Support for advanced GML objects

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” Copyright © 2011 Open Geospatial Consortium, Inc. All Rights Reserved. 21

7.1.2.5 Digital NOTAM generation

CADAS-AIMDB supports the generation of DNOTAM events following the specification given in the document Digital NOTAM Event Specification, Proposed Release, Increment 1. When TEMPDELTA time slices are inserted via the Transaction operation, DNOTAMs with digital encoding are created and sent to interested parties by a Web Service message. In OWS-8 these are event brokers that themselves categorize, forward and distribute events to registered clients on a subscriber-publisher basis. In the current implementation, only a basic support for DNOTAMs is available. No scenario identification or validation of business rules is performed.

7.1.2.6 Metadata support

In OWS-8, support for the storage and retrieval of metadata on time slice level was added to CADAS-AIMDB.

7.1.3 Luciad

Luciad provided an OGC Web Feature Service to retrieve Special Activity Airspace SAA data provided by the FAA. The WFS supports filtering on properties, spatial filtering for instance, to retrieve airspaces that potentially affect a flight route, and basic temporal filtering. One of the challenges with serving the SAA data is the handling of the separate airspace components that are combined using constructive geometry operations, such as unions and subtractions. The Luciad WFS supports the additional processing required for exact spatial queries on such complex airspaces. This allows you to evaluate exactly whether a point lies inside or outside of a particular airspace. The supplied data also contained links between features, for instance from an air traffic control service to the airspace for which it provides a traffic separation service. These links pose additional issues when they are encountered by client applications. This issue is avoided by the WFS by automatically converting all links to local links to features that were included with the response.

7.1.4 WFS Service Capabilities - Summary

A Web Feature Service supports a certain set of functionality, for example operations, operation parameters, feature types, filter operations as well as operands. This section provides a summary of the respective capabilities for each WFS that is introduced in the previous sections. Table 1 – WFS Service and Operation Capabilities Snowflake GO Publisher WFS Comsoft CADAS- aimdb-WFS Luciad WFS Service Version 1.0   Compliancy tested