Fractional Vegetation Cover Estimation from Proba/chris Data: Methods, Analysis of Angular Effects and Application to the Land Surface Emissivity Retrieval

RESUME In this paper, two different methods for fractional vegetation cover (FVC) retrieval from CHRIS (Compact High Resolution Imaging Spectrometer) data based on vegetation indices have been analyzed. The first method uses NDVI (Normalized Difference Vegetation Index) values, as suggested, among others, by Carlson and Ripley (1997) and Gutman and Ignatov (1998), and the second method uses VARI (Variable Atmospherically Resistant Index) values as suggested by Gitelson et al. (2002). In addition, a simple Spectral Mixture Analysis has been also carried out in order to obtain the fractional vegetation cover. The different methods have been tested using vegetation cover values obtained from hemispherical photographs. All methods provide differences with in situ values lower than 15%. The methods based on the VARI and the spectral mixture analysis show the best results, with differences lower than 10%. Substantial angular effects for the different view zenith angles of CHRIS acquisitions have not been found. The fractional vegetation cover has been used in order to obtain land surface emissivity maps with a very simplified approximation, which is only valid for flat surfaces because it does not take into account the cavity effect. In this way, land surface emissivity, which is a parameter used when working with thermal infrared data, can be retrieved from sensors with no available thermal bands.