explorer) because not all visual variables are under the direct control of the user. GIS provide only a partial set of visual variables to communicate meaning. Some representational techniques are not supported (e.g., transparency, movement, height); others are accessible only via predefined, hard-coded paths. On the positive

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