Comparing Automatic and Manual Zooming Methods for Acquiring Off-Screen Targets

Previous studies indicate that user performance with scrolling can be improved through Speed-Dependent Automatic Zooming (SDAZ), which automatically couples the document’s zoom-level with scroll-speed. These studies have compared traditional scrolling techniques (scrollbars and rate-based scrolling) with SDAZ, leaving a potential confound that the efficiency gains are due to zooming rather than the automatic binding of zoom-level with speed. It is therefore possible that decoupling zoom from speed, allowing users separate but concurrent control of each, could further enhance performance. This paper describes an experiment (n=35) that examines user performance, workload, and preference in tasks that involve scroll-based acquisition of off-screen targets using SDAZ and manual zooming. Three different types of document navigation are explored: text documents, ‘flat’ 2D maps, and a ‘globe browser’ that allows multi-level zooming of a globe-map of Earth and underlying city views. Results show that automatic zooming not only improves performance, but that it does so with substantially less subjective workload, and that it is strongly preferred. We also confirm limited previous work using Fitts’ Law as a model for off-screen target acquisition, and show that it applies even when zooming is employed.