GML, WFS and SVG: A New Frontier of Internet GIS Zhong-Ren Peng Director Center for Advanced Spatial Information Research Associate Professor Department of Urban Planning University of Wisconsin-Milwaukee zpeng@uwm.edu
Problems of Current Internet GIS Programs Interoperability problem The client can only talk with its own server; The client cannot access to distributed data in different format in real time. Quality of graphic images Most current Internet GIS programs display spatial data over the Internet in raster image. The raster images in GIF, JPEG format is not scalable. The raster image is not intelligent.
The Use of GML, SVG and WFS to address the issues use GML to model, code and transport geospatial objects to achieve data interoperability, use SVG to improve image quality on the Web, and use WFS to query, access and retrieve data at the feature level in real time on the Web.
Geography Markup Language (GML) An OGC Standard to model, encode, transport and store geographic objects. GML offers standard ways to describe spatial features and their corresponding properties in terms of GML Schemata, including schema to describe features, coordinate reference systems, geometry, topology, time, units of measure and generalized values.
What GML is not? GML Is NOT a Presentation Language for Data Display GML Is NOT A Programming Language for Data Processing
Why GML? ArcView shapefile Oracle Spatial ArcInfo coverage Intergraph CGM Mapinfo AutoCAD
Why GML? ArcView shapefile Oracle Spatial ArcInfo coverage GML Intergraph CGM Mapinfo AutoCAD
GML An Example <Point> <coord> <X>10.0</X> <Y>2.0</Y> </coord> </Point> OR <Point> <coordinates>10.0,2.0</coordinates> </Point>
GML Schema: -- An Example <element name="coord" type="gml:coordtype" /> <complextype name="coordtype"> <sequence> <element name="x" type="decimal"/> <element name="y" type="decimal" minoccurs="0"/> <element name="z" type="decimal" minoccurs="0"/> </sequence> </complextype>
XLink and XPointer to Form A Geospatial Web
How to Display GML data on the Web Use Scalable Vector Graphic (SVG). A W3C standard format. Works directly with the Web browser. Requires a SVG plug-in (free from Adobe or other vendors).
Advantages of SVG A vector graphic, NOT raster image formats like GIF, JPEG and PNG. Vector graphic is scalable SVG format can be displayed in any devices with any size (cell phone or 19 computer monitor) and any resolution without changing clarity. SVG can grow to a larger number of file, a large number of users, and a wide variety of applications on the Web.
Advantages of SVG AVG reduces the size of the map images by allowing complex scalable cartography in a highly compressed form. As an XML application, SVG provides hyper links to many other files and vector and raster graphics. since SVG file is XML file, it offers supreme portability. It can work directly with other XML-based technology. A SVG file could be edited and displayed in any environments regardless of computer operating systems and Web browsers.
From Ron Lake, 2001
How To Extract data OpenGIS Web Feature Server (WFS) An OpenGIS implementation specification Can extract data from the data sources in their original formats. Transform extracted data to GML. Transport the extracted data to the Web client.
Data retrieval process using WFS Client WFS Request WFS Response Web Feature Server (WFS) Opaque Feature Store
Functions of WFS WFS allows the client applications to access, query, create, update and delete data elements from the database server over the Web. Client applications can post request for feature level data stored in one or more distributed datastores. The WFS server reads and parses the request and returns the result in the form of GML.
Some Examples Put GML, SVG and WFS together to develop standard-based Internet GIS. Encode Spatial objects using GML. Use existing data (in Shapefile) to create GML and SVG.
Example 1
Example 2
Clear Vector Map
Advantages of GML+SVG+WFS Standard-based, Non-proprietary, Interoperable GIS components and data, Text-based, easy to understand, High quality maps on the Internet, Fast response on the client-side, Data can be in their original formats but could be retrieved using WFS and transformed into GML on the fly.
Further research Integrating geospatial data with different data models. Address the problem of semantic interoperability. Proper compressing mechanism for GML and SVG files. Desiring Multidimensional GML-based databases.
Questions and Comments Email: zpeng@uwm.edu http://www.uwm.edu/~zpeng