International Archives of the Photogrammetry, Remote
GetCapabilities operation makes it possible that SCS
can provide general information about service itself and
specific information about available sensors and observables.
The whole provided information by this operation is called
metadata. The result of invoking GetCapabilities operation is
an XML encoded document called Capabilities XML
Using the GetObservation method, the SCS provides
access to sensor observations and measurement data via a
spatio-temperal query that can be filtered by observables. In
addition, the SCS provides two other methods for retrieving
detailed information about the sensors making those
measurements and the platforms that carry the sensors. These
are the Describe Sensor and Describe Platform operations.
5. METADATA
In the GIS world, information is used for research and
manipulation everyday. This information comes from various
sources from various times. Not all data that is in use is
necessarily accurate, precise or up 10 date. But, how can one
determine if it is of the accuracy, precision or as up to date as
the project needs? This is determined simply through the use of
metadata on the particular dataset in use.
Metadata is data about the data itself. This information
helps the user of the data to determine whether the information
contained within the dataset will be useful or in good condition
to be used, by examining the quality, content and condition that
the data is in. The location of where to find the information can
also be a key factor in the use of the data. If it has to be sent
away for, but the data is needed in an instant, then that data will
not be of good use as access is poor to the user. This has the
same purpose as in a library catalog system. When searching
for a document one looks for the title, year it was published,
author, description, location to find the document and many
other attributes of the document to determine whether the
document is of any use [5].
It is obvious that each web service needs to be
described and published to one or more Internet registry service
so that the users can locate and bind. In fact, published
information is metadata about service (Figure 3). OGC has
determined the following roles for metadata:
e Metadata specifies the characteristics of service
provider. Service registry uses these characteristics to
categorize service providers to support the find
operation and Service requesters use them to match a
service provider to their requirements.
e Metadata specifies non-functional characteristics.
Non-functional characteristics may be used to help a
service requester find a service provider.
e Metadata describes the interfaces used to access the
service. The interface description includes its
signature, allowed operations, data typing, and access
protocols. Service requesters use this information to
bind to the service provider and invoke its service
using the published interfaces.
Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
6. DATA FORMATS AND PROTOCOLS
The need for format/protocols comes from data
heterogeneity between different programs and systems that are
working on the Internet. To success in transferring and sharing
data on the Internet it is necessary that component of the
network use a common and standard vocabulary
(formats/protocols) for their data encoding.
6.1. XML (eXtensible Markup Language)
"The Extensible Markup Language (XML) is the
universal format for structured documents and data on the
Web." [6].
The Extensible Markup Language (XML) is an
extremely simple dialect (subset) of SGML the goal of which is
to enable generic SGML to be served, received, and processed
on the Web in the way that is now possible with HTML. XML
has been designed for ease of implementation, and for
interoperability with both SGML and HTML.
XML documents are made up of storage units called
entities, which contain either parsed or unparsed data. Parsed
data is made up of characters, some of which form the
character data in the document, and some of which form
markup. Markup encodes a description of the documents
storage layout and logical structure. XML provides a
mechanism to impose constraints on the storage layout and
logical structure. À software module called an XML processor
is used to read XML documents and provide access to their
content and structure. It is assumed that an XML processor is
doing its work on behalf of another module, called the
application.
6.2. GML (Geographical Markup Language)
GML is one of the most important elements in
interoperability Program managed by Open GIS. It provides
standardized components to encode geographic information.
Geography Markup Language (GML) is an XML grammar
written in XML Schema for the modeling, transport, and
storage of geographic information developed by Open GIS.
GML provides repository of schemas available on the
Internet to support the encoding geographic information.
Measurement.xsd and Value.xsd are the schemas, which supply
the required elements to encode and describe sensor collection
service observations and measurements.
The main idea about measurement model comes from
OGC feature model. Based on the OGC definition feature is the
basic item for geographic information. A feature has a number
of properties, some of which may be geometric and spatial.
Within measurement model a measurement is defined as à
specific feature type with particular properties to encoding in
situ sensors observations. Measurement model determines
resultOf, MeasuredAt and timestamp properties to encode
observed value, type of observer (sensor), location ©
measurement, and time of measurement respectively.
Value model provides elements required to encode
observed value by the in-situ sensors. There are different kinds
of classes for observed value.
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