The problems associated with simple models for terrestrial and planetary surface reflectance are discussed and the value of using more rigorous expressions that properly describe the effects of sub-pixel shadowing at low solar zenith angles is emphasised. The use of such expressions will improve the accuracy of theoretical reflectance estimates and could provide information useful in geological mapping, geomorphology, vegetation classification, climatology and micrometeorology. An existing general-purpose roughness correction to radiative transfer models of surface reflectance is considered in some detail, and the relationship of its roughness parameter, θ, to sub-pixel terrain morphology is then explored through the collection and analysis of reflectance measurements from fractal digital terrain models. A well-defined relationship is found to exist between θ and fractal dimension, D, such that, under certain circumstances, knowledge of one may be used to infer the other.