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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
have also designed a toolbox for additional client side 
functionality. 
2. WEB SERVICES 
Web services may be realized as the next generation of Web- 
based technology for interoperability. Web services enable 
constructing Web-based applications using any platform, object 
model, and programming language (Barefoot, 2002). A service 
is a collection of operations accessible through an application- 
programming interface that allows users to invoke a service, 
which could be a response to a simple request to create a map or 
a complicated set of image-processing operations running on 
several computers (Hecht, 2002). Web services are for 
application-to-application communication over Internet. Web 
services are based on open standards such as XML to enable 
interoperability. 
required services. 
There are three components of the Web services architecture 
(fig. 1). These are service provider, service broker and service 
requestor. Service broker is sometimes referred to as service 
registry (Cerami, 2002). The interactions among these 
components involve publishing, finding and binding Web 
services. In a typical scenario, a service provider hosts a Web 
service. It provides service description and publishes it to a 
service broker. The service requestor uses a find operation to 
retrieve the service description locally or from the service 
broker and uses the service description to bind with the service 
provider and invoke the Web service implementation (Kreger, 
2001). Service broker is responsible for service registration and 
discovery of the Web services. The broker lists various service 
types, descriptions, and locations of the services that help the 
service — requestors find and subscribe to the 
  
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Figure 1. Web services architecture (Nagappan et al., 2003) 
Web services mean a shift from “human-centric” Web to a 
“application-centric” Web (Cerami, 2002). In its widespread use 
today, Web is human-centric, where the Web has been 
perceived as the means of merely disseminating information to 
as large an audience as possible. This view falls short in 
meeting the expectations of Today, in which users of the Web 
do not only want to get information but also want to do business 
or develop applications over the Web. There are three 
components of “human-centric” Web; The Web, user, and the 
application programs. The way of achieving a goal or “doing 
the job” is the user-Web interaction. What usually happens IS 
the user browses through many web pages, spending plenty of 
his time and hopefully gets some useful information. However 
this is not the end of the story, he might also need to use an 
application program to do the job. This means that the user 
might have to go through many lengthy searches and then 
transfer the results to the application programs. Besides being 
error-prone, this is an unacceptably time consuming process. In 
the “application-centric” Web, it is the applications that 
discovers and manipulates the information. In short, in "human- 
centric” Web it is the human who controls the processes and 
does the job, whereas in “application-centric” it is the 
applications that control and do the job by interacting with each 
other over the Internet and present the results to the users. Web 
services have emerged from the need for such applications. 
There are other names for the “application-centric” Web in the 
3 
literature. For instance, Ryman (2000) uses the term “Service 
Web”. Tim Berners-Lee, the original inventor of the Web, has 
coined the term “Semantic Web” and envisioned the Web 
services as an actualization of the Semantic Web vision Cerami 
(2002). See Berners-Lee et al. (2001) for more on Semantic 
Web. 
3. NATIONAL SPATIAL DATA INFRASTRUCTURE 
(NSDI) 
Interoperability can be defined as the ability by which different 
applications can talk and cooperate with each other. The 
differences may be in the hardware, software or the data 
formats. To date, spatial data interoperability solutions have 
been inefficient, proprietary, and complex. Nevertheless, it has 
been well accepted that the organizational problems are more 
difficult to overcome than the technical ones (MSC, 1993). 
National Spatial Data Infrastructure (NSDI) has been proposed 
to provide the legal framework in which institutional problems 
would be solved by law. 
NSDI has originally been envisioned in the US for “sharing 
data" to cut down the data production cost, improve spatial data 
access and use throughout networked systems in the country. 
Public and private sectors, local governments, universities, and 
finally citizens would have been connected to each other via