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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
and suitability of the images. It also configurable if the image
has to be sent to the end user or should be “forwarded” to
another service (saving that way time and bandwidth, especially
if the next service is running out of the same server, minimizing
interaction with the end user). Users can also select the required
area. The way this is done is left to the client application (the
web service cares only about the bounding
coordinates and not of the way they were chosen).
rectangle's
4.3 NDVI calculation service
This is the web service that actually implements online the
NDVI calculation algorithm described in Section 3. This is a
chained web service: it needs the satellite imaging web service
in order to get the satellite images in the required format. The
algorithm produces in turn a series of intermediate images,
which can also become available to the end user by user's
request through the web service interface. The output of the
web service is a processed NDVI image. The distinct steps of
the calculation algorithm can be transparent or not depending
on the user's request. The whole processing takes place at the
server and the user is only receiving the final image, thus
reducing the overall downloading times and wasted network
bandwidth.
There are several open issues in the area. It seems the ability to
provide a caching scheme for the produced images. Users might
select again and again the same images, so such a scheme
would have saved redundant and repeating computations. On
the other hand a balance has to be implemented in order for the
system to actually take into account the proposed use of the
service in order to decide if it needs to reproduce the same
image (for example one image for download purposes only
might be treated differently than an image that needs to be used
in NDVI calculations). Another interesting issue is the ability to
formally describe the produced NDVI images. This introduces
the necessity of a semantic description language like RDF/S or
OWL and the production of a corresponding ontology.
4.4 Publication and Discovery of services
Although the need to publish the web services in a service
repository is planned in the framework design, so as to allow of
easy and seamless discovery of the services, it is not
implemented yet. The small number of the implemented web
services does not make it necessary for such a publication since
a link can be provided to the requesting client through the Web
Service Description Language (WSDL).
But the lack of such a universal repository makes publication
and discovery rather a difficult task at the moment. This does
not affect the functionality of the proposed framework is
planned for future development, being associated with the fact
that the overall design calls for the broader possible availability
of the services. The AXIS web services framework supports the
addition of publication and discovery catalogues at a later stage
and this step will follow. The necessary messages (SOAP
messages) are already being produced by the framework,
making the adoption to the use of a cataloguing service
straightforward.
45 Client Applications
Clients are being developed in conjunction to the proposed web
services framework in order to provide a demonstration means
of it. Clients are showing the ability of almost anybody to
677
connect to the framework by calling and using the appropriate
web service. The developed so far clients are targeted the expert
environmental modelling community, instead they are just
demonstrating the feasibility and availability of services.
Anybody is invited to built its own client, either desktop or web
based, upon the framework, which is and will remain open to
the environmental community.
Two kinds of clients to the environmental monitoring web
services are being developed. The first one is a web based
client, which presents the information to the end users through a
standard web site. Users can supply through web forms the
required input (like the area of interest, the index they want to
be calculated, etc). The processed information is returned to
them through one or more web pages. The web-based client
application does not require any installation from the user side
and only allows the user to save the resulting image.
( AXIS
Discovery and Publication Services
LT 0
Figure 4. The chained web services communicate inside the
framework using SOAP messages.
The local client application is "calling" the web services as it
would have done with any software library bundled with it. The
results are downloaded through the internet to the local
application and extra processing can occur at the local level.
This application can combine these data with locally stored data
or with data form other external sources or web services. This
shows that superior functionality can be incorporated to a local
client application but on the other hand such an application is
more demanding form the client machine. Usually a desktop
application requires some kind of installation and of course an
internet connection in order to access the web services and
retrieve the results. It is important to note here that the client
application can be developed totally independently from the
services framework and can be in any language that supports
use of web services. The overall approach is based on the
production and exchange of the corresponding SOAP messages
that describe both the necessary results and the communication
