Captain H. Topley in his article on radar plotting errors (11, 167) has endeavoured to simplify the arithmetic and introduce an approximate formula which will be of practical use to the navigator. He admits that his approximation has limitations and it appears to me that some danger may arise from the difficulty which the non-mathematical navigator may have recognized when he has passed the point at which the formula will be safe to use. Further, Topley tends to concentrate on the errors in the estimated course and speed of the other vessel and to give rather less attention to the problem of errors in the closest position of approach (C.P.A.). The latter may be by far the most significant factor.