Since there is no precise map of the environment, it is impossible to express a goal state using coordinates in . However, a goal can be expressed in terms of the vertex that must be chased to make the state visible. For example, imagine showing the robot an object while it explores. At first, the object is visible, but a gap may appear that hides the object. After several merges, a vertex deep in the tree may correspond to the location from which the object is visible. The robot can navigate back to the object optimally by chasing the vertex that first hid the object by its appearance. Once this vertex and its corresponding gap disappear, the object becomes visible. At this time the robot can move straight toward the object (assuming an additional sensor that indicates the direction of the object). It was argued in  that when the robot chases a vertex in the GNT, it precisely follows the paths of the shortest-path roadmap, which was introduced in Section 6.2.4. Each pair of successive gap events corresponds to the traversal of a bitangent edge.
Steven M LaValle 2012-04-20