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XML resources, it is worth noting C-Web Portal
(Amann et al. 2001) and (Amann et al. 2002). C-
Web Portal supports the integration of non-spatial
resources on the Web, and C-Web provides the
infrastructure for (1) publishing information sources
and (2) formulating structured queries by taking into
consideration the conceptual representation of a
specific domain in the form of an ontology. On the
other hand, (Amann et al. 2002) proposes a mediator
architecture for the querying and integration of Web-
accessible XML data resources (non spatial data). Its
contribution is the definition of a simple but
expressive mapping language, following a local as
view approach and describing XML resources as
local views of some global schema.
In relation to spatial data integration, there are
approaches developed by (Gupta et al. 1999) and
(Boucelma et al. 2002). (Gupta et al. 1999) extends
the MIX wrapper-mediator architecture for
integrating information from spatial information
systems and searchable databases of geo-referenced
imagery. (Boucelma et al. 2002) presents a
mediation system that addresses the integration of
GIS data tools, following a GAV(Global as View)
approach.
In order to design approaches for querying spatial
GML resources, we have based our work on two
existing studies: (Amann et al. 2002) and (Amann et
al. 2001) mentioned above. We have selected the
first approach ((Amann et al. 2002)) because it is
focused on integrating XML resources, and it can be
extended in a simple way to query GML resources
with spatial operators. The second approach has
been selected because it is an interesting approach
that makes it possible to query different resources on
the Web. By this we mean that modifying it adds the
possibility of querying GML resources. In addition,
both approaches follow a LAV (Local as View)
integration. The LAV approach facilitates the
maintenance of the integrated schema and
mediation, although query evaluation is far more
complex than the global-as-view approach where the
integrated schema is defined in terms of source
schemas. The LAV approach is therefore favoured
in the context of the integration of resources that
change significantly over time, such as Web
resources. In short, by modifying these contrasted
approaches we exploit the solution to query XML
data on the Web.
The overview of both architectures and the
modification applied to query spatial information are
shown in Section 2 and Section 3. In Section 4 we
conclude with a comparison of the two approaches,
emphasising the most important advantages and
disadvantages, in order to obtain the best approach
for developing a real system for querying GML
resources stored in different sources.
2 APPROACH BASED ON RDF
A Community Web Portal(Karvounarakis et al.
2000)(C-Web) essentially provides the means to
select, classify and access, in a semantically
meaningful and ubiquitous way, various information
resources (sites, documents, data) for diverse target
audiences (corporate, inter-enterprise, …). The core
Portal component is a Catalog holding descriptions,
i.e. metadata, of the resources available to the
community members. In order to effectively
disseminate community knowledge, Portal Catalog
organises and gathers information in a multitude of
ways, which are far more flexible and complex than
those provided by standard (relational or object)
databases. It uses the Resource Description
Framework (RDF) standard (Brickley et al. 2000)
proposed by W3C, designed to facilitate the creation
and exchange of resource descriptions between
Community Webs. In order to query the Catalog, a
query language, called RQL, is presented in
(Alexaki et al. 2001) which allows semistructured
RDF descriptions to be queried using taxonomies of
node and edge labels defined in the RDF schema.
In order to integrate the spatial information of
several spatial XML documents (GML), we have
based our work on the Community Web Portal
concept (Amann et al. 2001) with RDF and RQL, a
declarative language for querying both RDF
descriptions and related schemas. To perform this
integration, it is necessary to make some
modifications to the original approach because in the
original approach the Catalog is considered as a
collection of resources identified by URIs and it is
described using properties. However, it does not
need to use operator over the resources, only over
the properties.
GML documents (or part of) are a resource. Unlike
the original approach, it is possible to apply spatial
operators (comparatives: cross, overlap, touch;
analysis: Area, Length) over the resources provided
they represent geometry information with GML. In
order to take advantage of this fact, we have
designed two modifications with respect to the
original approach:
Extension of RQL to support spatial operators over
the resources that represent spatial documents or part
of spatial documents. These operators must be the
same as those defined in (Corcoles et al. 2001) for a
query language over GML. There are two types of
operators: methods for testing Spatial Relations and
methods that support Spatial Analysis. This
extension is not dealt with in this paper.
Extension of the Community Web Portal architecture
to support the application of the spatial operators
STUDY OF DIFFERENT APPROACHES TO THE INTEGRATION OF SPATIAL XML WEB RESOURCES
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