Resolve Content Given By Reference

Copyright © 2010 Open Geospatial Consortium, Inc. 97 geometry, a time or some thematic property is the same as the one that will later be filtered upon. In other words, the filter operations can easily be performed. It is also possible to ignore the reference systems completely and to assume that they are the same. An example of an according filter statement would be “air temperature is below 10” and of an appropriate event “observation air temperature is 8.7 °C”. However, in an Event Architecture this may not always be the case. Here, an Event Service can offer functionality to automatically transform the values of the filter operands and those of the incoming events to a compatible reference system so that the filter operation can be executed. The Sensor Event Service OGC Discussion Paper 08-133 defines support for unit conversion. It does not define a generic mechanism to solve the issue. Further work to define such a mechanism would be beneficial, especially with respect to the heterogeneity of events and event sources in a cross-domain Event Architecture.

9.2 Filter Processing Languages

9.2.1 Filter Encoding

The OGC Filter Encoding Specification FES supports filtering of generic XML instances. This is achieved via XPath expressions that are used to point to attributes or elements in the XML. These properties provide the values that a filter operates upon. FES supports a wide range of filter functionality temporal, spatial and thematic filters. It also supports additional functions which can be vendor specific or defined in standards. Applications need to take care that the namespace-prefix binding used for XPath expressions of a filter statement are preserved – this is explained in more detail in the following section.

9.2.2 XPath

The XML Path Language XPath 4 supports filtering of XML instance documents. It provides simple comparison functions but also logical and temporal operations 5 . So far no spatial operations are supported. In addition, operations do not take into account the possible meaning of a complex XML element – like a gml:TimePeriod element contained in the XML instance. Applications need to take care that the namespace-prefix binding used in an XPath expression are preserved and not destroyed by changing the namespace prefixes of XML instances. This can result in the expression no longer being able to identify the correct XML attributes and elements in other words: the event properties. 4 http:www.w3.orgTRxpath20 5 http:www.w3.orgTRxpath-functions