Virtual Depth-based Representation of Cartographic Uncertainty Keith C. Clarke National Center for Geographic Information and Analysis University of California, Santa Barbara, CA 93106 Paul D. Teague National Center for Geographic Information and Analysis University of California, Santa Barbara, CA 93106 H. Greg Smith National Imagery and Mapping Agency Fairfax, VA 22031 Abstract Research at NCGIA, funded under the National Imagery and Mapping Agency's University Research Initiative, has concentrated on the elements, measurement, modeling, propagation, and cartographic portrayal of uncertainty in the national geospatial information infrastructure. As part of this work, cartographic methods for dealing with both positional and attribute components of uncertainty have been reviewed, and the most promising methods designated for implementation and testing. Our work has focussed on using a combination of methods, including color manipulation, animation, and visual depth to represent at least positional uncertainty in an active way; that is, the user has control within the display as to what or how much uncertainty becomes part of the visualization. The environment in which we are working is the virtual world, and interaction is established using the Virtual Reality Modeling Language as well as an immersive Head Mounted Display. Uncertainty within the three dimensional environment is reflected as a floating point value assigned to features, and features change their color properties, move, and are present or absent based on the user's chosen level of uncertainty portrayal. User responses are then to be measured based on interactions established to indicate virtual position of the user's eye focus within the virtual field. This paper presents some initial findings of building and testing a prototype system, and examines methods to be used for more rigorous human subjects testing. 1. Introduction Of growing interest in the cartographic community, the portrayal of uncertainty has lately been the subject of much research in the cartographic community. In 1997, a three year project began at the National Center for Geographic Information and Analysis examining five aspects (elements, measurement, modeling, portrayal and propagation) of uncertainty in the national geospatial information infrastructure. The fourth portion of the project was tasked with comprehensively determining suitable methods for uncertainty portrayal. Existing methods of uncertainty representation were revisited and the development of innovative techniques undertaken with the goal of eventual identification of the most effective methods through human subjects testing. Past research on the display of uncertainty or its components has centered on the use of traditional cartographic visual variables, animation, and three-dimensional representation techniques. Cartographic methods using traditional variables for uncertainty portrayal are based primarily on an extended set of Bertin's visual variables: position. size, value, texture, color, orientation, and shape (Bertin, 1983). Beard, Clapham, and Buttenfield (1991) suggested size, shape, and color as useful variables for uncertainty in point and line data, while suggesting color value, saturation, and possibly size and shape for continuous data. MacEachren ( 1992, 1994) favored the use of color saturation for uncertainty portrayal and added focus as a variable that could be changed by manipulating edge crispness, fill clarity, resolution or by adding tog. McGranaghan ( 1993) favored creating visual ambiguity using Benin's variables as well as focus, realism, time, and interaction. The use of time as a variable to represent uncertainty cartographically requires the use of animation. Animation has been used b\ several cartographers to communicate uncertainty information to map users by focusing on the different ways time can be used as a variable and by portraying multiple realizations of data at the same point in time. Peter Fisher ( 1993, 1994. 1996) used animation to display the set of possible maps created by multiple equiprobable outcomes or realisations using, both 253