The lengths of >100 exterior links for three catchments in Devon and Somerset in southwest England were obtained by field survey. These link lengths were compared with data obtained from the provisional edition of the 1:25,000 map series and the Old County map series at 1:2500. The degree of correspondence with the 1:25,000 series was very poor, but both regression and analysis of variance methods failed to reveal any major difference between the field data and the data based on the 1:2500 series. It is suggested that, other than complete field mapping, the 1:2500 series represents the most reliable data source available for the delineation of stream networks in the area.