  Visualizations such as bar charts, scatter plots, and objects on geographical
maps often convey critical information, including exact and relative numeric
values, using shapes. The choice of shape and method of encoding information is
often arbitrarily, or based on convention. However, past studies have shown
that the human eye can be fooled by visual representations. The Ebbinghaus
illusion demonstrates that the perceived relative sizes of shapes depends on
their configuration, which in turn can affect judgements, especially in
visualizations like proportional symbol maps. In this study we evaluate the
effects of varying the type of shapes and metrics for encoding data in visual
representations on a spatio-temporal map interface. We find that some
combinations of shape and metric are more conducive to accurate human
judgements than others, and provide recommendations for applying these findings
in future visualization designs.
