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9 Topological Path Planning
2. Associative. Relational techniques tend to focus on the graph-like rep-
resentation of spatial memory. Associative techniques focus on coupling
sensing with localization in a manner which parallels the tight coupling
of sensing to acting found in reflexive behaviors.
Because relational techniques use an explicit representation, they can sup-
port path planning. Associative techniques are better for retracing known
paths.

9.2 Landmarks and Gateways

LANDMARK Topological navigation depends on the presence of landmarks. A landmark is
one or more perceptually distinctive features of interest on an object or locale
of interest. Note that a landmark is not necessarily a single, self-contained
object like “red door.” A landmark can be a grouping of objects; for exam-
ple, “McDonald’s” means a tall sign, bright building of a certain shape, and
a parking lot with a lot of activity. Another outdoor landmark might be a
“stand of aspen trees.”
Landmarks are used in most aspects of navigation. If a robot finds a land-
mark in the world and that landmark appears on a map, then the robot is
localized with respect to the map. If the robot plans a path consisting of seg-
ments, landmarks are needed so the robot can tell when it has completed a
segment and another should begin. If a robot finds new landmarks, they can
be added to its spatial memory, creating or extending a map.
Dave Kortenkamp popularized a particularly interesting special case of
GATEWAYS landmarks: gateways. 79 A gateway is an opportunity for a robot to change its
overall direction of navigation. For example, an intersection of two hallways
is a gateway; the robot can choose to go straight or turn left or right. Because
gateways are navigational opportunities, recognizing gateways is critical for
localization, path planning, and map making.
ARTIFICIAL Landmarks may be either artificial or natural. The terms “artificial” and
LANDMARKS “natural“ should not be confused with “man-made” and “organic.” An ar-
NATURAL LANDMARKS
tificial landmark is a set of features added to an existing object or locale in
order to either support recognition of the landmark or some other perceptual
activity. An interstate highway exit sign is an example of an artificial land-
mark. It is put there with the purpose of being easy to see (retro-reflective)
and the white-on-green font is sized for optimal visibility (perceptual activ-
ity is reading the sign). A natural landmark is a configuration of exisiting
features selected for recognition which were not expressly designed for the