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developed around these issues of identity [REN 96, HOR 98]. Renolen proposed
formalizing these changes with graphs illustrating the genealogy of objects (“history
graph”) and introduced more complex changes as the division or union of objects.
Are the 13 states composing “European Union” the same object as the European
Union composed of 27 states? What happens to Germany's identity when it was
divided into two states in 1949? And at the reunification in 1990, do we find the
same object as before 1949? Or still, how can we handle the identity of a forest fire
that moves, changes shape and divides itself into several centers? These questions
go beyond mere computer management of identity and refer to the intrinsic nature of
the object. Hornsby and Egenhofer [HOR 98] introduced the concept of identity
states (existing, that never have existed and having already existed) and proposed a
visual language to represent the states and the transitions between these different
states. The objects may appear (birth or sometimes reincarnation) or disappear; it is
a question of grasping their “presence”. Germany could be modeled as a “having
already existed” identity state from t = 1990. The changing ownership plot could be
modeled as a new object created from another destroyed one.
The following positions raise once more the question of what it means to
“integrate time” in geographical information systems. While the first attempts of
developing “temporal GIS” drew upon the developments made in the area of
databases [ABR 99], some authors agree, however, to say that the representation and
reasoning about geographical dynamics require more than the simple introduction of
time through the notion of change. Indeed, such approach fits in an
endurantist
perspective (vision called 3D+1, 3D for space and 1 for time) while it should be
necessary to reflect on the formalization of
perdurant
objects (4D) [YUA 01,
REN 00, GAL 04, WOR 05]. Galton demonstrated that such a perspective would
allow reconciling “object” and “field” approaches from the spatial point of view, as
well as from the temporal point of view. The object approach enters into time
through events, whereas the fields approach enters through processes. The challenge
is then to make several points of view coexist: a cyclone could, according to the
observer, be represented in the form of an event or of a process, in the form of a
spatial object or defined by the values of a space field.
The first developments associated with time have introduced the need to work on
change, and then on the events and processes, in a “data management” kind of vision
based on the background history record and the reconstitution of evolution. These
different experiences rooted in the formal development of databases lead then to the
need to return to a conception of events and processes envisaged under the thematic
perspective, by linking them to the identification of the causes of space's
transformations.
