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7.5 Service Discovery Based on Spatial Extent
Within the scope of service discovery an additional goal was the capability to identify services which provide data for a certain spatial extent e.g. Europe, US. For WFS
instances this is achieved by providing an ows:BoundingBox for every item in the FeatureTypeList.
As an ES theoretically can take any data as an input it naturally does not have a spatial limitation. To support discovery based on the spatial extent, the WFS’s integrated an
additional Capabilities element to point to the ES endpoint where DNOTAMs are sent to see Listing 11Error Reference source not found.. This way, the registry is able to
map the ES instances to the WFS instances, hence providing discovery by spatial extent.
Listing 11 - Exemplary Event Service Reference for the IfGI ES.
ows:ExtendedCapabilities es:EventServiceReference
es:Publisher es:serviceNameSesServicees:serviceName
es:WSDLhttp:v-tml.uni- muenster.de:8080EventServiceservicesBroker?wsdles:WSDL
es:Publisher es:EventServiceReference
ows:ExtendedCapabilities
7.6 Accomplishments
Within this OWS, several new aspects and features which improve the data provision services were designed and implemented. Most of these new features were designed to
make the life of clients using these services easier e.g. the introduction of Stored QueriesFilter into the Aviation architecture, “AIXM Features as Geometry Operands” as
well as “Spatial Filtering on Non-spatial Features”. Additionally, a special focus was laid on performance and bandwidth improvement. This has been achieved for instance by
defining the explicit exclusion of metadata when receiving data on AIXM features but also through the introduction of “feature provision by reference” through the
gml:identifier and the design of “Update Intervals” for Event Service subscriptions. The latter also helps a client e.g. a pilot to focus on the most relevant information.
In contrast to previous OWS test beds efforts on creating filters which take the vertical extent of features into account have been made. In particular, the design of simple
altitude queries forms a valuable basis for future scenarios which require the filtering based on altitude limits.
7.7 Lessons Learned
Taking the vertical extent of AIXM features into account when applying filtering has been identified as a challenging task. Especially when dealing with complex Airspace
geometries or three-dimensional flight routes a simple approach was not sufficient. The work on altitude queries improved the overall comprehension on complex AIXM features
and is reflected in a matured filtering capabilities.
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The work on Selective Metadata Retrieval highlighted the complexity of metadata handling within data provision and Aviation architecture in general. A first approach
planned to provide functionality on both inclusion and exclusion of metadata. This could not easily be achieved as remote metadata should ideally not be provided through data
provision services but through a CSW. Additional efforts on embedding a CSW into the data provision workflow are required to foster this work area. Additionally, other
encodings for Selective Metadata Retrieval have been discussed e.g. defining the PropertyExclusion outside of the wsnt:Filter element. The agreement on the provided
encoding has been made as it can be used in combination with Stored Filters. A definition outside of wsnt:Filter is currently not supported through Stored Filters as these only
provide a shortcut to the wsnt:Filter element. Hence, one future work item is to develop a more general projection clause support for ES subscriptions e.g. in alignment with WFS
queries.
7.8 Future Work
During the design and implementation of the advanced filtering functionality several future work items have been identified. The following list provides a summary on these.
Stored Filters – Currently, the design of Stored Filter only takes the wsnt:Filter
part of an Event Service subscription into account. In order to provide other aspects of a subscription through a stored mechanism, a more general approach
should be designed, namely Stored Subscriptions. Thus it would be possible to define policies and projection clauses in addition to the filter part.
Selective Metadata Retrieval – The current design of Selective Metadata
Retrieval only provides capabilities for the exclusion of metadata. One future work item is to develop a mechanism to allow explicit inclusion of metadata
inline or by reference. Here, several aspects such as the dynamic creation of remote metadata instances e.g. through a catalog service need to be considered.
Another future work item is the general definition of projection clauses for Event Service subscriptions. The current approach has the drawback that it cannot be
used without a Filter.
Altitude Queries – A transition from “simple” to “complex” altitude queries is a
very interesting field of work. This requires detailed work on the various ways of how to encode altitude limits within AIXM e.g. taking the unit of measurements
into account. The most complex situations occur when one of two geometry operands provides altitudes as flight levels. Here, barometric pressure calculations
need to be taken into consideration. A complex altitude query design could also be aligned with future work on the Geometry Processing field of work.
AIXM Features as Geometry Operands – It has been identified within this test
bed that an AIXM might have multiple geometries defined. The current design does not take such cases into consideration. Future work should focus on this by
Copyright © 2013 Open Geospatial Consortium.
73 introducing, for instance, a “scenario” parameter which defines how the geometry
should be computed e.g. simple locating vs. complex visualization of a feature. Such work should also be aligned with Geometry Processing work areas.
Event Service Update Intervals – The design of Update Intervals already
provides encodings to define the behavior on how to treat non-related feature events as well as the detection of duplicate events. Nevertheless, the development
of algorithms for these cases is a complex venture and thus has been postponed. Future work on this feature should focus on the design and implementation of
such algorithms.
8 Geometry Processing via Web Processing Service
8.1 Overview