EFFECTS OF SPECTRAL, SPATIAL, AND RADIOMETRIC CHARACTERISTICS ON REMOTE-SENSING VEGETATION INDEXES OF FORESTED REGIONS

Vegetation indices derived from satellite image data have become one o f the primary information sources for monitoring vegetation conditions and mapping land cover change. The most widely used vegetation index in this context is NDVI, the normalized difference vegetation index, w hich is a function of red and near-infrared spectral bands. Given that the spectral and spatial characteristics of imagery in the red and ne ar-infrared vary from sensor to sensor, NDVI values based on data from different instruments will not be directly comparable. The present st udy demonstrates the impact of changes in spectral bandwidth and spati al scale on NDVI derived from Airborne Visible/Infrared Imaging Spectr ometer (AVRIS) data acquired at 20-m resolution over a forested region in southeastern British Columbia. For this purpose, the 10-nm AVIRIS data were spectrally and spatially aggregated in the red and near-infr ared to simulate bandwidths from 10 nm to 150 nm for ground resolution s varying from 20 m to 1100 m. Sensor-specific spectral bands and spat ial resolutions such as those for SPOT HRV, Landsat TM, NOAA AVHRR, EO S MODIS, and Envisat MERIS were also generated. NDVI values were then calculated using atmospherically corrected surface reflectances for fo restry-related targets for the entire simulated band set at the variou s scales. The results indicate that the NDVI is significantly affected by differences in spectral bandwidth, especially for the red band, an d that changes in spatial resolution lead to less pervasive but more l and cover specific effects on NDVI. Results for the forested regions a lso indicate that NDVI is not very sensitive to the location of the ne ar-infrared spectral band, provided that the bandwidth is no wider tha n 50 nm and the atmospheric correction for water vapor absorption is a dequate. If either proviso is relaxed, the wavelength placement of the near-infrared spectral band is more critical, the optimum location be ing in the 850-880 nm range. Finally, for the same forest targets, som e results were also generated for several other vegetation indices tha t make straightforward use of atmospherically corrected red and near-i nfrared spectral bands. (C) Elsevier Science Inc. 1997.