An examination of spectral band ratioing to reduce the topographic effect on remotely sensed data

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Abstract
Spectral hand ratioing in the form of radiance,/radiance, was examined as a proposed means for reducing the topographic effect from muitispectral data. The topographic effect is defined as the variation in radiances from inclined surfaces compared with radiance from a horizontal surface as a function of the orientation of the surfaces relative to the light source and sensor position (Holben and Justice 1980). A ground based nadir pointing two channel radiometer filtered for the red and photographic infrared portions of the electromagnetic spectrum was used to measure the topographic effect from a uniform surface inclined from horizontal to 60°, at 16 compass points, for several solar elevations. Spectral band ratioing reduced the topographic effect by more than a factor of 6 (i.e. 83 per cent) on the radiance data sets obtained in this study. The greatest proportional reduction of the topographic effect due to ratioing occurred where the topographic effect in the radiance was most pronounced, i.e. for slopes parallel to the principal plane, and least reduction for slope orientations perpendicular to the principal plane. A residual topographic effect was observed after ratioing the radiance data. This was reduced on an average of 50 per cent for all slopes and aspects by subtracting the diffuse skylight component from the radiances. Band ratioing of muitispectral satellite and aircraft data can be expected to be less successful than results presented in this study because of stronger additive radiance factors and sensor calibration and quantization effects. The degradative effects of Landsat sensor calibration and quantization are demonstrated for a range of. surface reflectances and irradiance levels.