Geoscience Reference
In-Depth Information
- developing a crossed approach. This has been the case of Elodie Buard who
adopted an object approach for defining the “places of animal frequentation” and a
field approach to study the change of the vegetation cover.
We are going to rely on this work to review the four stages around which this
chapter is organized.
2.3.3.1.
Step 1: construction of the entities (objects and properties) from the
empirical data: identifying the “places of animal frequentation” from GPS
readings, and characterizing the change in vegetation cover from satellite images
At one end of the chain, there are conceptual entities that represent places of
places of animal frequentation and the
field
of change of the vegetation cover. At the
other end, there is what is observable: - a series of vectors (id, X, Y and t) derived
from GPS readings; - satellite images corresponding to two dates t1 and t2. The first
step consists of using these observables to build up relevant empirical entities to
represent the defined conceptual entities.
The concept of trajectory to build up “places of frequentation” (places of staying
and places of transit)
Two complementary inputs can be envisaged to build the necessary entities: - start
from the herds of animals and conceptualize them as mobile objects (
endurant entities
)
moving in space and staying in different places; - or starting from the movements,
considered as trajectories (
perdurant entities
) and focusing on their spatial embedding,
on the distances traveled, on the stopping points and on the associated durations. The
first point of view corresponds to the phase of collecting information, from GPS
collars placed on an animal of each of the
k
followed up herds (during a period of
g
weeks). The second point of view is, however, privileged during the construction of
the “places of frequentation”. In physics, a trajectory is defined by the “curve
described by the center of gravity of a mobile” (Le Robert - French dictionary). Buard
[BUA 13] mobilizes the richer meaning developed by Hägerstrand in the framework
of time-geography [HAG 70]: the trajectory of a mobile object (in general, an
individual) is made up of all his movements and stopping points named “stations”
(Figure 2.11). The movements can be used to identify places of transit and the
“stations” to identify the places of staying. The series of vectors (id, X, Y and t),
collected each hour by GPS (series of events) for each of the observed herds, allows
decomposing the trajectories into temporal parts of 1 h and comparing this duration
with the distances traveled. This approach allows evaluating the speeds associated with
different time intervals, and therefore distinguishing the two types of practices,
respectively, corresponding to the travel (greater speed) and sojourn (the “station”,
corresponding to a very slow speed). The author then identifies the “sojourning
places” and “transit places” from the aggregation of information drawn from all the
trajectories of different herds (Figure 2.12). The purpose is to search for concentrations
of events (“hotspots”).
