Correction of topographic effects for the vegetation index obtained from NOAA/AVHRR data

Topographic effects included in the vegetation index derived from NOAA/AVHRR data are evaluated. Radiance variations caused by shadows cast by adjacent terrain and occlusions of light ray by adjacent terrain are called topographic effects in this study. The Monte Carlo method was used to compute the multiple scattering processes of light in the atmosphere bounded by rugged terrain. The digital terrain data sampled with the grid width 50 m (50 m-DTD) was used to estimate topographic effects included in one pixel area of AVHRR data. The ratio, Rn, of the apparent radiance reflected by the rugged surface to the radiance reflected by the flat surface (no shadow and occlusion) was computed. The following results were obtained. (1) For the vegetation field with the high reflectance in the near infrared band, the value of Rn changes from 0.8 to 1.0 at the channel 2 of AVHRR data. On the other hand, at the channel 1, Rn becomes more than 0.9 except for flat terrain. (2) It is possible to correct topographic effects included in NOAA/AVHRR data using the global 30 arc second elevation data (1 km-DTD) when the incident solar elevation is high.