An examination of transparency as a visual variable for the mapping sciences

Within the mapping sciences, transparency is a symbol form that until recently has largely been ignored. With recent technological advances, it has emerged as a logical choice for some cartographic design decisions. While it is being used with increased frequency, transparency has never been empirically tested for its usefulness as a visual variable for depicting spatial data. This study examines the use of transparency as a means to represent spatial data on maps. The use of transparency is examined through a review of its current utilization in cartographic displays. The two experiments conducted in this study are designed to contribute to a long tradition of empirical research in map design and cartographic symbolization. The first experiment is designed to elicit information about how we perceive transparent map symbols by looking at how efficiently and accurately we make decisions about its presence or absence. The second experiment is designed to determine how consistently and accurately we can estimate the amount or level of transparency used in map symbols. Results of the first experiment indicate that transparency can be attended to efficiently and accurately. Results of the second experiment indicate that we can estimate transparency level efficiently and accurately if only a few categories are used on a map. Suggestions for the use of transparency as a visual variable for the representation of spatial data on maps are provided