Overview Purpose in OWS-8

Copyright © 2011 Open Geospatial Consortium, Inc. All Rights Reserved. 19

7.1.2.3 WFS 2.0 conformance

One objective of OWS-8 is to stress the WFS 2.0 standard and to apply it on AIXM 5.1 data. During this project, not all features of WFS 2.0 could be linked to a use case. Instead of reaching a maximum of WFS 2.0 conformance, COMSOFT focused on the implementation of the actual features used in the demo scenarios. A detailed list of the supported WFS operations and capabilities can be found in Table 1.

7.1.2.4 New and specific functionality and other contributions to OWS-8

Basic WFS support for CADAS-AIM DB was developed in OWS-7. In OWS-8, the support was extended and new features were implemented.

7.1.2.4.1 Support for advanced GML objects

In the document Use of GML for aviation data, version 0.3, a detailed explanation of the usage of GML in the aviation domain is given together with implementation hints. The following table lists the compliance of CADAS-AIMDB with that document: Section Support in CADAS- AIM DB Details 3.1 Use of srsName Yes 3.2 Use of global srsName Partially No inheritance from gml:boundedBy. Developed for OWS -8. 4 Positions Yes 5 Lines and Surfaces Yes Geodesic interpolation is always used. The gml:Geodesic element is not supported. 5.1 Encoding parallels Yes Only EPSG:4326 and CRS:84 are supported. 5.2.3 Arc by edge Yes Developed for OWS-8 5.2.4 Arc by centre point Yes Developed for OWS-8 5.2.5 Circle by center point Yes 6 Point references No The specification is not yet clear about how to implement and interpret references at geometry level as this is a deviation from the general AIXM principle of having xlink:href associations towards the feature level only. References based on gml:ids cannot take the temporality of the information into account as gml:ids are linked to a 7 Geographical border references No 20 Copyright © 2011 Open Geospatial Consortium, Inc. All Rights Reserved. specific property in a specific time slice.

7.1.2.4.2 Challenges in the support for advanced GML objects

Common spatial databases do not natively support curved geometries. They support points, straight lines and complex geometries built upon them only e.g. line strings, polygons. Curved geometries can only be stored in an approximated form by dividing them into straight lines. While implementing advanced GML objects for OWS-8, COMSOFT discovered the need for a precision parameter to spatial query operators. These findings are documented in OGC 11-073, chapter “Precision of spatial filters”.

7.1.2.4.3 Generation and retrieval of SNAPSHOT time slices

SNAPSHOT time slices can be seen as virtual properties of a feature. This is because they can be calculated from BASELINE, PERMDELTA and TEMPDELTA time slices. The SNAPSHOT generation involves a complex merge process, in which time slices have to be ordered, overlaid and filtered. CADAS-AIM DB already supported the retrieval and filtering of SNAPSHOT time slices through its CAW interface. In OWS-8, COMSOFT made the SNAPSHOT support available for the WFS 2.0 interface. As SNAPSHOTs play a central role in the AIXM temporality model, this work included effort in the domain of extending the WFS specification to support it, which can be found in OGC 11-073, chapter Configuring a WFS 2.0 to serve AIXM 5.1”. CADAS-AIM DB also already supports a proposed extension of the SNAPSHOT definition which is SNAPSHOTs for time periods. See sections 10.1.3 and 10.1.3.4.1 for details.

7.1.2.4.4 Advancement of the AIXM profile for WFS 2.0

COMSOFT contributed to the work on the proposed AIXM profile for WFS 2.0, which benefitted from its operational experience. This included a proposal for a new temporal query type as an alternative to the complex extensions needed otherwise to enable a WFS to serve AIXM 5.1 data. This alternative interface is derived from COMSOFT’s CAW interface which proved its value in productive systems. Details can be found in OGC 11-073 chapter “Use case oriented approach to time slice retrieval with WFS 2.0”. Further conceptual work was done in the fields of gml:id uniqueness see OGC 11-073 chapter “Guidelines for ensuring gml:id uniqueness” reverse associations see OGC 11-073 chapter “Implications of Reverse Associations” excluding of optional properties see OGC 11-073 chapter “Excluding of optional properties”