We investigate the relationship of perceived distances to judged similarities between document points in various types of spatialized displays. Our findings suggest that the distance–similarity relationship is not as self-evident to viewers as is commonly assumed in the information visualization literature. We further investigate how participants interpret instructions to judge distances when those instructions do or do not specify the type of distance. We find that in all types of spatialization displays, there is no significant difference between default and direct judgements of distance; people clearly interpret default distance instructions to refer to direct (straight-line) distance. These findings provide direct
evidence on the conditions under which people employ distance when assessing similarity between data objects in various types of spatialized views and, when they do, which type of distance. They also give insight into how people explore
the similarity of geographic features depicted in cartographic maps or GIS displays.