Abstract. The objective of the presented work was to develop a method to acquire Manning's n by creating very high-resolution surface models with structure-from-motion methods. As hydraulic roughness is an essential parameter for physically based erosion models, a practical measuring technique is valuable during field work. Data acquisition took place during several field experiments in the Lainbach valley, southern Germany, and on agricultural sites in Saxony, eastern Germany, and in central Brazil. Rill and interrill conditions were simulated by flow experiments. In order to validate our findings stream velocity was measured with colour tracers. Grain sizes were derived by measuring distances from a best fit line to the reconstructed soil surface. Several diameters from D₅₀ to D₉₀ were tested with D₉₀ showing best correlation between tracer experiments and photogrammetrically acquired data. Several roughness parameters were tested (standard deviation, random roughness, Garbrecht's n and D₉₀). Best agreement in between the grain size and the hydraulic roughness was achieved with a non-linear sigmoid function and D₉₀ rather than with the Garbrecht equation or statistical parameters.