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Visualisation of elevation information on maps: An eye movement study

Traditionally, information about elevation variation on a topographic map has been represented with contour lines. Compared to plain contour lines, depicting landform and elevation through slope shading or oblique view representation may provide more intuitive and realistic representation of the environment. However, more realistic, detailed representations may not be appropriate for map tasks (Hegarty et al., Cartographica, 44, 2009, pp. 171-186.). The aim of the study was to evaluate the effect of elevation visualisation on eye movements and performance in four different map-related tasks.

The stimuli maps depicted the same real geographical locations in three visualisations. Twenty-six participants were divided into three groups according to these visualisation types. The visualisation types were (only) Contour lines, Slope shading (in addition to contour lines), and Oblique view (triangle grid instead of contour lines).

The experiment consisted of four map tasks.

1. The search task: Search for a pentagon-shaped symbol on the map
2. The height comparison task: Judge which one of the two points is at higher elevation
3. The area selection task: Select a hiking area from the map.
4. The route planning task: Plan a realistic route from a starting point to a destination.

The response time were recorded in all the tasks. In the search task and height comparison tasks the accuracy of performance was measured and in the selection and planning tasks the participants explained their choices verbally. From the verbal data, the words related to elevation were extracted, and the number of trials containing these words were compared between the groups. The eye movements of the participants were recorded during all the tasks. From the eye movement data, the number of fixations and fixation durations as well as saccade durations and amplitudes were analysed.

The response times differed between visualisation types only in the height comparison task: Oblique view was fastest and Contour lines slowest. However, the Oblique view produced more incorrect answers than the two other visualisations. The results showed no differences in the number of trials containing words related to landform or elevation. Compared to other visualisation types, the Slope shading produced shortest average fixation durations in all the four tasks. On the other hand, in the height comparison task, all the eye movement measures differed between the visualisations: Contour lines required more fixations and induced shortest saccade amplitudes.

The visualisation of elevation information affects eye movements, particularly the average fixation durations. The shortest average fixation durations in the slope shading visualisation may indicate that it was regarded less complex than the other visualisations (Hooge et al. in Eye movements: A window on mind and brain. 2007), or that the contrast and spatial frequencies differed between the visualisations. However, due to the research design the individual variation in eye movements may have affected the results. The results also showed that the experimental task affects eye movements as has been showed in previous research (Rayner, Quarterly Journal of Experimental Psychology, 62, 2009, pp. 1457-1506).