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9.2 Landmarks and Gateways
perceptual activity. If someone gives directions to their house, “take the sec-
ond right after the McDonald’s,” the McDonald’s is being used as an orienta-
tion cue for navigation to their house. Clearly, the McDonald’s was not built
with the purpose of being a navigational cue to a private home. The fact that
it was used for another purpose means it is a natural landmark.
CRITERIA FOR Regardless of whether a landmark is artificial or natural, it must satisfy
LANDMARKS three criteria:
1. Be readily recognizable. If the robot can’t find the landmark, it isn’t useful.

2. Support the task dependent activity. If the task dependent activity is simply
an orientation cue (“take the second right after the McDonald’s”), then
being recognized is enough. Suppose a landmark is intended to provide
position estimation to guide docking the space shuttle to a space station.
In that case, the landmark should make it easy to extract the relative dis-
tance to contact.

3. Be perceivable from many different viewpoints. If the landmark is widely vis-
ible, the robot may never find it.

Fig. 9.1 shows artificial landmarks constructed for use in the 1992 AAAI
Mobile Robot Competition. 42 Each robot was supposed to follow a route be-
tween any sequence of waypoints in an arena. The teams were allowed to
mark the waypoints with artificial landmarks. Each waypoint is readily rec-
ognizable by the checkerboard pattern. The task dependent activity of going
to the correct waypoint is facilitated by the cylindrical barcodes which are
unique for each station. Notice that the use of a cylinder guaranteed that
landmarks would be perceivable from any viewpoint in the arena.
A good landmark has many other desirable characteristics. It should be
passive in order to be available despite a power failure. It should be perceiv-
able over the entire range where the robot might need to see it. It should have
distinctive features, and, if possible, unique features. Distinctive features are
those which are locally unique; they appear only as part of the landmark
from every viewpoint of the robot in that region of the world (e.g., there is
only one McDonald’s on Busch Boulevard). If the feature occurs only once in
the entire region of operations (e.g., there is only one McDonald’s in Tampa),
then the feature would be considered globally unique. In addition to being
perceivable for recognition purposes, it must be perceivable for the task. If
the robot needs to position itself to within 0.5 meters of the landmark, then
the landmark must be designed to achieve that accuracy.