Remote Sensing Geology by Ravi P. Gupta

Author:Ravi P. Gupta
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

14.4.2 Radiance-to-Reflectance Transformation

The hyperspectral image data carry the influence of a number of external factors, which may mask the fine spectral features of ground objects. These factors are: effects of the solar irradiance curve, atmosphere and topography. The effect of the solar irradiance curve arises from the fact that solar radiation intensity peaks at 0.48 µm and the radiation intensity drops off towards longer wavelengths (Fig. 2.​2); therefore, the effect of solar irradiance is not uniform throughout. Atmospheric effects arise due to the fact that hyperspectral image data are collected over a wide wavelength range, which includes atmospheric windows as well as parts of the EM spectrum affected by atmospheric absorption and scattering. Topographic effects, in hyperspectral sensing, are similar in nature to those in multispectral sensing. All these factors are external to the target reflectance, and influence the spectral and spatial variations in at-sensor radiance. Therefore, these must be adequately normalized in order to compute ground reflectance values.

A variety of techniques have been developed to remove the above effects of external factors. A comparative review is provided by Rast et al. (1991). The various techniques can be categorized into two groups: (a) those using radiative transfer codes (atmospheric models) and (b) those using parameters derived from the scene (including flat-field correction, internal average relative reflectance correction and empirical line methods). The atmospheric correction procedures have been discussed in Chap. 10.

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