Abstract

In applied geology/geomorphology tridimensional reconstruction of the ground surface (Digital Terrain Model, DTM) is used since a few years; it provides an accurate definition of the topographic features as well as the management of a large amount of data. At present, the DTM is obtained using digital photogrammetric stations to process stereoscopic pairs. In the last years, marine technology allows to realize digital bathymetry (i.e. digital terrain model of the seafloor) using multibeam or interferometer instruments. The article describes the methodology we used to generate a joint digital model of the seafloor and of the subaereal surface along the eastern and southern flank of Palmarola Island; we call such model DTMM (Digital Terrain and Marine Model). This methodology can be especially useful in describing volcanic islands and coasts, that are usually affected by conspicuous erosional and instability processes. The DTMM combines aerial photogrammetric data and swath bathymetry data (where possible, as far as the shoreline). Aerial photos (1:13.000 scale) were digitized and processed by a digital photogrammetric unit with a 1000 dpi resolution; in this way the pixel size is approximately 0,3 m on the ground. The DTM was georeferenced by means of a GPS network of ground control points established on natural features clearly visible on the photos. Thus the DTM coordinates are derived in the WGS84 reference system. The seafloor model was obtained by a swath bathymetry using a sonar interferometric technology. Data were collected from the shoreline to a depth of 60 m. Data were georeferenced using the DGPS with a reference station located at a known position; thus bathymetric data are referred to the WGS84 ellipsoid too. In order to generate the DTMM, the two sets of data should have: 1) a congruent reference system, which is possible connecting the reference station used for the marine survey into the ground GPS network; 2) an homogeneous spatial resolution, achieved by re-sampling and/or interpolation procedures; 3) a good height/depth matching, achieved by mosaicking the two data sets along the coastline; 4) a congruent file format to perform the same image processing and mapping. DTMM can be used in large scale geomorphological mapping, for a homogeneous description of the whole earth surface, highlighting features continuity between terrestrial and shallow-marine environment. The Palmarola Island (eastern and southern flank) DTMM allows to identify two different processes active respectively on cliffs and pocket beaches, controlled by wave erosion and mass instability, and on the inner shelf (-15/-50 m), controlled by downwelling currents re-distributing the debris produced on the coast.

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