A technical session on the above subject was held in London on 13 April 1983. The topic was introduced by Mr M. St Erme Cardew, who first stressed the distinction between navigational data and positional data. The former, unlike the latter, are referenced by time so that there is a sense of urgency in processing navigational data. Inherently these are always out of date, possibly only slightly, due to the finite processing time required. Some extrapolation is always required. He recognized the importance of errors, possibly gross ones, and distinguished these as due either to the assumptions made by the navigator being incorrect or to errors in the data. He advised checking the calculational methods, possibly by undertaking, via a computer, two or more independent analyses. Checking that incoming data were both stable and within specification was imperative. In an emergency it was necessary that human intervention into computer-produced information be accepted.