Various applications such as meteorology, climatology or hydrology require information about the soil hydraulic properties over large areas. Microwave radiometry is a promising approach to gather this type of information. The microwave emission from soils is strongly affected by the roughness of the soil surface. This effect has therefore to be quantified to get a reasonable estimation of the hydraulic properties. In a cooperation of the Institute of Terrestrial Ecology, Soil Physics with the Institute of Geodesy and Photogrammetry digital surface models of soils were generated to study the influence of the surface roughness on the soil measurements. Accurate Digital Surface Models (DSM) can be derived by the application of photogrammetric measurement techniques and provide the spatial basis to extract roughness information. In this paper an approach to determine the roughness of the topsoil surface is presented.