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open platform to publish and consume interoperable sensor data is described
in [116].
In general, associating sensor and sensor network data with other concepts
(on the Web) and reasoning makes the data information widely available for
different applications, front-end services and data consumers. The semantic
description allow machines to interpret links and relations between the different
attributes of a sensor description and also other data existing on the
Web or provided by other applications and resources. Utilising and reasoning
this information enables the integration of the data on a wider scale, known
as networked knowledge [115]. This machine-interpretable information (i.e.
semantics) is a key enabler for the semantic sensor networks.
2.8.4 Virtual Sensors
A virtual sensor can be considered as a product of spatial, temporal and/or thematic
transformation of raw or other virtual sensor producing data with necessary
provenance information attached to this transformation. Virtual sensors
and actuators are a programming abstraction simplifying the development of
decentralized WSN applications [117].
The data acquired by a set of sensors can be collected, processed according
to an application-provided aggregation function, and then perceived as the
reading of a single virtual sensor. Dually, a virtual actuator provides a single
entry point for distributing commands to a set of real actuator nodes. The
flow of information between real devices and virtual sensors or actuators is
presented in Figure 2.30. We follow that statement with this definition:
• A virtual sensor behaves just like a real sensor, emitting timeseries
data from a specified geographic region with newly defined
thematic concepts or observations which the real sensors may not
have.
• A virtual sensor may not have any real sensor’s physical properties
such as manufacturer or battery power information, but does have
other properties, such as: who created it; what methods are used,
and what original sensors it is based on.
The virtualization of sensors can be considered at different levels as presented
in Figure 2.31. At the lowest level are those related with the more local