Database-driven Generation of SVG and other Graphical Representations

Dr. Martin Huber, CEO GeoTask AG, Güterstrasse 253, CH-4053 Basel, Switzerland, mhuber@geotask.ch

 This paper presents an architecture to manage large amounts of (geo-)graphical data, to edit it in an interoperable way with desktop and Internet based client systems and produce high-quality and interactive renderings in different formats on request. Based on a first prototype in early 1999 producing VML output, the architecture has since evolved and has been used in several real-live applications.

 Data Management

 To make graphical and especially geographical data available for interoperating applications, system independent data management bears considerable advantages. Using standard object-relational database technology, a database extension has been developed to store and geo-reference graphical data types like point, polylines, polygons, Bézier curves, circular arcs, raster images etc. All these types can be manipulated and analysed using the Structure Query Language (SQL) on the common database management system interfaces like ODBC or JDBC. Spatial indexing functions speed up the selective extraction of spatial objects.

 A crucial point for interoperability is the management of metadata on the DBMS. All information about rendering styles and data driven style modifications, map image composition etc. is stored in a system independent way in the database and is similarly interpreted by all client systems.

 Rendering and Interaction

 On this common base, different architectural layouts are possible to render graphical images in different formats and to interact with the graphical data. For Internet applications, an application server interprets user requests, selects the respective geographical data from the database and adds the rendering information. Depending on the format, either the server (e.g. GIF, JPEG, PNG) or the client (SVG, PDF, VML) renders the final image for the user. For interactive web applications, e.g. for map navigation or to link graphical objects with extended object information, client-side rendering has the advantage that object information is directly and interactively available at the client. Using server-side rendering, a considerable scripting and programming effort is necessary to make pictures as interactive as with client-side rendering.

 Desktop clients for graphical editing like Adobe Illustrator or MapInfo use the same rendering style information as the application server. Depending on the graphical capabilities of the client, maps might look different on different systems. Common to all editing clients is the need to support consistent transactions in a multi-user environment. A locking mechanism was therefore implemented that operates across the different client systems. All data access functionality has been grouped in a library so client interface development is simplified to managing a user session, retrieving data from the database and render them in the graphics engine of the client, listen to user events and treat them for data retrieval, locking and carrying out transactions on the database.

For the editing client to run in a browser over the Internet, an additional protocol has been implemented. For security reasons, the application server based on information stored in the database handles user management. Transactions are packed in an XML protocol (e.g. OpenGIS web feature server specifications). Though already very appealing, graphical data editing over the Internet is not limited to the capabilities of the browser (e.g. JavaScript, Java, ActiveX). Using TCP/IP and a web transaction protocol, also the desktop clients can cooperate over the Internet and carry out transactions on a multi-user database.

 Applications

 Automated visualisation of large databases simplifies many business and decision making processes. Several XML based visualisation systems have been deployed based on the architecture presented here. Graphical data editing over the Internet opens new perspectives for cooperation in distributed teams. An application for the coordination of public utility construction has been deployed for the City of Bern in 2000. The two awards the application has received since show the added value in graphics based cooperation on the Internet.