The basic values for dead reckoning, course and speed, are ascertained by inadequate references. The log does not measure the ground-speed but the speed relative to the water. The accuracy of magnetic compasses is limited by the instability of variation and deviation; gyro-compasses are better, but neither does away with steering errors. All classes of position-fixing permit the derivation of course and speed from two fixes but are influenced by the errors inherent in the methods. An astro fix is subject to irregularities of dip and the inefficiency of artificial horizons. The uncertainty of course and speed errors in running fixes is estimated and does not give very satisfactory results for astronomical and radio fixes, with the exception of methods giving an accuracy of 1 n.m. (e.g. Decca). Standard errors are referred to throughout.