DEM extraction with digital photogrammetric tecniques: examples of landslides monitoring and applications on volcanic areas

Digital Photogrammetry represents one of the most powerful tool for acquiring, through semi-automatic procedures, a large amount of 3D points for the generation of high spatial resolution Digital Elevation Models (DEM) and the relative rectified images. DEMs are regular or irregular planimetric grid points with associated elevation values: they are three-dimensional ground surface digital reconstructions. Digital models can be obtained by means of several geodetic techniques like digital aerial and terrestrial photogrammetry, airborne and terrestrial laser scanning, SAR satellite interferometry and GPS kinematic. DEMs internal precisions are strictly related to the selected surveying method, to the operative working conditions and to morphological features of the studied area. The digital approach for measurement of points from photogrammetric stereopairs make use of image correlation processes which are based on well defined shape comparison techniques or on the grey level distribution in the corrisponding zones of the images. The precision of a photogrammetric DEM depends on the morphology of the terrain, on the inclination, on the images quality, on the presence of break-lines and vegetation, on the resolution at ground (and therefore on the scale of aerial photograph and pixel dimension), on the ratio between the distance (camera-terrain) and the base (distance between the two acquisition points). 3D data derived by digital photogrammetric processing have recently demonstrated remarkable potentialities for the study of landslides and volcanic areas; models generated at different epochs can be differenced in order to determine displacements and large morphological variations