Database Reference
In-Depth Information
In summary, WFS is used for object-based data, and WCS is used for
gridded/raster data. Displaying such data and their overlays is done using
WMS. To illustrate the functionality of these OGC services, consider the WCS
standard as an example. Like the other two services, it defines a mandatory
GetCapabilities operation, which allows clients to get WCS server metadata,
including an optional list of the offered coverages with some metadata for each
coverage. In addition, WCS also defines a mandatory DescribeCoverage opera-
tion that allows clients to get more metadata about identified grid coverage(s),
including details about the spatial extent of the coverage. A WCS GetCover-
age operation requests and returns coverages representing space-time varying
phenomena. In general, through a sequence of GetCapabilities
→
GetMap
(WMS), GetCapabilities
→
DescribeCoverage
→
GetCoverage (WCS), and
GetCapabilities
GetFeature (WFS), client appli-
cations are able to retrieve both metadata and data subsets of interests in a
standard way.
The realization of the above services typically occurs in the form of middle-
ware layers that clients can access through the Web. Among the most promi-
nent representatives of such middleware layers are the open source systems
GeoServer
71
and MapServer.
72
Either system provides a client with transpar-
ent access (using the above OGC services) to diverse types of data stores. That
is, these servers can be configured to access geographic data managed in, for
example, PostGIS, Shapefiles, or Oracle Spatial, and to provide clients with
access to the data through WFS, WMS, and WCS interfaces. In this sense,
such a type of middleware layer already realizes an important component to
data integration scenarios, namely the transparent and uniform access to the
diverse geospatial data sources. For example, using the services, one can re-
quest data in a particular (common) coordinate system. Thus, the services
help in resolving some data heterogeneity issues.
→
DescribeFeatureType
→
10.3.5 Sensor Web Enablement
Of particular relevance are the activities recently taken by the OGC Sen-
sor Web Enablement (SWE) program, one of the OGC Web Services
initiatives.
73
,
74
The SWE initiative seeks to provide interoperability between
disparate sensors and sensor processing systems by establishing a set of stan-
dard protocols to enable a “Sensor Web,” by which sensors of all types in
the Web are discoverable, accessible, and taskable. The SWE standards al-
low the determination of the capabilities and quality of measurements from
sensors, the retrieval of real-time observations in standard data formats, the
specification of tasks to obtain observations of interest, and the asynchronous
notification of events and alerts from remote sensors.
SWE components include models and XML Schemas (SensorML, Obser-
vations & Measurements, TransducerML) and Web service interfaces (SOS,
SPS, SAS, WNS), which are briefly described as follows (see References 73
and 74 for more details):
