## 5 Challenges and Future Work for SWE
### Enabling the Semantic Sensor Web and Linked Sensor Data  
Research on the Semantic Sensor Web [140] investigates the role of semantic annotation, ontologies, and reasoning to improve Sensor Web functionality such as sensor discovery and sensor integration. It combines OGC's vision of a Web of sensors with the reasoning capabilities of the Semantic Web [141]. Related work in this field includes methods for linking geosensor databases with ontologies [142], a semantically-enabled Sensor Observation Service (SemSOS) [143], an analysis of the challenges to realize semantic sensor plug and play [126], or the semantic annotation of sensor services with terms from ontologies [144]. Recent approaches to enrich geospatial services with semantics include an OWL-Profile for the Catalogue Service Web (CSW) suggested by [PERSON] _et al._[145] and the development of a transparent semantic enablement for SDIs [146]. The latter approach defines specific profiles for Web Processing Service (WPS) and CSW to serve functionality for reasoning and ontology look-up, respectively.  
Ontologies need to serve as the basis for semantic reasoning. Hence, thoroughly defining models for sensors from an ontological perspective is a challenging research task. Various research groups started to specify sensor, stimuli, and observation ontologies. Examples include the Semantic Web for Earthand Environmental Terminology (SWEET) (_[[http://sweet.jpl.nasa.gov/ontology_](http://sweet.jpl.nasa.gov/ontology_)]([http://sweet.jpl.nasa.gov/ontology_](http://sweet.jpl.nasa.gov/ontology_))) focusing on modelling of observed properties, observation based ontologies influenced by O&M [147,148], and a sensor-centric ontology with a strong relation to SensorML [149]. Also, there are domain-specific ontologies, such as the approach of the Marine Metadata Interoperability project (_[[http://marinemetadata.org/_](http://marinemetadata.org/_)]([http://marinemetadata.org/_](http://marinemetadata.org/_))), which is particularly designed for oceanographic sensors [150], but could be adapted to other domains in the future. Promising is the observation-centric ontology recently developed in a consensus process within the W3C Semantic Sensor Network Incubator Group (_www.w3.org/2005/Incubator/ssn_) [151].  
Another important research direction in the context of enabling the Semantic Sensor Web is applying the Linked Data principles to make sensor resources available on the Linked Open Data Cloud [68]. Those sensor resources are identified by URIs which can be dereferenced over simple HTTP calls to retrieve representations of those resources in machine-interpretable formats such as RDF [152]. Research on linked sensor data has for example addressed the design of meaningful URI schemes for sensor resources [153] as well as the filtering and retrieval of linked sensor data through RESTful interfaces [154].  
The presented approaches of the Semantic Sensor Web and linked sensor data build a solid basis for future efforts in this research area. Much work still remains to be done. From the perspective of SWE, a central challenge is semantic enablement of SWE specifications and incorporation into the OGC standardization process. This is needed to pave the way to utilization of those methods in real world applications.