Dead reckoning — DR plot maintenance

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

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

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

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.

Bowditch Pub. 9 Ch. 9 §900

Definition and Use Dead reckoning (DR) is a method for determining the estimated position of a vessel by advancing from a known fix of position along the vessel’s ordered course and speed. This can be used to determine where a vessel currently is or where it will be. This is an approximate position; it does not allow for the many errors that can cause a vessel to veer off course such as helmsmen error, compass error, or current and wind. Dead reckoning helps in predicting landfall, determining distances to objects, predicting arrival times, and evaluating the accuracy of electronic positioning information. It also aids in predicting which celestial bodies will be available for future observation. However, its most important use is in projecting the ship's position in the immediate future …

Bowditch Pub. 9 Ch. 9 §901

Measuring Courses and Distances To measure courses, use the chart’s compass rose nearest to the chart area currently in use. Transfer course lines to and from the compass rose using parallel rulers, rolling rulers, or triangles. If using a parallel motion plotter (PMP), simply set the plotter at the desired course and plot that course directly on the chart. Transparent plastic navigation plotters that align with the latitude/longitude grid may also be used. The navigator can measure direction at any convenient place on a Mercator chart. All the meridians are parallel to each other and a line making an angle with any one makes the same angle with all others. When measuring direction on a conformal chart having nonparallel meridians, use the meridian closest to the area of the chart in use.…