Magnetic surveys carried out with an automatic caesium gradiometer in the 1980s suffer from systematic and unsystematic data errors. Recently–developed algorithms for restoring erroneous magnetic data allow postprocessing of the early surveys. The restored data are thereafter suitable for a 3D–modelling algorithm. The resulting model is used for 3D–interpretation and reconstruction of the respective monument. The processing is demonstrated on a circular ditch system dating from the Middle Neolithic period about 6500–6800 BP at Strögen/Austria.