Evaluating MODIS, MERIS, and VEGETATION -: Vegetation indices using in situ measurements in a semiarid environment

New and improved satellite sensors specially designed for vegetation monitoring have been launched in recent years; including the Moderate Resolution Imaging Spectrometer (MODIS) onboard Terra and Aqua, the Medium Resolution Imaging Spectrometer (MERIS) on the ENVISAT satellite, and VEGETATION onboard the Systeme Pour l'Observation de la Terre (SPOT) satellite. The aim of this paper is to evaluate two different vegetation indices of these new sensors; the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI). This is done by radiometric in situ measurements covering natural grass savanna in Senegal. Variations in the dynamic range of in situ NDVI was found caused by the differences in spectral response functions, MERIS NDVI characterized by the largest dynamic range. Both daily MERIS and MODIS NDVI mirrored accurately in situ measured NDVI (MERIS r(2) = 0.97 and MODIS r(2) = 0.96). VEGETATION NDVI (only available as ten-day composites) was found to be significantly lower than MODIS NDVI due to lower VEGETATION near-infrared (NIR) reflectance values. A good agreement between the NDVI/EVI relation from satellite and from in situ measured MODIS NDVI/EVI was found. This indicates an accurate atmospheric correction of the MODIS red, NIR, and blue spectral bands, also confirmed by in situ measured reflectances. EVI is sensitive to variations in blue band reflectance, and the consistency between EVI from the different sensors is reduced when compared to NDVI due to the different atmospheric correction schemes of the blue band. Thus, it is recommended that vegetation index cross-sensor algorithms should be based on NDVI over EVI.