Relations between tropical forest biophysical properties and data acquired in AVHRR channels 1–5

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Abstract
The remote sensing of biophysical properties has generally relied on the use of data acquired in red and near-infrared channels only, often combined in a vegetation index such as the Normalized Difference Vegetation Index (NDVI). This is wasteful of information acquired in other channels and may prevent the accurate estimation of biophysical properties. The use of vegetation indices based on red and near-infrared radiation to estimate biophysical properties of tropical forests has met with little success and this may be due to the asymptotic nature of the relation between the indices and biophysical properties, the variable sensitivity of vegetation indices to vegetation biophysical properties in different environments, the low radiation reflected in red and near-infrared channels, and severe attenuation by atmospheric water and aerosols. For tropical forests the only feasible way to estimate biophysical properties at regional to global scales is through the use of the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) which operates in three channels in addition to the red and near-infrared. The potential of the data acquired in all five AVHRR channels for the estimation of tropical forest biophysical properties was investigated. Radiometrically calibrated AVHRR imagery of Ghana were related to ground data on tree density and mean basal area. Radiation measured in the middle-and thermal-infrared channels were more strongly correlated with the forest biophysical properties than radiation acquired in red and near-infrared channels. Moreover, vegetation indices containing data acquired in the middle- and thermal-infrared were also more strongly correlated with biophysical properties than the widely used NDVI. Correlation coefficients of 0·71 and 0·23 were derived between the NDVI and tree density and basal area respectively, while the corresponding correlation coefficients for indices based on data acquired in AVHRR channels 3–5 were up to −0·88 and −0·52 respectively. Since current and proposed sensors operate in visible to thermal-infrared channels users should consider the potential of data acquired all channels and not just a subset.