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and Grabisch [LAB 03], it is necessary to identify such links and measure them,
which is generally difficult for a non-specialist.
3.3.2.3. Filtering and GIR
In Chapter 2, we have defined geographic information composed of three
dimensions: spatial, temporal and thematic. We have presented various existing
GIRSs and noted that many systems were only interested in two of these dimensions
(generally spatial and thematic). Palacio [PAL 10a] proposes regrouping the
approaches to result combination implemented by these GIRSs in three categories:
filtering (sequential or parallel), linear combination and projection. Let us note that
the filtering approaches are the most widely employed. A presentation of these
categories, illustrated with examples, is proposed in [PAL 10a].
The approach corresponding to sequential filtering consists of performing a first
retrieval based on criteria of a given dimension: the result is a set of ranked relevant
documents. Then, a new retrieval, based on criteria relative to another dimension, is
carried out on this set of results. Finally, a last retrieval concerns the third geographic
dimension. The first two retrievals allow us to reduce the set of resulting documents
(filtering approach), and the last retrieval allows us to obtain a set of ranked results
with respect to the criteria relative to the last processed dimension. Thus, sequential
filtering allows us not to combine the three dimensions directly. The
STEWARD [LIE 07] and SINAI [GAR 09] systems implement such an approach for
a thematic and spatial filtering. Lieberman et al. [LIE 07] argue in favor of a
contextual configuration of the order of use of dimensions in the process of
sequential filtering.
The approach corresponding to parallel filtering consists of querying the
document repository with respect to each of the dimensions of the query separately,
then performing the intersection of the results. Thus, the query is subjected to three
processes proper to each geographic dimension. For every dimension, a set of results
is retrieved. Only the documents present in the three sets are kept. No ranking is
performed. However, in the set of results obtained, some documents are evidently
morerelevantthan others and should appear on top of the list. This approach does not
favor any dimension as does sequential filtering, for example. The SPIRIT [VAI 05],
GRID [VAL 06], GEOTRACKER [CHE 07], WatWasWaar [LIB 08],
CITER [PFO 09] and Document Trajectory [STR 10] systems implement parallel
filtering.
Other approaches to combination are applied to GIR. Linear combination allows
us to combine scores from several IR systems [VOG 99], the objective being to
obtain a unique score. For example, the arithmetic average corresponds to a linear
combination associating an equal weight with each dimension: GEOSEM [BIL 07]
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