2.2 DEM VISUALIZATION 
The quality of the landscape rendering is important to get a realistic perception of the 
characteristics of the geographic area. In fact most of the external information concerning our 
environment is received through visual perception: the higher the photorealistic quality of the 
rendering the greater the level of immersion that can be experienced through the virtual reality 
simulation. 
A meaningful colour code must be assigned to each element (pixel) of the DEM by 
associating to each element (pixel) of the terrain model a colour code which corresponds to the 
spectral reflectance of the earth sampled area, generating a Photorealistic rendering of the 
reconstructed DEM with its proper colour codes and by simulating different illumination 
conditions. 
Two methods were used in the simulation of the natural environment: 
- texture mapping of remotely sensed multispectral images. Integration of information from GIS 
systems and remotely sensed data, once correctly georeferenced on a DEM. allows us to create a 
computer representation, which is best suited for displaying, analysis and understanding of the 
geographical data. 
- colour coding based on spectral reflectance of the actual area. The computation of colour 
properties of DEM elements takes into account interaction between light sources and terrain 
characteristics, i.e. spectral response of the surfaces. Photometric methods can be adopted in order 
to quantify light interaction, while well known shading techniques can be used in order to display 
a rendered DEM (Brivio et al, 1994). 
The natural behaviour captured by fractal interpolation method is confirmed by rendering 
the DEM with high quality photorealistic techniques. Evocative potentialities are confirmed in 
Figure 1, where a perspective 3D representations of the Aurina and Isel valleys area is displayed: 
in this figure it is also shown the TISS interface. The mountain range is shown as observed from 
bottom of the valley and as the observer is approaching the tongue of a glacier and the higher 
ridges of the mountains. Final representation was improved by adding sky and fractal clouds. 
2.3 TERRAIN ANALYSIS 
Digital terrain models are most valuable as a basis for the extraction of terrain related 
attributes to support GIS modelling such as environmental impact studies, hydrological 
simulations and many others (Weibel and Heller. 1992). A first objective of DEM interpretation is 
the derivation of geomorphometric parameters through the quantitative analysis. According to 
Evans (1972) general geomorphometry concerns measurements and analysis of those 
characteristics of landform which are applicable to any continuous rough surface. 
Topographic functions are used to calculate values that describe the topography at a specific 
geographic location or in the vicinity of the location (Aronoff. 1989). Slope and"aspect, which are 
computed using the elevation data of the neighbouring points, are the two most commonly used 
terrain parameters and are sufficient for many purposes, being the first derivatives of the altitude 
surface.