International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
imaging (Pantham, 2000). AVHRR channels 1, 2 and 5 are
converted to “Bufteredlmage” objects using BufferedImage
class functionality. A BufferedImage object contains two other
objects: a "Raster" and a "ColorModel". The Raster class
provides the image data management, whereas the ColorModel
class provides a colour interpretation of the pixel data provided
by the Raster. The Java2 code implementing the NDVI
algorithm consists of 5 different modules, which are activated
remotely by the GIS web services:
a) Image Viewer Module: Used to view the produced
images. Not used for the web service implementation.
b) Calibration Module: It consists of three different
classes performing the image calibration with the use
of pre-launch calibration coefficients. It produces a
new Buffered [mage for each AVHRR channel.
c) NDVI Module: This module uses the calibrated
channels 1 and 2 and produces the intermediate NDVI
image according to the Eq. (1).
d) Cloud Mask Module: It uses the calibrated channels 1
and 5 and produces the cloud mask image according
to Chrysoulakis and Cartalis (2002).
e) Export Module: This module combines the cloud
mask image and the intermediate NDVI image and
produces the final NDVI image. In the latter, all
pixels corresponded to clouds have been masked as
shown in Figure 2. Finally, the NDVI product is
stored in the application's database.
AT
Figure 2. The NDVI product for Crete, Greece on June 18, 2003
(NOAA17/AVHRR).
4. DESIGN AND IMPLEMENTATION OF
ENVIRONMENTAL WEB SERVICES
THE
In this section an overall description of the design of an
environmental web services framework is provided. Moreover
both the implementation and deployment of the services are
discussed. Specifically the tools and the framework utilized in
the implementation are presented, along with the developed
services. The NDVI algorithm was chosen as a demonstration
because it is a simple algorithm and rather simple to implement
using only satellite data, since our focus is on the services of the
framework. Furthermore, it can be used to calculate other
parameters like land surface temperature.
4.1 Servers and services
As described in Section 2, there are various actions taking place
in the server side of the web services interaction process.
Mainly the “service” component is a server based component,
which inevitably will return some results to the requesting
client. Many tools are available in order to facilitate building of
web services. Our goal was to focus on open source tools and
676
environments on the one hand (making sure beforehand that all
the required functionality will be there) and on easily deployed
and used services on the other, since we are targeting a non
computer science audience.
To create a server environment for the proposed framework a
simple LINUX box is adequate to perform the server role. The
ApacheTM Web server (Apache Foundation, 2004), along with
TOMCATTM (Apache Foundation, 2004) as the servlet
container, was used to facilitate web server support. TOMCAT
is a full blown servlet container that provides support through
AXIS (Apache Foundation, 2004) for web services. It provides
a mechanism to accept and dispatch SOAP messages in order to
facilitate communication among the server and the requesting
client.
The overall implementation for both the server and the client is
done in Java. Java consolidates almost all of the required
capabilities for such a system, being both platform and system
independent, having an extensive background support and use
in the web services arena, being extensively used and supported
by the open source community and finally having already in
place the required APIs for such a task.
For the services implementation the J2EE (SUN Microsystems,
2004) platform was the underlying toolkit. On J2EE is based the
implementation of AXIS, that is used for supporting web
services creation and discovery. AXIS provides also tools for
automatic generation of the description XML based documents
for each service. These documents describe in WSDL the
required inputs and expected output of the service. The overall
architecture of the framework is depicted in Figure3.
TOMCAT
XERCES
(XML Parser) nv...
SOAP
A Messages
AXIS
(Web Services
Catalog)
INT
I
>
LI
«
NSDL
(
Environmental
Monitoring
Services
Figure 3. The overall web services’ architecture.
4.2 Imaging service
The first building block of the framework is the imaging web
service. This service takes as input NOAA satellite images as
described in Section 3. These images are undergoing a specific
transformation procedure, making them available for use by the
NDVI calculation web service. The satellite imaging service
can also be used as a standalone service in case users would like
to take into their hands not the processed NDVI images but the
raw satellite data, with or without a performed transformation.
This service handles satellite images in order to provide them
either to the user directly or to the service following this one. It
can participate in services chaining usually being the ring in the
chain to be formed. The necessary have been
implemented in the service's code to check for the availability
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