Visualizing the Off-Screen Evolution of Trajectories

In the context of volunteered geographic information, large sets of trajectories of humans and animals are collected. Analyzing these trajectories visually is often complicated due to limited display sizes. For instance, when a user chooses a large map scale to inspect the details of a trajectory, only a small part of the trajectory is visible in the map. Therefore, in this article, we present an approach for visualizing the off-screen evolution of trajectories, i.e., their continuation outside of the displayed map. We propose visual cues in the form of glyphs that are displayed at the map’s boundary and that consist of one or multiple disk sectors of varying size and opening angle. These glyphs indicate the direction and variability of direction of a trajectory’s continuation outside the map frame. We present an algorithm for computing the glyphs efficiently and evaluate them in a user study. The results show that the glyphs are intuitive to understand even without explanation. We further present suggestions for improving the glyph design based on the results.