XXXIX-B4, 2012 
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International Archives of the Photogrammetry, Remote Sensin 
g and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
Standard visualization tools are often not sufficient for an 
adequate visualization of sensor data. However, the use of SWE 
services restricts and formalizes the syntax and semantics of the 
sensor measurements and of sensor metadata. Therefore, open 
toolkits are well-suited for developing flexible solutions that 
can be easily adapted to the requirements of special 
applications. The WEBBOS project followed this approach by 
using a JavaScript mapping library. The web client developed 
by the project allows retrieving sensor data and metadata as 
well as controlling and evaluating the SES. 
The rest of the paper is organized as follows: In the next section 
we present the overall architecture of our early-warning system. 
This is followed by the presentation of the data model for the 
sensors. Section 4 deals with the event service and alert 
generation. The web client is presented in Section 5. In the sixth 
section we discuss related work. The paper concludes with a 
short summary and an outlook to future work. 
2. OVERALL ARCHITECTURE 
2.1 Sensors and Wireless Sensor Network 
The observation of the soil humidity for early flood warnings 
requires (i) the measurement of soil humidity in different depths 
and (ii) the observation of the soil humidity in areas with a high 
variety of soil humidity (Hiibner et al. 2010). Moisture sensors 
are based on the dielectric properties of water. The higher the 
content of water, the higher is the permittivity (i.e. the electric 
conductivity). However, there is no unique relation between 
water content and permittivity. This relation depends on factors 
like soil density, soil type, grain geometry, temperature and so 
on. Thus, a material-specific calibration is required. Within the 
WEBBOS project, two specific sensor types have bcen 
developed and evaluated: 1.) a ring sensor like it is depicted on 
the left side of Figure 1 and 2.) a multi-part planar sensor 
shown on right side of Figure 1. Both sensor types measure the 
permittivity. 
  
Figure 1. Two types of humidity sensors. 
The demand for Observing areas requires connecting the 
humidity Sensors via a wireless sensor network. In one 
Observation area about 10 to 20 sensors are placed on a strip of 
3 to 100 m length. The sensors are connected to one single 
base unit that enables external communication. Within the 
Project, different network topologies, sensor boards, sensor 
node Operating systems and protocols have been tested. Areas 
With a high variety of soil humidity are typically found near 
43 
small rivers on sloping terrains. Figure 2 shows such an 
observation area. 
  
Figure 2. Observation area with sensors. 
2.2 Web Services and Client 
For systems like carly-warning systems and disaster 
management systems, a standardized and interoperable access 
to the sensor measurements is important (Annoni et al., 2005). 
As mentioned in the introduction, the geospatial web services 
introduced by the OGC SWE initiative allows a web-based 
incorporation of sensor data into specific applications. 
WEBBOS integrated — like many other geospatial sensor web 
projects (see Section 6) — the Sensor Observation Service (SOS) 
(Na & Priest, 2007) for requesting sensor measurements 
according to the Observations and Measurements (O&M) 
specification (Cox, 2007) and sensor metadata according to the 
Sensor Model Language (SensorML) specification (Botts & 
Robin, 2007). In addition the Sensor Event Service (SES) 
(Echterhoff & Everding, 2008) was used for detecting events 
and alert — details are presented in Section 4. 
WEBBOS implemented its SWE services by using the popular 
open-source Sensor Web framework of 52° North 
(http://52north.org/communities/sensorweb/). This allows a fast 
development of operating SWE services. In return, sensors and 
measurements have to observe the given 52°N database schema. 
The resulting data model is discussed in Section 3. 
For visualization and management purposes web-based map 
clients are a suitable choice. For this purpose, standard 
visualization tools like WMS clients miss functionality. This 
had led to the development of special SWE solutions (see also 
Section 6). For the WEBBOS project we extended the popular 
open-source JavaScript mapping library "OpenLayers" 
(http://www.openlayers.org). Beside the visualization of sensors 
and sensor data, the web client allows also controlling and 
evaluating the SES. Details are presented in Section 5. 
2.3 Integration and Evaluation 
The overall architecture is illustrated by Figure 3. It shows the 
data flow from the sensors via SWE services to a web client. 
1