Atmospheric effects on vegetation indices of TM and ETM+ images from a tropical region using the 6S model

Vegetation indices have been widely used for monitoring vegetation and to estimate vegetation biophysical parameters. The atmosphere affects differently sensor bands depending on the waveband, thus affecting the indices obtained from top of the atmosphere reflectances. The objective of this paper is to evaluate the atmospheric effects on the NDVI and SAVI using the 6S model adapted for atmospheric correction of images of eight bits digital numbers. Images from the TM and ETM+ sensors of the same area were corrected for the atmosphere and the vegetation indices were compared with those from noncorrected images. For these images the results show that the vegetation indices at the top of the atmosphere were lower than those from the surface. This underestimation of the vegetation indices due to the atmosphere is attributed to a higher absorption of radiation in the near infrared than in the red spectral region. This is exceptionally pronounced on tropical atmospheres where absorption by water vapor tends to be higher than in mid-latitudes. It is concluded that atmospheric correction is necessary when calculating vegetation indices over tropical regions and the adapted 6S model is suitable for this task.