Comparison of data classification techniques applied to TLS point clouds

Marshes are ubiquitous landforms in estuaries and lagoons, where important hydrological, morphological and ecological processes take place. These areas attenuate sea action on the coast and act as sediment trapping zones. Due to their ecosystem functions and effects on coastal stabilization, marshes are crucial structures in tidal environments, both biologically and geomorphologically, and fundamental elements in wetland restoration or coastal realignment schemes. Observational fluvial geomorphology has greatly benefited in the last decades from the wide availability of digital terrain data obtained by orthophotos and by means of accurate airborne laser scanner data (LiDAR). On the contrary, the spatially-distributed study of the geomorphology of intertidal areas, such as tidal flats and marshes, remains problematic owing to the small relief characterizing such environments, often of the order of a few tens of centimetres. Here, we present the results of the application of terrestrial laser scanning (TLS) for the generation of a DTM of the bare soil within a tidal marsh in the Venice lagoon. To this aim, different procedures for clustering of TLS data have been investigated. Post-processing of the very high resolution data obtained shows that the laser returns coming from the low vegetation present (about 0.3-1.0m high) can be satisfactorily separated from those coming from the marsh surface, allowing to generate detailed DSM and DTM. Furthermore, the DTM is shown to provide unprecedented characterizations of marsh morphology, e.g. regarding the cross-sectional properties of small-scale tidal creeks (widths of the order of 10cm), previously observable only through conventional topographic surveys, thus not allowing a fully distributed description of their morphology.