International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
environmental modelling community. The framework will
support functions that can be used by desktop and wcb-based
applications alike in order to perform environmental modelling
tasks using satellite images. In the next section a more detail
presentation of the value, applicability and functioning of GIS
web services is presented and reviewed. Section 3 describes the
methodology used to calculate the Normalized Difference
Vegetation Index (NDVI), along with the necessary satellite
images. Section 4 presents the design and implementation of the
proposed web services framework and describes in detail the
interconnections among the different pieces of software and
hardware. Finally, section S concludes the work presented in
this paper by summarizing the research conducted so far and
suggesting future work.
2. GIS WEB SERVICES
2.1 Web Services
Web Services is a constantly emerging technology that allows
many diverse internet based applications to interact in order to
exchange data and software. They are one, rather significant,
instance of the new service based computing paradigm. Web
services are actually providing application-to-application
communication over the internet, although applications
involved do not have to be explicitly web based. Software
developers do not have any more the restriction of only local or
restricted remote access to software modules. Applications can
now automatically search and discover other software modules
that can be used to perform specific tasks. These modules are
well-described following technologies and standards that,
although still in the standardization process, are well and widely
accepted. Both data and software can be provided through a
service. The way the service operates is transparent to both the
developer, who only describes his own needs, and the end user.
Figure 1. The web service access model - taken from (Saganich,
2001).
Web services are based on XML (eXtensible Markup
Language), which is a description language widely used
(xml.org, 2004). Many XML based technologies are used to
describe, communicate with and finally use a web service.
These technologies described in the next paragraph have
become the de facto standards in the area and enjoy almost
global adoption even form rival companies (like Microsoft,
674
SUN, IBM, etc.). Having described that, one can point out a
twofold benefit situation for developers: The architecture is
both (a) language and system neutral and ignorant and (b)
software and/or data neutral (in the sense described above).
But how do web services work? As it can be also seen in Figure
2, there are three major players involved in every web service
interaction. There is first of all the client who issues the request
describing the required service (the requestor). This request is
passed to a Service Repository, which handles it. Depending on
implementation, the repository will either provide a link to a
suitable service, if it finds onc, or if it does not, it will either
pass it to another known service repository or just inform the
requestor that such a service is not available. The link that the
service repository returns to the client corresponds to the
service provided by the service provider, who is the last vertex
of the triangle. At the provider's location the requestor gets
information on what to expect form the service and how to
actually call it from within its own code.
2.1.1 — Chaining of Web Services: Finally one of the biggest
advantages of web services is their ability to be chained. By
creating a web services chain one can expand the possibilities
of the developed application. Many different and diverse
information sources and software can be combined by forcing
the one to use as input the output of the other, providing in that
way an application (or broader speaking an information source)
with extreme value added.
2.2 Related Technologies
There are many different technologies and languages — mostly
based on XML - involved in the construction, publication and
use of a web service. The technologies discussed herein are not
related with the actual construction of a web service per se. For
that purpose any programming language can be used (and in
this case any is literally used). Most of these technologies are
self-descriptive and they describe the usage of the service or the
messages being passed among the services and the applications.
Each one plays a specific role and each one is both agnostic and
transparent to the other. These technologies include:
* UDDI (Universal Description, Discovery and
Integration) (uddi.org, 2004), is used by services in
order to register themselves at one or more
repositories. It is absolutely cross-platform and
provides a platform independent open framework for
describing, discovering and integrating services.
e WSDL (Web Services Description Language) (W3C,
2004), is an XML based file which is used to describe
network services as a set of points exchanging
messages that can be either data or procedure
oriented.
e SOAP (Simple Object Access Protocol) (W3C, 2004),
which is the communication protocol of choice
although it is not the only one used. The biggest
advantage of SOAP (apart the fact that it is XML
based and thus universally and seamlessly used) is
that it can use various different transport protocols as
the messages transport mechanism, including HTTP.
The previously described technologies allow the seamlessly
integration of services. information about services and usc of
services. They are playing a crucial role in the proper use of the
service and the dissemination of the information or the
procedure. Taking into consideration fact that the same service
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