separate indicators. The computer graphic display that shows three aircraft symbols (past, present, and predicted future) in roll, pitch, and yaw relative to glide slope, command heading, and altitude integrates even more information. The plan view or map display that shows waypoints, heading, other aircraft, predicted trajectories, and weather is another advanced visualization aid.

The digital representation of altitude on the radar display has remained a feature of the air traffic control workstation that is less than optimal. Although controllers adequately handle digital flight-level data, the fact remains that it is difficult to visualize vertical trends or the magnitude of altitude differences from such a representation. Designers have realized the possible advantages to visualization by representing the vertical dimension in analog format. There are in fact two ways in which this might be accomplished (Wickens, 1997). One is through addition of a vertical "profile" display, and the other is through a three-dimensional or "perspective" display.

To date, most experimental research has compared conventional plan view displays with perspective displays. Such comparisons have generally not been favorable for the latter (Wickens, Miller, and Tham, 1996; May et al., 1996). Although a perspective view does indeed represent the vertical dimension, it also compresses the three-dimensional airspace onto a two-dimensional viewing surface, leaving a certain amount of perceptual ambiguity regarding the precise lateral and vertical distance separating a pair of aircraft (McGreevy and Ellis, 1986; Merwin et al., 1997). This ambiguity can disrupt the controller's judgments of predictive separation.

Three solutions may be available. First, as noted, a profile display could be coupled with the plan view display to represent, without ambiguity, the vertical separations. This approach has proven quite successful for representing traffic separations and terrain awareness in cockpit displays (Merwin et al., 1997; Wickens, Liang, et al., 1996). Second, some designers have proposed using holographic or stereo techniques to create displays, in which the ambiguity is lessened (Wickens et al., 1989). Third, it is possible to provide a controller with interactive tools, whereby the three-dimensional viewpoint of the display can be altered, making the position of aircraft less ambiguous through the perceptual cue of motion parallax (Sollenberger and Milgram, 1993; Wickens et al., 1994); this can also be provided by holographic displays.

A more radical form of interaction is created by allowing the controller to change the viewpoint position and "immerse" himself within the airspace, thereby approximating the technology of virtual reality (Durlach and Mavor, 1995; Wickens and Baker, 1995). Certain limitations of this technology for air traffic control, however, appear evident. First, by immersing oneself within the traffic volume, aircraft to the side and behind are "out of sight," a factor that is of considerable concern if the safe separation of all traffic is to be maintained. Second, such immersion can be disorienting to one who is not in active control of the viewpoint and hence would tend to be disruptive to efforts to coordinate a

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