The problem of reproducibility of results, due to different observers taking part in the measurement and recording of rock discontinuities, is discussed. The article gives details of an investigation carried out by the TRRL into the reproducibility of rock joint spacing and orientation. The studies were carried out in two tunnel drives in sandstone, mudstone and limestone in the kielder aqueduct tunnels. Examples are shown of the results of scanline surveys in which six observers recorded rock joint orientation from which methods of measuring the degree of agreement between the observers are examined. Joints recorded by the observers were shown to vary by a significant amount showing the difficulty of interpretation which indicates a possible problem in the visual monitoring of tunnels or slopes. (TRRL)