What is dead reckoning (DR) in navigation?
Dead reckoning (DR) is the practice of advancing a known position using only the course steered, the speed through the water, and the time elapsed. It deliberately ignores current and leeway, which is what separates a DR position from an estimated position.
Related terms
How it works
Start from a trusted fix, apply distance = speed × time along the course steered, and plot the result. Navigators keep the DR running continuously, plotting a new position at regular intervals and at every change of course or speed, so there is always a best-guess position even when electronics fail.
Because DR uses course steered and speed through the water, it drifts away from reality as current and leeway accumulate. Comparing each new fix with the DR reveals what the water has been doing; applying estimated current and leeway to the DR produces an estimated position (EP), the corrected cousin of the DR.
Where the name comes from
The term is often said to descend from “deduced reckoning,” shortened in old logbooks to “ded. reckoning,” though the etymology is debated. Whatever its origin, the technique predates every electronic aid: with a compass, a log, and a watch, mariners crossed oceans on dead reckoning, and Bowditch still treats the DR plot as the backbone of the navigator’s work.
On the water
- Steering 090° at 5 knots for 30 minutes advances the DR 2.5 nautical miles due east of the last fix.
- The GPS fix plots south of the DR—current has been setting the boat toward the shoal, so the skipper alters to compensate.
- When the chartplotter blanks out, the paper DR from the last fix still says where to look for the next buoy.
Common mistake
A DR position is not where you are. It deliberately ignores current and leeway, so its error grows with time; an estimated position (EP) applies those corrections, and only a fresh fix resets the plot.
Remember it
DR = Direction + Rate: course steered and speed through the water, nothing else.