What is dead reckoning in navigation?

Dead reckoning is the running discipline behind almost every fix a navigator takes. Starting from a known position, usually the last confirmed fix, the officer advances that position using the course actually steered (not just the course ordered) and the distance run, calculated from speed and elapsed time or read off the log. On a paper chart this is plotted as a straight line from the fix along the true course, with the DR position marked at intervals, typically every hour and at every alteration of course or speed, using the conventional symbols: a dot with a semicircle for a fix, an open triangle for an estimated position, and a small dot alone for a pure DR position. On ECDIS the system does this continuously and automatically from gyro and log input, but the underlying principle and the watchkeeper's obligation to understand it are unchanged.

Pure DR only accounts for the ship's own movement through the water; it ignores set and drift. The estimated position, or EP, refines the DR by applying the known or forecast current, tidal stream, and leeway, the sideways drift caused by wind pushing on the hull and superstructure. Working an EP well requires reasonably reliable current and tidal data, so on coastal passages where tidal streams are strong and change hour by hour, the EP is worked far more carefully than in open ocean where set is usually slow and steady.

Errors creep into DR from several directions: a compass or gyro error that goes uncorrected, a log that under- or over-reads due to fouling or calibration drift, steering error where the helmsman does not hold the ordered course precisely, and simple arithmetic or plotting mistakes. None of these errors are individually large, but they accumulate the longer the ship runs without a fix, so DR must be reset to a confirmed position at every opportunity, whether from visual bearings, radar ranges, celestial observation, or GPS.

On watch, the officer is expected to maintain a continuous DR track as a standing practice, not just an exercise for exams. It is the safeguard against instrument failure or GPS outage, spoofing, or jamming, all of which have become real operational concerns, and STCW watchkeeping standards expect officers to be able to fall back on manual DR without hesitation. A ship whose bridge team has lost the habit of running DR is one that can be caught badly out when the electronics fail at the worst possible moment, typically in restricted visibility or congested waters.