Radiometric correction of multispectral scanner data requires physical models of image formation in order to deal with variations in topography, scene irradiance, atmosphere and viewing conditions. The scene radiance equation is more complex for rugged terrain than for flat terrain since it must model elevation, slope and aspect dependent effects. A simple six parameter model is presented to account for differential amounts of solar irradiance, sky irradiance and path radiance across a scene. The model uses the idea of a reflectance map to represent scene radiance as a function of surface orientation. Scene radiance is derived from the bidirectional reflectance distribution function (BRDF) of the surface material and a distribution of light sources. The sun is treated as a collimated source and the sky is treated as a uniform hemispherical source. The atmosphere adds further complication and is treated as an optically thin, horizontally uniform layer.
The required six parameters account for atmospheric effects and can be estimated when a suitable digital terrain model (DTM) is available. This is demonstrated for Landsat MSS images using a test site near St. Mary Lake in southeastern British Columbia. An intrinsic surface albedo is estimated at each point, independent of how that point is illuminated and viewed.
It is argued that earlier conclusions about the usefulness of the Lambertian assumption for the radiometric correction of multispectral scanner data were premature. Correction methods proposed in the literature fail even if the surface is Lambertian. This is because sky irradiance is significant and must be dealt with explicitly, especially for slopes approaching the grazing angle of solar incidence.
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