Academic journal article Cartography and Geographic Information Science

The Search for Boundaries on Maps: Color Processing and Map Pattern Effects

Academic journal article Cartography and Geographic Information Science

The Search for Boundaries on Maps: Color Processing and Map Pattern Effects

Article excerpt


Keates (1982) argued that map reading could be considered as a number of map-use tasks that take place in the process of obtaining desired information. These tasks are often related and, frequently, the result produced by an earlier task becomes input for a later task. Keates argued, "whatever task is concerned, the initial stage depends on the two processes of detection and discrimination" (Keates 1982, p. 12). Symbols on maps must stimulate the visual system enough to be detected and they must have some characteristics, e.g., location, color, shape, size, or orientation, so that one symbol can be distinguished from another. Keates further links these two processes as part of a more general identification process. As he stated, "whereas detection and discrimination can take place without understanding what the symbols represent, identification is clearly learned behavior" (Keates 1982, p. 12). Comparison of currently detected information to previously acquired information from the map legend or prior experiences is required for identity to be established. Once identification has been established, this information could be passed to a later stage and used for further interpretation or decision making.

If the general task is to find a specific object, e.g., map symbol or boundary, on a map, the identification through detection and discrimination activities might be repeated numerous times as the search for the target object is completed (Figure 1). Assuming one knows the characteristics of a target that might be found on a map, a visual search of the map is an activity requiring one to locate and identify that target. Specific objects that might be the target must be detected and discriminated from other objects that also might be the target. The efficiency of the search can be measured by how quickly an accurate result occurs. Clearly, one must look at or focus attention on the target object before it can be identified (Figure 1). To understand and explain how maps are searched one must consider the effects of patterns put on maps by cartographers as well as the visual processes used by map readers. The former might be called in-the-map effects and the latter in-the-head effects.


The purpose of this paper is to investigate the effect of color in controlling attention to achieve an efficient visual search. The basic problem considered was an experimental search task with realistic choropleth maps that required map readers to find a target boundary defined by two colors among other boundaries that were not the target. Colors that varied in hue and luminance were used to construct realistic experimental maps. Colors were selected so that visual search could be investigated as the interaction between patterns on cartographic maps and human cognitive processes. This interaction is clearly the key to understanding the behavior of map readers and the production of efficient cartographic displays. This paper first offers discussions of color theories and visual search theories related to map reading. Following these reviews, an experimental map-reading study is presented that considered specific hypotheses related to color processing and visual search.

Color Theory

We experience color in our brains by detecting and processing electromagnetic radiation (light) through the visual system (Palmer 1999). These color experiences occur because the receptors in our eyes are sensitive to wavelengths which range from about 400 to 700 nanometers in the electromagnetic spectrum. Three types of cones in the retina are initially responsible for the perception of color. Each type of cone has different photo pigments and, consequently, different spectral sensitivity. This results in cones that are maximally responsive to short (S), middle (M), and long (L) wavelengths of light (Albright 1991; De Valois and De Valois 1993; Neitz et al. 1993). Monochromatic light at the maximum of the absorption curves for the S, M, and L cones appears violet, blue-green, and yellow-green respectively (Figure 2). …

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