The soil line concept in remote sensing

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Abstract
The soil line concept, a linear relationship between red and near infrared reflectance of bare soil is widely used for the interpretation of remotely sensed data over the Earth's surface. The slope and intercept of soil lines presented in the literature are reviewed. They are found to be inversely related. A numerical experiment shows that part of this relationship is due to statistical artifacts occurring when estimating these parameters. However, observed variations correspond also to the various experimental conditions and to actual differences in soil types and surface status. The fundamentals of the soil line are analyzed from a simple radiative transfer model in which the bidirectional reflectance is split into its single and multiple scattering components. The slope of the soil line corresponds to the ratio of the single scattering albedos characterizing the 2 wavebands where the soil line is observed. The intercept originates from the difference in multiple scattering observed in each of the two wavelength bands used. For pure ponctual or diffuse light sources, the concept of the soil line is very robust over the whole spectral domain if soil types are separated, and when the effect of the view and source configurations as well as the surface roughness are considered. However, in the middle infrared spectral domain, important scatter appears around the soil line when soil moisture is a factor of variation. Scatter is also expected in natural conditions particularly for shorter wavelengths due to variation of the diffuse fraction as a function of the wavelength.