Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 114

by W3C, mainly due to the fact that these two standards were developed
in parallel by different organizations.
Basically, the fi rst important difference between the two types of
services is represented by the strong standardization imposed by OGC
regarding the public interface of a geographic service. As a matter of fact,
unlike W3C services, each OGC service represents a separate standard
designed to handle a specifi c kind of data (Ioup et al. 2008). Each of these
standards describes how a service should perform its tasks following a
neutral approach for technology implementation, describes the service
public interface and specifi es additional parameters and data structures
needed in all request and response operations. Furthermore, each OGC
service that implements a particular standard presents a fi xed interface
whose functionality (and the type of data returned) is defi ned a priori. On
the contrary, each W3C service can expose its own interface in a WSDL
document, so that two services offering the same functionality could have
two totally different public interfaces.
As for the OGC specifi cations, the consortium has proposed over time
a quite wide and complete set of them, where the most widespread and
commonly used are: Web Map Service (WMS), Web Feature Service (WFS)
and Web Coverage Service (WCS). In particular, to facilitate the development
process of these three types of services, the OGC has developed a Common
Standard that defi nes all aspects that should be common to these three
implementations in addition to their single operational requirements
(Whiteside and Greenwood 2010). These common structures relate to
some of the parameters and data types used in the various request and
response operations. Moreover, every OGC service must provide a standard
way to describe its capabilities to its clients and a client may obtain such
a description invoking the standardized GetCapabilities operation. The
implementation of such an operation is mandatory and, for a client, there
is no other way to know what the capabilities offered by a specifi c server
are.
The Common Standard requires that a request message for the
GetCapabilities operation is encoded in an XML document or using the
Key-Value Pair (KVP) encoding. A KVP is a set of two data items linked
each other: a unique identifi er (the key) for some data item and the value
associated to it. This technical feature represents another important
difference between the two types of services under discussion. W3C services
only use XML, while OGC services are not limited to it. The response to a
GetCapabilites request is represented by a document containing metadata
about the operations supported by the server that implements a particular
OGC specifi cation. Such documents are usually encoded in XML, and
use XML Schemas to specify the correct document content and structure.
Moreover, the OGC Common Standard describes how the encoding of the