International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanhul 2004
artefacts of an infrastructure of interconnected nodes that
include, among others, data repositories, data brokers, ser-
vice providers, service brokers and clients. Within a GSI,
large geo-processing tasks are achieved by combining or
chaining artefacts located along the distributed nodes. The
GSI system enables Geo-Service Providers to make use of
each others functionality to supply a wide range of services
and possibly to reach larger groups of users.
For this system to work service providers must create and
make available descriptions or models of their individual
services, which can be used as the basis for the specifica-
tion of complex services. A service repository is therefore
required as a central component of the system. The ser-
vice repository supports the exchange of service models
between different service providers. If a model properly
describes an individual service, that is, with the relevant
information at the correct level of detail to enable one to
determine what it does and how to access the function it
provides, then this service can be easily reused. By reuse
of services, we mean the inclusion of a previously designed
service in multiple combinations of more specialised ser-
vice definitions.
2 GEO-INFORMATION SERVICES
The role of the GDI is currently changing, from it being
a simple data discovery and retrieval facility to become an
integrated system suitable for the provision of customised
information and services. We consider a service as the con-
tribution of a system or part thereof to its surrounding en-
vironment. This contribution can be defined in terms of
data, operations, processes, resources, value-added prod-
ucts or'any combinations of them. This For the sake of
simplicity we use the term services to denote geo-informa-
tion services.
Normally developers address the issue of designing com-
plex services by stringing together groups of functions in
an ad hoc manner. This approach may satisfy a partic-
ular need but doing this separately for different services
hampers reusability. Moreover, lack of descriptions of the
solutions obtained makes it hard to aggregate solutions to
execute complex tasks.
Research is therefore focusing on the development of mech-
anisms to describe, combine and manage independent col-
lections of services. Here, we introduce a concept that aims
at facilitating the generation of sophisticated value-added
services. We call it the Geo-information Service Infras-
tructure or GSI for short. The idea of the GSI is that el-
ementary services can be described, accessed, combined
and managed to deliver complex content. Within the GSI,
a common method is used to describe elementary services
and their interfaces, and then these services are made avail-
able for users to create service chains that perform complex
geo-processing tasks.
2.1 GSI
A Geo-information Service Infrastructure (see Figure 1)
is a system from which specialised geo-information prod-
User (2)
Network (Internet)
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7
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Fi gure 1: The GSI system concept
ucts and services can be obtained by exploiting the arte-
facts (data, operations, processes, resources, value-added
products or any combinations of them) of an infrastruc-
ture of interconnected nodes that include data repositories,
data brokers, service providers, service brokers and clients.
We call the above mentioned artefacts of the infrastructure
architectural elements (see section 3). This service frame-
work builds upon the layer of interoperability of informa-
tion as defined by the OpenGIS implementation specifica-
tions (OGC, 1999), therefore separating the actual imple-
mentation of services from their definitions and the percep-
tion of these services by the users.
Large geo-processing tasks can be constructed by com-
bining or chaining sets of architectural elements located
along the distributed nodes. Such combinations of archi-
tectural elements provide diverse functionality that satis-
fies particular sets of requirements. Every architectural el-
ement represents an artefact that has an economic value;
these architectural elements are assembled to perform op-
erations within the infrastructure, resulting in a specialised
architectural element (artefact) that has a value equal or
larger than the combined value of the architectural ele-
ments used. This architectural approach can be regarded
as a “value-added system.” By chaining architectural ele-
ments one can provide a service. A service is defined as
a behaviour of value to the user, which is accessible or in-
stantiated through interaction points (Quartel et al., 2002).
This behaviour is exhibited through an appropriate combi-
nation of elementary architectural elements.
In order to bind multiple architectural elements into a chain
that accomplishes a large geo-processing task, a proper de-
“scription of the participating architectural elements is re-
quired. These descriptions focus on exposing the artefact’s
internal behaviour, its intended effect and its interaction
points or points of composition. These descriptions, which
are presented as instances of well-defined models, make
it possible to interchange and reuse architectural elements.
We call these descriptions system metadata; they are stored
and made accessible through a service repository.
The GSI system enables Geo-Service Providers (GSPs) to
make use of functionality offered by others to supply a
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