Surface roughness defines the geometry between the pedosphere and the atmosphere (soil-air boundary).
In the field of microwave remote sensing, surface roughness affects scattering and emission characteristics of natural surfaces. The degree of roughness of a random surface is determined by statistical parameters, measured by the units of wavelength of the observing sensor. The two fundamental surface roughness parameters are the standard deviation of the surface height variation (RMS height) s, with its related surface correlation function p(ξ), and the horizontal surface correlation length l. Additional, a third roughness parameter, the root-mean-square (RMS) slope m, is important for some surface scattering models to simulate electromagnetic wave scattering of surfaces.
Surface roughness determines the variation of surface height within an imaged resolution cell. The transition from smooth to rough is qualitative, and is function of both wavelength and incident angle. With decreasing frequency the soil surface appears rather smooth to microwave sensors. This results in the fact, that while one surface appears smooth when sensed at L-band (λ ≈23 cm), the same surface appears rough when sensed at X-band (λ≈3 cm). Hence, in the field of microwave remote sensing, the ‘effective’ surface roughness parameters are scaled by the wave number k= 2π/λ. Surface roughness can be observed at single or multi-scale.