Global Positioning System Reference
In-Depth Information
request and response operations should be implemented when HTTP is used
as transport protocol. The supported HTTP methods are GET and POST and,
for each operation, the server must support at least one of these methods. In
particular, for the request operations, the standard states that the encoding
of parameters and operations within the POST method can be done by using
either KVP encoding or an XML document. When using XML the operation
request can be transferred “as is” over HTTP or contained in a SOAP
envelope. On the other hand, with the GET method, only a KVP encoding
can be used since a valid HTTP GET request can be seen just as a URL with
a fi xed prefi x where additional parameters are added. Upon receiving a
correct request, the service must send a response or an exception report
if unable to correctly serve the query. In the following subsection, a quick
overview of the Geographic Markup Language (GML), the cornerstone of
the whole OGC architecture is given. Then, a brief description of the basic
characteristics of the three main types of services is provided.
The Geographic Markup Language
Geography Markup Language (GML) is 'an XML grammar written in XML
Schema for the description of application schemas as well as the transport
and storage of geographic information' (Portele 2007). In other words,
GML is an XML language to manage geographic information, where the
GML schema describes the document and the instance document contains
the actual data.
The main goal of GML is to provide a means to exchange and
manipulate geographic information in a standard way that is guaranteeing a
programming language and source format independency. In general terms,
the role played by GML in the context of geographic information can be
split into three main categories (Lake 2005):
an encoding standard for the transport of geographic information from
one system to another,
a storage format for geographic information, and
a modelling language for describing geographic information types.
As for the fi rst two aspects, GML can be considered as the OGC answer
to the need of representing geographic information in a standard manner
in order to facilitate communication and data exchange among a wide
variety of autonomous and distributed data sources thus contributing to
the reduction of costs related to the management of spatial information.
Like XML, GML represents information in a textual form, focusing on the
content description and relying on other mechanisms for data visualization.
Another advantage is that being XML-based, people can immediately
use the plethora of available XML tools to perform all common XML
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