SVG as graphical metadata for distributed spatial data processing

Martin Brändli, Jörg Sparenborg
Swiss Federal Institute WSL
Zürcherstrasse 111
CH-8903 Birmensdorf Switzerland
phone: +41 1 739 23 92, +41 1 739 23 77
fax: +41 1 739 22 15
{martin.braendli, joerg.sparenborg} @wsl.ch

Today, most of spatial data handling applications on the internet concern web mapping. They allow for viewing operations on spatial data and for submitting simple queries. Current standardization efforts such as the initiatives by the Open GIS Consortium (OGC) support this type of geospatial data handling. OGC released the Web Map Service Implementation Specification (WMS) which standardizes the way in which maps are requested by clients and the way that servers describe their data holdings. A main problem of this specification as well as for most of available web mapping applications is the restricted access of data. Data are sent as images and thus cannot be used for spatial analysis and modeling functions such as overlays.

The need for exchanging and sharing of spatial data on the internet that goes beyond images and maps is well recognized, however. For instance, OGC recently released a request for comment on the Web Feature Service Implementation Specification (WFS). A web feature service is a program or module that implements support for query and optionally for transaction operations on web accessible spatial features. The specification enables exchange of spatial data in an XML-compliant way. In contrast to web mapping applications, however, such feature services are data-oriented and neglect the graphical representation of spatial data. Rendering hints may be useful, though, since many geospatial application fields such as geology have agreed-on cartographic symbols for mapping specific data.

This paper proposes the use of Scalable Vector Graphics (SVG) as metadata for the description of the graphical representation when spatial data is exchanged and shared via the internet. We report on a project called Virtual Database which has the aim of integrating different databases of the Swiss Agency for the Environment, Forests and Landscape storing fauna and flora data. The architecture of the Virtual Database follows the principle of loose coupling of individual data components (databases). Every component provides its data according to uniformly specified interfaces. An integration layer collects the data from the internet, integrates and processes them. In a first step, data processing is implemented to enable web mapping functionality. Analysis and modeling capabilities will be added in a second step.

The interfaces for accessing the individual components are implemented according to the OGC's WFS. As a minimum, a service must be able to process requests concerning its capabilities, the description of the feature types and the feature data. Requests and responses are transferred as XML, in particular as GML for feature data.

Providing the description of graphical representations for feature data requires to extend the basic capabilities of the WFS. For the implementation of the Virtual Database, we specify an additional capability that allows to query the graphical metadata. Graphics are defined using SVG elements, in particular symbols, markers, patterns, and the more general presentation attributes.

Rendering spatial data using SVG as graphical metadata is performed in two alternative ways: As a first possibility, a spatial server such as ArcIMS serves as rendering engine. SVG elements are used to construct symbology and legend of the spatial data based on the syntax of the spatial server. As a second possibility, spatial data available as GML is matched with SVG elements and translated into an SVG document in order to enable data display by an SVG rendering engine.

As a conclusion, using SVG as the syntax for specifying graphical metadata allows for the strict separation of data and its graphical representation. Additionally, it allows to profit from the rich and standardized set of features to describe graphics in XML.