Abstract
Flow maps allow users to perceive not only the location where interactions take place, but also the direction and volume of events. Previous studies have proposed numerous methods to produce flow maps. However, how to evaluate the usability of flow maps has not been well documented. In this study, we combined eye-tracking and questionnaire methods to evaluate the usability of flow maps through comparisons between (a) straight lines and curves and (b) line thicknesses and color gradients. The results show that curved flows are more effective than straight flows. Maps with curved flows have more correct answers, fixations, and percentages of fixations in areas of interest. Furthermore, we find that the curved flows require longer finish times but exhibit smaller times to first fixation than straight flows. In addition, we find that using color gradients to indicate the flow volume is significantly more effective than the application of different line thicknesses, which is mainly reflected by the presence of more correct answers in the color-gradient group. These empirical studies could help improve the usability of flow maps employed to visualize geo-data.
Highlights
Flow maps are effective tools to represent connections and interactions volume between geographical regions [1], and are widely applied to research related to spatial trajectory and interactions, such as migration [2], transportation [3], population movement [4], disease spread [5], and social communication flows [6]
We mainly focused on comparisons between (a) straight lines and curves to indicate flows and (b) line thicknesses and color gradients to indicate the flow volume
In our study, according to the questionnaire results (Figure 5b), users think maps using color gradients are clearer than maps using line thicknesses, and they feel that it is more difficult to identify volume correctly on maps using line thicknesses
Summary
Flow maps are effective tools to represent connections and interactions volume between geographical regions [1], and are widely applied to research related to spatial trajectory and interactions, such as migration [2], transportation [3], population movement [4], disease spread [5], and social communication flows [6]. The usability of flow maps influences the effectiveness and efficiency of map users processing and interpreting cartographical flow information. Previous cartographical studies have demonstrated the influence on effectiveness and efficiency in maps caused by shape, color, and size [14,15,16]. Researchers have conducted experiments to evaluate the representation of these three visual variables in flow maps [1,11,17,18] We believe that these three visual variables have significant influences on the usability of flow maps. These studies did not reach consistent conclusions on the effectiveness and efficiency of flow shapes. The differences in usability between using line thicknesses and color gradients to represent flow volume remain to be studied
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