Effects of Band Positioning and Bandwidth on NDVI Measurements of Tropical Savannas

Abstract

The consequences of shifting the spectral location of narrow and broad red (R) and near-infrared (NIR) bands on measurements of the normalized difference vegetation index (NDVI) of Brazilian savannas were investigated for different plant communities and seasonal phenology. The basic data came from an Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) image obtained during the regional dry season and two Landsat 5-Thematic Mapper (TM) images from the dry and rainy seasons. Principal components analysis (PCA) was applied to a set of 64 AVIRIS bands from the interval of 500–1100 nm in order to select the R–NIR band pair of greatest spectral contrast and respective endmember spectra for the R and NIR band variations. For the endmember spectra, modeled AVIRIS-derived NDVI values, obtained by changing the spectral positioning and bandwidths of R–NIR band pairs, were compared with NDVI calculated from simulated R–NIR band pairs of orbital sensors. Furthermore, the suitability of different positions and bandwidths was also examined employing, as criterion, the calculated NDVI contrast between the endmember spectra. The results revealed that the NDVI of the green vegetation was essentially affected by the proximity of the R and NIR bands to the spectral interval of the red edge (690–750 nm). The strongest effects were observed for the R bands that partly extended longwards the 690 nm wavelength. On the other hand, the NDVI of the nonphotosynthetic vegetation varied up to 100% within the studied interval, especially as a function of the R–NIR spectral distance. The largest NDVI contrast between green and dry vegetation was obtained with the convergent displacement of the R and NIR bands, respectively, towards the 690 nm and 750 nm wavelengths, as predicted from PCA. Results from simulated orbital sensors demonstrated that bandwidths have no significant influence on the NDVI provided the R and NIR bands are not extended into the red edge domain. The variable NDVI contrasts along traverses presented the smallest values when associated with the conspicuous green vegetation, and the largest values when associated with the dried-out grasslands or exposed soils.