98 
The principal approach to this new possibility is offered by VRML language, which has been 
proposed in 1994 by Mark Pesce (Pesce. 1995), and which has been subsequently developed by 
many research and commercial centres, giving rise to a complete and powerful language to 
describe 3d data sets and to support interactive navigation in such environments. 
The kind of navigation which is supported by VRML language is. first of all, the visual 
navigation by moving a virtual camera in the three dimensional scenario. But more powerful is the 
possibility to enrich a 3d model as an hypertext, by adopting the HTTP standard which is 
supported by all Web navigation browsers. Therefore a VRML based navigation browser can be 
activated by a Web browser, and connecting URL links to relevant parts of the 3d model makes 
possible to get back the required Web page. The adoption of this methods, standards and tools 
opens up the possibility of creating a distributed environment to support co-operation among 
researchers in the field of earth research. 
The approach that we adopt to the development of a virtual environment for earth landscape 
analysis and navigation is base on the following main stages: first of all a DEM is constructed by 
interpolation methods; the DEM model is subsequently enriched with colour rendering parameters 
derived from analysis of satellite images; the rendering of the DEM is performed so that a 
perspective projection with moving camera position can be generated; a variety of analysis tools 
can be furtherly applied to generate different views of relevant parameters. 
All the above mentioned stages can be performed using the interactive system TISS, 
which, moreover allows us to create animation sequences that can be coded into standard video 
files that can be played inside TISS or by using Mpeg or Quicktime plaversc (Mpeg and Quicktime 
are standard methods to code an image video sequence). 
To further extend the level of interactivity with the reconstructed landscape, we have also 
implemented a simple program that converts the DEM data structure into the standard VRML 
representation. This recently standardised 3D description and modelling language is the best 
environment to insert into the landscape model other data, creating, for example. 3D tematic maps, 
or integrating man-made buildings coded as geometric models. 
The system is based on a set of modules that allow us to: 
- reconstruct the Digital Terrain Model (DEM) by adopting a variety of interpolation 
techniques (kriging and fractal fBM), 
- to associate to each element (pixel ) of the terrain model a colour code which corresponds 
to the spectral reflectance of the earth sampled area. 
- to generate Photorealistic rendering of the reconstructed DEM with their proper colour 
codes and by simulating different illumination conditions. 
- to navigate interactively in the reconstructed and rendered landscape. 
- to display a navigation sequence pre-determined along an exploration path, producing an 
image sequence like a movie film. 
2.0 MODELLING THE EARTH LANDSCAPE 
In order to make a Virtual Reality model of the Earth landscape, a fundamental step is the 
reconstruction of a three dimensional geometric model of the real world, both natural objects such