Building Objects in Time - Spatio-temporal Approaches - page 2

morphological 1 criterion; - finally, the third used the population statistics of the
“ urban area ”, an entity constructed by the INSEE according to a functional
criterion. According to this criterion, a city incorporates the totality of the
communes of which part of the active population works in the urban unit. The
conceptual object on which the decision maker poses its questioning is, therefore,
the city A of which he/she wishes to know the population evolution. In the empirical
domain, each of the three “observables” (all three supplied by INSEE, but each
corresponding to a different delimitation of city A) referred to in this example is
a priori relevant to answer the question. The ambiguity comes from the fact that
the city concept has not been sufficiently specified. Depending on the nature of the
establishment X and the targeted customers (the inhabitants of the central commune
of A or the active population working in the urban area of A), one of these
“observables” will be more appropriate than the others.
E XAMPLE 1.2.- in the fields of geosciences and environment, this type of question
arises with the same sharpness. In a similar way to the previous example, let us raise
the question about monitoring the evolution of glaciers. Indeed, it is essential in
order to evaluate the associated risks. The dynamics of glaciers is an indicator of
climate fluctuations. It is associated with a number of events that are sources of risk
for the surrounding human activities: avalanches, flash floods, inundations and
mudslides. In addition to these disasters, a number of consequences also affect
economical activities such as that of mountain tourism. Even if the expert agrees on
the conceptual definition of a glacier, irrespective of his disciplinary training,
glaciologist, geomorphologist or geophysicist, the specialist does not use the same
approach to analyze its dynamics and displacement. The points of view will differ
both in the choice of information sources and in the methodology for dynamics
monitoring: some will favor the monitoring of field measurements as that of
glaciological beacons; others will use high-definition satellite images to delineate
the glacier at different dates and observe the evolution of the surface; get others
choose not to start from the delimitation of the glacier, but to build an indicator
whose monitoring in time provides information about the movement of the glacier,
for example, the glacier equilibrium line altitude ( ELA) 2 [COS 11]. All these
approaches allow apprehending the same question (the dynamics of the glacier), but
the quantitative results will be different, since they correspond to different measures,
each appropriate to a particular scale, the fine-scale of the glacier for the
glaciologist, and more often the regional scale for the geophysicist.
The first example illustrates how an insufficient specification of the city entity
leads to the observation of different empirical objects (the commune, the “urban
1 In this framework, the morphology of a city is defined from the built up area. A
morphological criterion is then based on the continuity of the built up area.
2 “The glacier ELA refers to the altitude of the theoretical line between the zone of glacial
accumulation and the zone of glacial ablation” [COS 11].