Dead reckoning — DR plot maintenance

Bowditch Ch. 1 §102

Bowditch Ch. 1 §102 — Introduction to Marine Navigation Dead reckoning (DR) is the determination of position by applying course steered and distance run from a known starting point. DR accuracy degrades over time and must be confirmed by external fixes whenever possible.

Bowditch Ch. 8 §801

Bowditch Ch. 8 §801 — Dead Reckoning A dead-reckoning (DR) plot projects the vessel's position from the last fix using only course steered and distance run through the water, ignoring current and leeway. A new DR position is plotted at every course change, at every speed change, at the time of each fix, and at least once each hour — more often in pilot waters.

Bowditch Ch. 8 §802

Bowditch Ch. 8 §802 — Dead Reckoning DR plotting symbols are standardised: a fix is a dot inside a circle, a DR position a dot with a semicircle, and an estimated position a dot inside a square. The course line carries the true course as "C 090" above it and the speed as "S 8" below, and each position is labelled with the 24-hour time. Consistent labelling lets any watchstander read the plot at a glance.

Bowditch Ch. 8 §803

Bowditch Ch. 8 §803 — Dead Reckoning The estimated position (EP) is the DR position corrected for the estimated set and drift of the current. From the DR position lay off the current's set and drift for the elapsed time; the end of that vector is the EP, plotted as a dot inside a square. Set is the direction toward which the current flows and drift is its speed. The EP is the best position available between fixes when only the current can be estimated.