Tuesday, February 26, 2008


The following are some terms used when using dead reckoning:

Heading - The horizontal direction in which the ship points or heads at any given second, expressed in angular units clockwise from 000° through 360° . The heading of the ship is also called ship’s head. The heading is always changing as the ship swings or yaws across the course line due to the seas or steering error.

Course - As applied to marine navigation, the direction in which a vessel is to be steered or is being steered, and the direction of travel through the water. The course is measured from 000° clockwise from the reference direction to 360° . Course may be designated as true, magnetic, compass, or as determined by the reference direction.

DR Track Line - In marine navigation, the graphic representation of a ship’s course normally used in the construction of a dead reckoning plot.

Speed - The ordered rate of travel of a ship through the water is normally expressed in knots. In some areas where distances are in statute miles, such as on the Great Lakes, speed units will be "miles per hour." Speed is used in with time to establish a distance run on each of the consecutive segments of a DR plot.

Fix - A position established at a specific time to a high degree of accuracy. It may be determined by any of a number of methods. A running fix is a position of lesser accuracy, based in part on present information and in part on information transferred from a prior time.

DR Position - A position determined by plotting a vector or series of consecutive vectors using only the true course and distance determined by speed through the water, without considering current

Estimated Position - The more probable position of a ship, determined from incomplete data or data of questionable accuracy. In practical use, it is often the DR position modified by the best additional information available.

Dead Reckoning Plot - called DR plot. In marine navigation it is the graphical representation on the nautical chart of the line or series of lines, which are the vectors of the ordered true courses and distance run on these courses at the ordered speeds while proceeding from a fixed point. The DR plot originates at a fix or running fix, it is labeled as to courses, speeds, and times of various dead reckoning positions, usually at hourly intervals or at times of change of course or speed. A DR plot represents courses and speeds that have been used. A similar plot may be made in advance for courses and speeds that are expected to be used.

Estimated Time of Departure - The estimate of the time of departure from a specified location according to a scheduled move to a new location.