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Slope at a point is the maximum gradient of the plane fitted to all elevation points in a 
window, normally a square kernel, centred on a geographic location. Slope map can be calculated 
everywhere from a DEM and it is scale dependent, i.e. depends very strongly on the sampling 
interval between grid points. 
Aspect or orientation of a surface is the direction that slope faces; this direction is given by 
the angle, measured clockwise from a reference direction, usually true north. 
In TISS system these terrain attributes, slope and aspect, are evaluated by considering the 
variations of the plane tangent to the surface in each point and represented by its normal vector 
(Chorowicz et al, 1989). Slope and aspect map derived from the altitude matrix may find 
applications also for DEM quality assessment (Wood and Fisher. 1993). 
Civil engineering, land planning and surface climate modelling applications require more 
specialized interpretation functions. One category of the involved procedures is used for visibility 
and relief shadow analysis, and for solar radiation analysis which is the major component in the 
energy balance models at the boundary layer interface. 
The relief shadow computation is closely related to visibility analysis; the given location 
from where areas are invisible is any selected position of the sun. These problem require the 
solution of the horizon. The algorithm adopted in the system derives from the solution proposed 
by Dozier et al (1981) which allows to reduce the horizon problem to its one-dimensional 
equivalent. In Figure 2 a shaded relief image derived from the elevation matrix of a mountain area 
of Northern Italy (Pian di Spagna) is presented simulating the illumination condition, i.e. sun 
position, at the time of the Landsat overpass in mid-September. 
Moreover a module is included which allows to estimate net solar radiation for a different 
time period, as hourly, daily and monthly basis. The model beside the necessary terrain attributes, 
elevation, slope aspect, accounts for eccentricity of the Eaith orbit and for the atmospheric 
influence. Figure 3 shows a 3D visualization of daily insolation map of Pian di Spagna area, 
computed for the first of November. 
3.0 INTERACTING WITH A DEM 
To implement higher level interactivity with a DEM. aiming to use it as a fundamental tool 
to access any possible knowledge about a given landscape area, we are adopting standard methods 
derived from Virtual Reality techniques. 
Interaction with a DEM can be divided into the following aspects: 
- navigation and exploration 
- query 
- presentation of results of numerical simulations 
- "what if" simulation 
Some of the above interaction methods can be easily implemented, others require complex 
software tools. In particular; navigation and exploration can be easily implemented by recurring to