Database Reference
In-Depth Information
type of resource cataloged by the repository server (Content), and the query
language and its functionality supported by the repository. The GetRecords
operation allows users to specify query constraints and metadata to be re-
trieved and returns the number of items in the result set and/or selected
metadata for the result set. The DescribeRecord operation allows a client to
discover elements of the information model supported by the target catalog
service. The optional GetDomain operation is used to obtain runtime infor-
mation about the range of values of a metadata record element. Finally, the
mandatory GetRecordByID request retrieves the default representation of cat-
alog records using their identifier. Through the GetCapabilities
→
GetRecords
→
GetRecordByID sequence, users are able
to probe the repository server's capabilities, search the repository, negotiate
the format of the metadata and finally retrieve the metadata of the dataset(s)
of interest.
DescribeRecord
→
GetDomain
→
10.3.4 Geospatial Web-Services and Standards
The OGC was founded with the mission of advancing the “development of
international standards for geospatial interoperability.”
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The OGC currently
comprises, at time of writing, over 350 companies, universities, and govern-
ment agencies from around the world. In the Earth sciences in particular,
the role of standard data and interface protocols is crucial in the context of
climate monitoring and forecasting. The National Weather Service,
70
for ex-
ample, has recently started to make forecast data available to users using Web
Feature Service (WFS) and Geography Markup Language (GML), two of the
open standards developed by OGC.
In this Web service framework, the concepts of coverage, feature, and layer
play a key role in publishing and accessing diverse types of geospatial datasets
through OGC Web services. Both coverage and feature provide associations
between observed or measured values with a geographical domain, such as a
particular region or spatial extent (see also Section 10.2.4). A coverage can be
thought of as a measurement that varies over space, while a feature is a spatial
object that has associated measurements. In the context of remotely-sensed
data, for example, a satellite image covering an area can be represented as a
coverage. On the other hand, the observation values of a (point-based) weather
station can be represented as a feature. A layer, which basically corresponds
to the concept of a theme, can be either a gridded coverage or a collection of
similar features.
The ability to map heterogeneous forms of geospatial datasets to a few sim-
ple types (such as features, coverages, and layers) greatly reduces the com-
plexity of diverse data types in application and data integration scenarios in
particular, and it makes it possible to standardize publishing datasets using
Web services. Although it is beyond the scope of this chapter to give a de-
tailed technical description of OGC Web standards, Figure 10.3 shows the
