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Figure 1.6 Individual (Tourist) versus collective (GroupOfTourists) Behaviors.
it shows this behavior or not. Should a GroupOfTourists behavior be defined
(as a group of trajectories that represents a group of persons moving together
and each of them complies with the Tourist behavior), it would be a collec-
tive behavior. Well-known collective spatio-temporal behaviors, such as Meet
(a group of trajectories that simultaneously end up at the same place) and
Flock (a group of trajectories that travel together), are described in Chapters 6
and 7 .
Figure 1.6 illustrates that, for finding which moving objects show a given
individual behavior, each trajectory has to be individually checked against the
behavior predicate. On the opposite, finding which moving objects show a given
collective behavior implies checking a group of trajectories.
Generally, collective behaviors are observed in groups of trajectories that are
simultaneously run by various moving objects. But collective behaviors may
also be defined for a set of trajectories run by a given moving object at different
times. A typical example is the Commuter behavior that characterizes a group
of trajectories made by the same object on working days and that show the same
peculiar trace: they start from a point P1 and go to another one where they stay,
then they end by going back to P1.
A case where the classification individual versus collective behavior is not
necessarily intuitive is when the behavior involves multiple trajectories with
one of them playing a special role in the group. For instance, given a group
of tourists, the tourists and their guide move together but the guide's trajectory
obeys additional rules: During the stops the guide is in the middle of the group;
during the moves, the guide moves a few steps ahead of the other members.
The group of tourists (guide included) shows the GroupOfTourists collective
behavior, yet the guide trajectory complies with the individual TouristGuide
behavior. Both the group and the guide's trajectory are needed in order to get
the TouristGuide behavior.
Other cases exist where a fixed number of trajectories is needed for the
behavior. For instance the CourtshipDance behavior of some birds, such as
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