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					    Dynamic retrieval of Web Feature Service data by Google Maps
                          and Google Earth
                                    Krzysztof Kuśnierek1
   1
       Institute of Physical Geography and Environmental Planning, Adam Mickiewicz
                                 University, Poznań, Poland


The geographic information distributed dynamically by web servers is becoming
a crucial data source for the internet cartography applications. The advantage of pro-
viding dynamic content over static data files concerns not only the optimization of data
transfer between the server and the user but also the emerging possibility of the
advanced on-the-fly data processing.

The Open Geospatial Consortium’s Web Feature Service (WFS) is a promising tool for
dynamic data distribution which is currently being implemented within the Spatial Data
Infrastructure. In its basic form, WFS provides the interface for retrieving and
displaying the geographic features stored in the vector format. Advanced implementa-
tions of WFS allow editing and spatial analysis of data. Along with the development of
the WFS data providers network and the increase of the amount of vector data served to
the users, this type of data distribution may become a major component of web-based
cartographic and geographic services.

To date, Google Maps, which is the extensible and scriptable cartographic content
system is one the most popular browser-based cartographic applications. Among the
digital globes, Google Earth is the best known and mostly used one, since it is already
fueled in with a multitude of different types of data. These applications are capable of
retrieving the dynamic data content, i.e. the content that is retrieved from the server after
each request of the end user. The capabilities of Google Maps and Google Earth are
limited, though. They feature networking interface, through which the spatial
information is dynamically presented to the end user. To accomplish the visualization
process the retrieved data must be yet transformed into the Google’s native Keyhole
Markup Language (KML) data format.

Using the Web Feature Service the data are distributed in the Geographical Markup
Language (GML) file format. The aim of using one standard data format within the
servce is to help to both increase interoperability between WFS servers and to improve
the ease of implementa-tion of the WFS standard. As mentioned above, the data written
in GML are not supported neither by Google Maps nor Google Earth. To allow the WFS
data parsing by these applications, the GML output data must be translated into the
KML format.

The method of dynamic retrieval of WFS data using Google Maps or Google Earth
incorporates the event-handling parameters generated by these applications after each
user’s request. The bounding box event-handler, which represents the area, presented on
the user’s screen is forwarded to the Web Feature Service using HTTP. The WFS
generates the output data in the GML format. The restyling of the spatial data into KML
format is carried out dynamically by the transformation stylesheet writtern in the
eXtensible Stylesheet Language (XSL) and the data is then appended to Google’s
cartographic applications by the custom PHP program. The whole transformation pro-
cess runs dynamically, with no user’s interaction.

The working example of the dynamic WFS data retrieval method is based on the polish
Geoportal Web Feature Service providing the vector data of the administrative and
cadastral boundaries and the associated attribute data (http://www.geoportal.gov.pl).
The proposed concept may be utilized not only in amateur or commercial mapping
services but also by the spatial data providers on both commercial and governmental
levels, who would like to provide their users with the spatial data in the most popular
form, within the Google Maps and Google Earth. The components integration schema
as well as the selected source code excerpts are presented here to give the background
for both methodological and technical discussions.

				
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posted:12/24/2010
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