Estimation of topographic effects in NVI data obtained from satellite images

The topographic effects in Normalized Vegetation Index (NVI) data obtained from Landsat TM data and NOAA AVHRR data are estimated. It is assumed that the topographically induced radiance variations can be estimated by the cosine of the angle, γ, between the direction of the Sun and the local surface normal. Digital terrain data sampled with the grid width 50 m are used to evaluate γ. By comparing NVI data derived from satellite data with γ-data, the following results were obtained: (1) The correlation between NVI and VI data derived from Landsat TM data and γ-data is high. (2) The correlation between NVI data derived from NOAA AVHRR data and γ-data is also significant