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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