Estimation of multiple reflection and lowest order adjacency effects on remotely-sensed data

Abstract

The aims of this paper are (i) to investigate the lowest-order adjacency effect and the effect of multiple reflections between the ground and the atmosphere on the Advanced Very High Resolution Radiometer (AVHRR) channel 1 and channel 2 reflectances and on the normalized difference vegetation index (NDVI) and (ii) to make a comparison between global irradiances calculated from two empirical relations, one theoretical and the other experimental. Derivation of the theoretical empirical relation for global solar irradiance is in accordance with Iqbal (1983) whereas the experimental empirical relation is taken from 'Singh et al. (1985).The effect of multiple reflection on each channel reflectance is about 1·2 per cent so that the NDVI is not affected by including a multiple reflection term in the atmospheric correction algorithm. The lowest-order adjacency effect in channel 1 is about 7 per cent and in channel 2 is about 5 per cent and the NDVI changes by about 0·5 per cent which is negligible. These results show that the multiplereflection and lowest-order adjacency effects can be ignored if one is interested in the NDVI. But such effects could be important if one is using reflectances, say, for scene classification. The variations of global solar irradiance with the solar zenith angle for both empirical relations are presented. These relations vary in a remarkably similar manner. However, the experimental relation underestimates global irradiance compared to the value obtained from the theoretical empirical relation. As a result of this, channel 1 and channel 2 reflectances differ by about 16 and 10 per cent, respectively. But the effect on the NDVI is only about 2·4 per cent.