Geoscience Reference
In-Depth Information
But, it would be awkward or even sterile to consider the computing stage without
the conceptualization stage.
2.1.2.
Representing and exploring change and movement (challenge 2)
This stage concerns the construction of synthetic “views” of the database. Each
of these views relates to a corresponding precise question. This phase is closely
linked to the design phase of the database. The questions will have to be made
relative to the structure of this database, namely according to a grammar compatible
with the formal syntax that represents the data model implemented. In this sense,
this stage helps validate the consistency between the data model and questions. As a
result, the exploration of data will be facilitated. As has been said in Chapter 1, most
data models do not represent change directly, but rather sequences of states or
events. The “views” are the result of queries on the database and may involve either
a state (where was the Queen Mary on 1st May 2014?) or a change or a
movement (what was the itinerary of the Queen Mary in May 2014?). It is necessary
that the data model allows answering these questions.
The query is the expression of a question according to a syntax incorporating the
objects and their relationships, their position in time and space. It is important to
identify in the question the different dimensions mobilized and, in this sense, the
decomposition according to the dimensions introduced by Peuquet (space, theme
and time), constitutes an extremely useful grid. The questions are being based on
these different dimensions, or combination of dimensions: it concerns identifying
the dimension targeted by the question (
where? what? and when?
), the other
dimensions then defining the reference (that which is fixed) ([PEU 94]). “What”
refers to the thematic dimension (an attribute or the very existence of the objects).
First, we illustrate simple questions, where only one of the dimensions is queried.
Let us consider the case where the objects are municipalities described by their
localization (space), their urban or rural status and their population (thematic), over
the period 1990-2000 (time). The questions are then decomposed according to the
three dimensions (3Ds), according to a syntax
4
inspired by Peuquet [PEU 94] and
Andrienko
et al.
[AND 03]:
- Where are the urban municipalities in 2000? (theme +time=> space)
- What were the statuses of these municipalities in 1990? (space+time=> theme)
4 The operator “+” corresponds to the fact that both involved dimensions are linked by
“AND”. “
” indicates a variation that is tested on the following dimension. In this last case,
the dimension according to which the variation is calculated, is following, preceded by the
sign “/” (for instance “
Δ
temperature/time” means a variation of temperature over time; while
a variation of temperature over space would be written “
Δ
temperature/space”. The sign “=>”
symbolizes the result of the query and is followed by the dimension concerned by the result.
Δ
