Geoscience Reference
In-Depth Information
- The entities called “span 11 ” (also named “occurrent” or “perdurant”) refer for
their part to events and processes. It concerns entities that have an extent in time. In
fact, processes do exist only because of the succession in time of their temporal parts
[GRE 04, LIV 09]: they are characterized by a beginning and an end, and a
succession of time intervals, each corresponding to a change. If such a perdurant
entity is observed at a given time t, its entirety cannot be accessed. As a matter of
fact, what would aging, growth or acceleration in a time snapshot mean? An entity,
such as a process, is built in time, it rolls out in some way, “it IS its history”
[LEN 09]. However, the events correspond to a phenomenon that can be regarded as
punctual. While event and process are often distinguished by differences in duration,
Galton and Mizoguchi [GAL 09] instead insist on a difference of temporal
delimitation, the process spreading over time (outflow of water in a river and
spreading of the built area) while the event is dated (for example, fusion of two
communes or avalanche). Let us notice that the difference between an event and a
process is a matter of perspective and choice in the granularity of the observation.
Therefore, the avalanche is an event at the spatial and temporal scale of the lives of
the inhabitants, but a process at the scale of the monitoring of the snow along the
slope during the seconds or minutes that it lasts. Finally, in some cases, the event
marks the beginning or end of a process (for example, starting a new construction).
From the point of view of the domain expert who is interested in the evolution of
a spatial phenomenon, who is in other words adopting a dynamic perspective, the
two approaches are complementary, each offering a vision on the phenomenon of
interest. An individual is a “snap” entity, his/her life trajectory is a “span” entity:
this trajectory is composed of different phases of youth, maturity and old age of the
individual. A city is also a “snap” entity, it exists at each step of time (of its
existence) maintaining its identity. Its “growth” process, as it can be measured by
the evolution of its population, of its economic activity or of the extent of its spatial
insertion, for example, is a “span” entity. For mobile objects, the “span” conception
of certain entities is required from the outset: such is the case of a crossing (for
example, maritime), a ride or a pedestrian walking, whereas the objects associated
with them, the vessel that crosses the sea and the individual who walks on the street,
are “snap” entities. A double temporality must sometimes be envisaged, for
example, for a vessel, the one associated with its aging process and the other
corresponding to the crossing from one port to another.
11 In English, “SNAP” has the meaning of photo, snapshot, whereas the term “SPAN” refers
to the duration, to the scope. This play on words puts the emphasis on two complementary
sides of temporality [LIV 10].
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