View-illumination effects on hyperspectral vegetation indices in the Amazonian tropical forest |
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Institution: | 1. Instituto de Agricultura Sostenible (IAS), Consejo Superior de Investigaciones Científicas (CSIC), Alameda del Obispo s/n, 14004 Córdoba, Spain;2. Departamento de Agronomía, Universidad de Córdoba, Campus Universitario de Rabanales, 14014 Córdoba, Spain;1. National Institute for Space Research (INPE), Avenida dos Astronautas, 1758 São José dos Campos, SP, Brazil;2. Oregon State University, College of Forestry, Corvallis, OR 97331, USA;3. NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA;4. National Centre for Monitoring and Early Warning of Natural Disasters (CEMADEN), Estrada Doutor Altino Bondensan, 500, São José dos Campos, SP, Brazil;5. Environmental Change Institute, University of Oxford, Oxford OX1 3QY, UK |
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Abstract: | Because of the pointing capability of the Hyperion/Earth Observing-One (EO-1) to improve the revisit time of the scene, temporal series of narrowband vegetation indices (VIs) can be generated to study the phenology of the Amazonian tropical forests. In this study, 10 selected narrowband VIs calculated from Hyperion nadir and off-nadir data and from different view directions (forward scattering and backscattering) were analyzed for their sensitivity to view-illumination effects along the dry season on the Seasonal Semi-deciduous Forest. Data analysis was also supported by PROSAIL modeling to simulate the spectral response of this forest type in both directions. Hyperion and PROSAIL results showed that the Enhanced Vegetation Index (EVI) and Photochemical Reflectance Index (PRI) were the two more anisotropic VIs, whereas the Normalized Difference Vegetation Index (NDVI), Structure Insensitive Pigment Index (SIPI) and the Vogelmann Red Edge Index (VOG) were comparatively less sensitive to view-illumination effects. When compared to the other VIs and because of the greater dependence on the near-infrared (NIR) reflectance, EVI showed a different spectral behavior. EVI increased from forward scattering to backscattering and with decreasing solar zenith angle (SZA) towards the end of the local dry season, due to reduction in shading and enhancement of the illumination effects. On the other hand, PRI was higher with increasing shading in the forward scattering direction, as deduced from the PROSAIL simulation. Results emphasized the importance of taking into account bidirectional effects when analyzing temporal series of VIs collected over tropical forests by imaging spectrometers with pointing capability or even by multispectral sensors with large field-of-view (FOV). |
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Keywords: | Hyperspectral remote sensing Hyperion Vegetation indices PROSAIL Tropical forest View angle Directional effects Dry season |
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