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Figure 3. Overall architecture and data flow.
Beside laboratory tests, the sensors and the service connection
were successfully tested in two field tests in the area of the
Sulm River and — beyond the original project objectives — for
the a soil moisture monitoring of vineyards. Figure 4 shows the
base station and radio module, which is connected with the
humidity sensors. The vineyard application is later depicted in
Figure 7.
Figure 4. Base station and a radio module.
3. DATA MODEL
The evaluation of soil moisture profiles requires temporal 3D
measurements and 3D models of soil humidity for cach sensor
network: Only the relation and difference between the humidity
in different depths and their development over the time allow
detecting relevant events. The measurements in Figure 5
illustrate this: After a rainfall only the two sensors near the
surface (in red level 1 and in yellow level 2) show a significant
growth of soil humidity. After several rainfalls (in the right part
of the diagram) also the humidity rises in deeper regions (in
cyan level 3 and in green the deepest level 4). The soil is no
longer able to absorb water and the danger of floods increases
significantly.
40
i BF-S-Mierstein-06 R2 BF-S-Niarstein-06_RZ
stain-06 1 3i: BF-S-Nie 6 R4
Sep 13 Sep 16 Sep 15 Sep 22 Sep 25 Sep 28 oci oct 4
Figure 5. Exemplary measurements of soil moisture in different
depths by one sensor.
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
As mentioned in the introduction, the temporal aspect is
sufficiently covered by the current SWE specifications. It is
also quite typical to measure different physical phenomena at
one 2D-location (e.g., temperature, humidity, air pressure etc.).
In our case, however, the soil humidity is measured by several
sensor rings or sensor parts at the same 2D location in different
depths. This property demands for a suitable 3D representation.
For using an existing SWE framework, the given database
schema implementing the current service specifications should
not be modified. As a result, a complete multi-part sensor or a
ring sensor with all of its rings is considered as one procedure.
The 3D position of a single ring is described as a
feature of interest. An observation (Le, one
measurement of one ring at one instant of time) is assigned to
exactly one procedure and to one feature of interest.
Thus, all required properties can be derived. All sensors of one
sensor network are combined by an offering. One — from the
point of view of the authors — unnecessary and objectionable
restriction is that one observation can only belong to one
offering. As consequence, the request other combinations (e.g.,
all measurements of the sensors of level 4) require more
expensive filter definitions.
In addition to soil humidity, temperature and voltage recordings
are included into model as phenomena.
4. EVENT SERVICE
The Sensor Event Service (SES) fits almost perfectly to the
needs for an early-warning system, especially the benefits of
complex event processing for the evaluation of potentially
dangerous situations. The SES does improve the former Sensor
Alert Service (SAS) (Simonis 2006) with various enhancements
for event processing and communication protocols. The only
available reference implementation of the Sensor Event Service
has been developed by 52° North (hitp://52north.org/
communities/sensorweb/ses/0.0.1/). Although the framework is
in an early development state, it has turned out as usable for
event processing within the WEBBOS project.
The first step was to incorporate the SES into the WEBBOS
infrastructure. The SES can be easily integrated in the existing
Sensor Web infrastructure in addition to a common Sensor
Observation Service because of the uniform communication
formats used for SWE services. The SES works as a standalone
service handling different types of messages like O&M
documents with observation data and SensorML documents for
sensor registration. It supports three different levels of filter
definitions to define an event for subscription. There exist
several methods to connect the SES to an existing sensor
observation system. One possibility is to customize the sensor
software so that the sensor system itself sends its data
additionally to the service. Another way to connect the service
is to use a gateway service that receives the sensor data and
forwards it to all services. With this solution no customization
is needed. A third approach is an indirect forwarding to the
event service by retrieving new sensor data from the existing
observations database. No changes to the existing service
architecture are needed for this alternative. Since the data for
the SOS is directly transferred into the database of the SWE
server, the third solution has been chosen for integrating the
event processing service into the WEBBOS system.
For supporting this approach, a solution from 52? North was
developed (https://svn.S2north.org/cgi-bin/viewvc.cgi
incubation/SES-SOS-Feeder/?root»sensorweb). This so-called
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