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"SES-SOS-Feeder" can be configured to automatically register 
available sensors from existing observation services. After 
registration, the feeder starts to query for new sensor 
observations within a user-defined minimal time interval. The 
interval will automatically be adjusted within the update cycles 
according to the availability of new values. 
A user can subscribe for an event that he defines in a 
subscription request. This request can be created and subscribed 
using the administration client, which is described in the next 
section. The subscription contains information of the receiver, 
where the notification will be sent, besides the event 
specification itself. Within the WEBBOS project, a special SES 
client has been developed to receive the event notification from 
the SES. It is used to store events locally on the receiver's 
computer system. If the event server sends a notification, the 
SES client informs the user that a new event occurred and 
caches the message for further analyses and actions. 
5. WEB CLIENT 
On the top of the SWE services, a client is required for 
visualizing and analyzing the results of the measurements as 
well as for management purposes. This client should be flexibly 
usable and extensible. Therefore, a system-independent web 
client was designed and developed. For evaluating and 
comparing the properties of different solutions, several 
prototypes were implemented that are based on different 
software frameworks. First, a client (depicted in Figure 6) was 
developed using the UMN Map Server (http://mapserver.org) 
and the ,,Mapbender* framework for spatial data infrastructures 
(http://www.mapbender.org). Sensor information was integrated 
by using WMS and WFS. For the second solution, 
“Mapbender” was replaced by the JavaScript mapping library 
“OpenLayers” (http:/www.openlayers.org). Also this approach 
requests the SOS indirectly by using WMS and WFS. Both 
solutions have shown that a seamless integration of a SOS into 
existing geospatial web services can be achieved with low 
efforts. However, because these services do not consider the 
specific properties of sensor data, both implementations are 
relatively inflexible and complicate more SWE-related 
extensions. 
      
   
   
  
  
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hd-Pfalz ; 
   
  
  
Sensoren Weinsber 
  
Sensoren Weinanbe 
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Figure 6. Web client using “MapBender”. 
The final client is also based on OpenLayers. But in contrast to 
the previous approaches, the extensibility of the open-source 
software OpenLayers has been exploited. A direct interface to 
the SOS service was integrated into OpenLayers by a new class 
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 
OpenLayers.Layer.SOS inheriting from the existing class 
OpenLayers.Layer.Marker. Marker was selected as 
superclass because sensors are represented in the corresponding 
map by point geometries. The processing of the service result 
messages is delegated to a specific class 
OpenLayers.Format.SOS. Because the results of SWE 
services are XML documents, again existing functionality is 
reused by inheritance from the  OpenLayers class 
OpenLayers.Format.XML. 
The measurements are visualized by the JavaScript plotting 
library “flot” (http://code.google.com/p/flot/) which is based on 
“jQuery” (http://jquery.com/). Figure 7 gives an impression of 
the web client depicting SensorML and O&M results received 
by SOS requests. 
  
  
  
  
  
Figure 7. Web client using “OpenLayers” extended by SOS 
functionality. 
An event can be subscribed from the SES via a web request that 
contains its definition. Therefore, an event administration client 
has been developed. It can connect with an SES for requesting 
the service capabilities which include registered sensors and 
available filter functionality for the event definition. The client 
then extracts the capabilities from the result message returned 
from the event service (see Figure 8). 
13 Get Capabilities — (2 Create Subsription — i2 Subscribe Topic 13 Edit Subsription | Sas Rosourne 23 = 
Capabilities 
  
SensoriD: Sensor WESSOY: Motstne-Sianon-DE 1SAIIDSUTADEAS = 
Phenomenon: 
  
  
ComparisonOperator 
GeometryOperand: 
SpatialOperator: 
TemporalOperand: 
TemporalOperator 
  
  
[- EML Capabilities 
  
DataView: 
  
BinaryPatternOperator: DAUSE 
  
Select Function: 
  
  
  
  
Figure 8. Administration client for event subscription. 
To define an event, the user creates a filter request within a 
subscription document. The phenomenon can be chosen from a 
list of phenomena specific for each registered sensor. The 
subscription document is visualized as a tree and can directly be 
created, customized, submitted with the administration client;