Introduction to OGC SWE and SOS
Institute for Geoinformatics, University of Münster, Germany.
The sos4R package provides simple yet powerful access to OGC Sensor Observation Service instances. The package supports both encapsulation and abstraction from the service interface for novice users as well as powerful request building for specialists. sos4R is motivated by the idea to close the gap between the Sensor Web and tools for (geo-)statistical analyses. It implements the core profile of the SOS specification and supports temporal, spatial, and thematical filtering of observations.
This document introduces core concepts of OGC, SWE, and SOS standards.
The package is published under GPL 2 license within the geostatistics community of 52°North Initiative for Geospatial Open Source Software.
If you are familiar with the OGC SOS standard specification, know how to use content assist in your favourite R editor, and you do not need to extend the functionality of
sos4R, then take a look at the “Quickstart” vignette and get started straightaway.
Terms and Definitions
The OGC has a particular set of well-defined terms that might differ from usage of words in specific domains. The most important are as follows and are based on https://en.wikipedia.org/wiki/Sensor_Observation_Service.
- Feature of Interest (FOI): The FOI represents the geo-object, for which measurements are made by sensors. It is ordinarily used for the spatial referencing of measuring points, i.e. the geoobject has coordinates like latitude, longitude and height. The feature is project specific and can be anything from a point (e.g. the position of a measuring station) or a real-world object (e.g. the region that is observed).
- Observation: The observation delivers a measurement (result) for a property (phenomenon) of an observed object (FOI). The actual value is created by a sensor or procedure. The phenomenon was measured at a specific time (sampling time) and the value was generated at a specific point in time (result time). These often coincide so in practice the sampling time is often used as the point in time of an observation.
- Offering: The offering is a logical collection of related observations (similar to a layer in mapping applications) which a service offers together.
- Phenomenon: A phenomenon is a property (physical value) of a geographical object, e.g. air temperature, wind speed, concentration of a pollutant in the atmosphere, reflected radiation in a specific frequency band (colours).
- Procedure: A procedure creates the measurement value of an observation. The source can be a reading from a sensor, simulation or a numerical process.
A more extensive discussion is available in the the O&M specification (Cox, 2007). The Annex B of that document contains the examples of applying some terms to specific domains, aerosol analysis and earth observations.
A very good and extensive introduction into the whole field of SWE, including its history, and an analysis of the current state of the art and future developments is provided in Bröring (2011).
sos4R supports the core profile of the SOS specification. But the possible markups for observations is extremely manifold due to the flexibility of the O&M specification. Sadly, there is no common application profile for certain types of observations, like simple measurements.
Therefore, the undocumented profile of the 52°North SOS implementation was used as a guideline. It is not documented outside of the source code. Observations returned by instances of this implementation are most likely to be processed out of the box.
In the author’s experience, OOSThetys SOS implementations utilise the same or at least very similar profile, so responses of these service instances are probably parsed without further work as well.
An incomplete list of **tested services} can be found in package documentation. Please share your experiences with other SOS implementations with the developers and users of
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