Copyright © 2011 Open Geospatial Consortium,
Inc. All Rights Reserved.
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9.5.1 Dealing with the AIXM Temporality Model
In the OWS-7 testbed a workaround was applied to provide all data to the Event Service needed for applying filtering. In particular, the geometries of features were included in
DNOTAMs containing delta timeslices, although the geometries themselves did not change. This is not conform to the intention of the AIXM Temporality Model and
treating events published to an Event Service this way should be avoided as emphasized by the Digital NOTAM Event Specification. Still, Event Service instances should be able
to do filtering and complex processing on any property of a feature to support more complex filtering. To overcome this issue an approach has been discussed within this
testbed. It is described in the following sections.
9.5.1.1 Event Service Conformance Classes
Conformance classes define the level of information needed by an Event Service to apply appropriate filters. Three conformance classes have been defined for the Event Service:
basic, snapshot and full-info.
Basic means that an Event Service can only filter on the data provided by thin
DNOTAM events following the AIXM Temporality Model. In particular, only the changed data is available for filtering.
Full-info means that an Event Service has enrichment capabilities. The Event
Service is aware of an appropriate WFS data store and requests all needed data for an AIXM feature of a thin DNOTAM.
Snapshot is a conformance class in between the former two. An event publisher
should provide snapshots of features to the Event Service who can then support subscriptions that access all properties of an AIXM feature at a given time
though only at the time instant represented by that snapshot.
The communication patterns for the “Snapshot” conformance class do not follow the AIXM Temporality Model as event publishers would provide information which does not
change. But still there are use cases probably beyond AIXM where such Snapshots can be used in an Event Architecture. Within OWS-8 the Event Services either implemented
the Full-info class or stuck to the Basic class.
9.5.1.2 WFS Support for Dynamic Features
To support the Full-info conformance class a WFS data store must support the retrieval of all information of an AIXM feature e.g. Snapshot. The support for snapshots has been
discussed and implemented within this testbed. Details can be found in the OWS-8 Aviation: Guidance for Retrieving AIXM 5.1 data via an OGC WFS 2.0 Engineering
Report OGC 11-073. Nevertheless, it has to be analyzed if a single snapshot is sufficient for such use cases. An event has a time period as its validTime. Hence, a single
snapshot does not always represent every possible state of a feature for the time period of an event. An Event Service should therefore be able to request all states of the feature and
based on these states decide if an Event matches certain filter criteria. A general solution is needed for such decisions e.g. an Event matches if there exists one state of the feature
for which the criteria match.