FRACTIONAL VEGETATION COVER ESTIMATION FROM PROBA/CHRIS DATA:METHODS, ANALYSIS OF ANGULAR EFFECTS AND APPLICATION TO THE LANDSURFACE EMISSIVITY RETRIEVAL

摘要

In this paper, two different methods for fractionalrnvegetation cover (FVC) retrieval from CHRIS (CompactrnHigh Resolution Imaging Spectrometer) data based onrnvegetation indices have been analyzed. The first methodrnuses NDVI (Normalized Difference Vegetation Index)rnvalues, as suggested, among others, by Carlson andrnRipley (1997) and Gutman and Ignatov (1998), and thernsecond method uses VARI (Variable AtmosphericallyrnResistant Index) values as suggested by Gitelson et al.rn(2002). In addition, a simple Spectral Mixture Analysisrnhas been also carried out in order to obtain the fractionalrnvegetation cover. The different methods have beenrntested using vegetation cover values obtained fromrnhemispherical photographs. All methods providerndifferences with in situ values lower than 15%. Thernmethods based on the VARI and the spectral mixturernanalysis show the best results, with differences lowerrnthan 10%. Substantial angular effects for the differentrnview zenith angles of CHRIS acquisitions have not beenrnfound. The fractional vegetation cover has been used inrnorder to obtain land surface emissivity maps with a veryrnsimplified approximation, which is only valid for flatrnsurfaces because it does not take into account the cavityrneffect. In this way, land surface emissivity, which is arnparameter used when working with thermal infraredrndata, can be retrieved from sensors with no availablernthermal bands.