2D vs. 3D visual cues for altitude maintenance in low-altitude flight

Previous research on altitude maintenance in low-altitude flight has focused either on cues provided by 2D features in the visual scene (e.g., splay angle) or on visual cues provided by the presence of 3D objects in the scene (e.g., occlusion). Therefore, little is known about the relative importance of 2D and 3D cues in altitude maintenance. We systematically varied the position variability, height, and pattern of surface elements in a simulated low-level flight environment to vary the salience of 2D and 3D visual cues. For 2D objects, altitude variability increased as a function of object position variability indicating that splay and depression angles are not reliable cues for terrains with irregularly spaced objects. For 3D objects, altitude variability increased less (or not at all) as a function of position variability indicating that the cues provided by 3D objects such as occlusion and motion parallax are the dominant visual cues for altitude maintenance for natural terrains with irregularly spaced objects.