Trajectories and Their Representations - Mobility Data - page 15

When the trajectory analysis uses only raw trajectories and no contextual
data, the analysis can produce only spatio-temporal patterns, as the Loop pattern
above. Since semantic trajectories and contextual data have gained attention,
several research groups have been trying to identify patterns that would have
a more semantic flavor than the traditional spatio-temporal patterns. For exam-
ple, a frequently discussed pattern in more recent works is the HomeToWork
pattern that applies to the daily routine trips of people going to work. Its defi-
nition requires knowledge of the home-place and the work-place of the moving
person, which is contextual knowledge that is likely to be available from pub-
lic people and company repertoires or inferable from the analysis of people's
trajectories. As the definition of these patterns relies on the semantic and con-
textual information associated with the trajectories, we refer to them as semantic
patterns .
It is worth noting that currently there is no consensus on the terminology, and
recent research tends to use either pattern or behavior, as well as behavioral pat-
tern, without a clear distinction among the concepts. This topic is no exception,
and in the following chapters the reader will find the terms pattern and behavior
as denoting the same concept. In this chapter we use behavior, with the following
definition that covers both semantic and spatio-temporal behaviors/patterns.
Definition 1.6. A trajectory behavior (or behavior , in short) is a set of trajectory
characteristics that identifies a peculiar bearing of a moving object or of a set
of moving objects. The behavior is defined by a predicate that says if a given
trajectory (or a given set of trajectories) shows the behavior. Synonym: trajectory
pattern .
Definition 1.7. A trajectory semantic behavior is a trajectory behavior whose
defining predicate includes conditions on some semantic representation of the
trajectories and/or conditions on some contextual data that are spatio-temporally
related to the trajectories. Synonym: trajectory semantic pattern .
Definition 1.8. A trajectory spatio-temporal behavior is a trajectory behavior
whose defining predicate bears only on the raw representation of the trajectories,
excluding any contextual data. Synonym: trajectory spatio-temporal pattern .
The predicate defining a behavior can rely on any characteristic of the trajec-
tories (e.g., spatio-temporal positions, episodes, annotations), contextual data
linked to the trajectories (e.g., the type of the geo-objects linked to some epi-
sodes or some of their attribute values), spatial relationships with geo-objects
(e.g., stopping near some given geo-object), temporal relationships with events
(e.g., moving during some given event), and relationships with other moving
objects (e.g., moving ahead of a given group of trajectories).