The OpenGIS Consortium has been developing, through time, the Geography Markup Language (GML), a non-proprietary language (public domain) specifically defined to codify, archive and transfer (via Internet too) spatial data. Based on the XML standard, the GML handles both the geometry and the properties of the geographical elements; this allows the various data providers to share heterogeneous data sets and the users to access the data in a completely transparent way. From the application point of view, it simplifies and standardises the operations in many sectors, from map building to data format transformation, from spatial query to geographical analysis, including the emerging applications in mobile systems.
Since the GML data structure is XML-compliant, it can be transformed in a SVG document format and then easily displayed on a standard web browser.
Because this technology is continuously evolving, this work presents an overview on some aspects of the state-of-the-art, an evaluation of the advantages and disadvantages, and a demo application of the use of this technology in the field of building and querying thematic maps.
The SVG Explorer application is running on a web browser (Microsoft Explorer) and all the functionalities are working at the client side on local and/or remote (WWW Internet) data. The application's main goal is the processing of GML documents and their visualisation in a graphical way, with the interaction of the user. A set of basic functionalities that handles both the graphical (zoom, pan, symbols, styles, ...), the geographical and thematic aspects (multilayer organisation, elements classification and aggregation by attributes, ...) have been developed.
SVG is mainly oriented to vector data but can define also raster data as JPEG,PNG files containing georeferenced images; with this approach the overlay of vector data on a raster data background is allowed.
All the functionalities are operating in a ECMA Script environment, making use of libraries implementing base primitives working on GML and SVG data structures (as recommended by W3C).
The aim of this work is not only building a specific application, but is for three-fold:
These items are checked in terms of capability, efficiency and performance.
The SVG Explorer can also be seen as an evolved application using the Web Feature Service (defined by the OpenGIS project) for the features (GML coded) presentation and manipulation at client side.