A low-magnetic-field digitising device combined with AMAPmod, a software designed to analyse plant architecture, provided a very efficient method for measuring and studying the geometry and topology of the structural roots of trees. The digitising device measures co-ordinates in a 3D space. AMAPmod was used to assess several characteristics of the root architecture including spatial position, root lengths and volumes, branching order and branching pattern, and to reconstruct 3D images from the data to check for measurement errors. Structural root systems of three 20 to 28-year-old Quercus petraea and thirty 5-year-old Pinus pinaster were uprooted by using rapid mechanised techniques. Only roots with a diamter larger than 2 mm were measured. A fast and precise spatial localisation in combination with the topological characterisation of all root segments was carried out. Oak showed a stronger more oblique and vertical rooting, more branches, more forks and narrower branching angles than pine. Oak had only few small roots in the north west direction whereas half of pine root volume was located in the 10 cm upper soil volume. The contribution of this new method in the characterisation of structural root systems is discussed and other possible applications of this method in root studies are proposed. Since this method is precise and fairly rapid, it may be used for agronomic testing (i.e. comparing treatments) involving several dozen root systems. Almost all parameters needed for tree root system simulations can be estimated from such data.