Handbook of best practices

observations from heterogeneous sensors, is often missing in sensor data archives.
Questions like “Which sensor has been producing the value with which quality?”, or “How
has the sensor, which has generated this time series, been calibrated?” may not be
answered, as the required metadata is missing. To address these issues, the Sensor Web
Enablement (SWE) initiative of the Open Geospatial Consortium (OGC), launched in 2003,
provides a suite of technical standards (Bröring et. al. 2011).
The core goal of the SWE initiative is enabling discovery, access, tasking, and analysis of
sensors and observations via the internet. Therefore, the initiative has developed several
standards (i) to model and encode information about sensors and observations and (ii) to
provide sensor-related functionality via web services in the internet. These are illustrated in
the following paragraphs.
6.1.2 SWE information models
In order to ease the exchange of sensor data in the Web, standardized information models
and encodings are needed. Existing models and formats are either missing explicit fields for
spatial information, e.g. Excel or CSV sheets, are only suited for vector data, e.g. ESRI’s
shape file format, or better suited for gridded data, e.g. NetCDF. Nearly all of them are
missing pre-defined fields for sensor parameters and information about what has been
observed. Hence, the OGC SWE initiative has defined the Sensor Model Language
(SensorML) and Observation & Measurements (O&M specifications). O&M provides a
standard data model as an ISO standard and a corresponding XML-encoding as an OGC
standard 3 . According to O&M, an observation consists of:
• Spatial location (optional)
• Timestamps (sampling time and time when result has been generated, if these differ)
• Observed phenomenon
• Sensor that has produced the value
• Result values (sensor output)
• Optional additional metadata, e.g. quality information
Several specializations of the basic O&M observation are available, e.g. Temporal- or
Spatial Observation or Time Series Observation. Furthermore, it is encouraged to define
domain-specific profiles of O&M by restricting the optional elements and adding domainspecific
parameters. An example for such a profile is the Water Markup Language
(WaterML) 2.0 from the domain of hydrology 4 .
For providing a common model and format to describe sensors, SensorML has been
specified by the OGC 5 . It allows to provide sensor metadata, such as:
• Type of sensor
• Spatial location
• Inputs and outputs
• Operator
• Additional parameters like calibration information
3 http://www.opengeospatial.org/standards/om
4 http://www.opengeospatial.org/standards/waterml
5 http://www.opengeospatial.org/standards/sensorml
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