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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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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;
