[NOTA ENRICO: SECONDO ME CI VORREBBE QUALCOSA SU global terrain database PER CONSISTENZA CON L'INTRO. SE DECIDIAMO DI FARLO BISOGNA VEDERE ATTI SDH ULTIMI ANNI, VIS98 (FORSE QUALCOSA NEI CASE STUDIES?), ULTIMI ANNI IJGIS, GEOINFORMATICA, LAVORO GOODCHILD E SHIREN, LAVORO HANAN E MIKE SU QUATERNARIA, CERCARE SU WEB]
Terrain generalization is a classical problem and a number of algorithms exist in the literature, which are generally aimed at producing a TIN that approximates the elevation of a set of input samples (see [Garland and Heckbert1995,] for surveys). Most such methods are based on the iterative application of local modifications on an initial TIN, in order to either refine or coarsen it. Here, we do not make any new proposal in terrain generalization, but we rather exploit basic principles of existing algorithms based on local modifications in order to obtain general techniques for building our multiresolution model.
There exist a number of multiresolution terrain models, which are based on subdivisions of the domain either through nested regular grids [,,,,Lindstrom et al.1996,,,,], or through TINs [Cignoni et al.1997,de Berg and Dobrindt1995,,,,,,,,,,,].
[NOTA ENRICO: MI SEMBRANO TROPPE CITAZIONI, NON TUTTE APPROPRIATE. ELIMINEREI QUELLE CHE NON TRATTANO ESPLICITAMENTE DI MODELLI DI TERRENO.]
Regular multiresolution models have the main advantage of requiring simple structures, since geometry and connectivity can be represented implicitly. On the other hand, multiresolution models based on irregular triangulations are more flexible since: they naturally adapt to different surface morphologies; they can be built from scattered data samples; they can include explicitly point features (e.g., maxima, minima, saddle points) as well as line features (e.g., ridges, valleys, coastlines) represented at multiple resolutions; and they can be defined for non-convex domains.
[NOTA LEILA: METTERE PIU' RASSEGNA DI MODELLI (DALLA VERSIONE VECCHIA)]
[NOTA ENRICO: LASCEREI COSI']
Some prototype systems which use multiresolution concepts have been developed, based on both regular and irregular triangulated models. Topovista [Topovista????] is a system developed at the University of Arizona, in which the underlying multiresolution model is a hierarchy of regular right triangles []. FlyAway [FlyAway????] has been developed at the University of Tübingen, using a multiresolution model based on Delaunay triangulations of irregularly distributed points [].
[CITARE ANCHE IL SISTEMA DI GEORGIA TECH?]
[CHIEDERE A HANAN SE IL SUO SISTEMA SAND GESTISCE MODELLI DI TERRENO]
Apparently, the only application proposed for such systems is interactive terrain visualization, where multiresolution is used for adapting the rendered model to view-dependent thresholds. We are not aware of systems that make explicit use of multiresolution in terrain analysis.