Synergy between LIDAR and RADARSAT-2 images for the recognition of vegetation structures in the coastal wetlands of the Danube Delta

Wetlands are among the most productive environments in the world and are characterized by exceptional biologic diversity. Despite their indisputable importance, these environments remain among the most endangered ecosystems in the world due to drainage, drying out, pollution or overexploitation of resources. The Danube Delta, a coastal wetland of the Black Sea cannot escape these dangers and, to preserve its resources, it has been declared a Biosphere Reserve (in 1993). The biodiversity of this area is remarkable and it possesses one of the largest reed in the world (a continuous reed cover on a surface of 2,700 km²). The essential goal of this project is to recognize, define and state the essential descriptors of the vegetation structures, the Phragmites australis species, of the Danube Delta being prevalent. For this purpose, this project aims, on the one hand, at interpreting the LIDAR measurements (carried out in May 2011) in correlation to the radar observations of the RADARSAT-2 satellite (made at the beginning of June 2011), and, on the other hand, at validating the results obtained by the introduction of the spectral measurements of the main vegetation classes in a Spectral Angle Mapper algorithm on a SPOT-5 image (May 2011). LIDAR data allow the assessment of the vegetation height with an accuracy of a few centimeters. Therefore, the diverse vegetation layers can be accurately represented on charts. Nevertheless, the differentiation of the diverse vegetation formations within the same layer requires the contribution of certain complementary information sources, such as the RADARSAT-2 data. This radar measurement is carried out here, starting from the C band (the wavelength λ = 5.3 cm), providing therefore additional information on the structure of the vegetation blanket, with regard to the roughness, humidity and biomass. The simultaneous procurement of the HH, HV and VV polarizations allowed us the differentiation of the surfaces according to their response to different polarizations, establishing their polarimetric signatures. Indeed, starting from these gross data, we were able to deduce other indices, such as, for instance, the intensity of the four polarizations, the span and the polarimetric entropy. The entropy is very sensitive to the vegetation density; the thicker the vegetation is, the bigger the entropy becomes. This approach allowed us to obtain valuable information regarding different types of exploitation of the reed (cut or burned reed). On the other hand, the exploitation of the spectral information starting from the SPOT 5 data was possible due to the calibration carried out by means of the spectrometers, used in order to perform spectral measurements in the sectors previously identified on images.