Tuesday, March 11, 2008

Dead Reckoning Terms

The following are some terms used when using dead reckoning:

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.

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, 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.

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

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 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.

The DR track (or DR track line) is the path or course the ship is expected to follow. It is plotted from a known position using courses and speeds through water. When plotting a DR track, no consideration is given for current and wind.

There are three basic principles when plotting a DR track:

A DR track is ALWAYS started from a known position.
Only true courses are plotted.
Only speed through water is used for determining distance traveled.

The purpose of the DR track is to show the navigator basically where you are planning to go, the rate of advance, and the ETA at various points along the way and at the final destination. After the DR track has been plotted, the navigator determines whether or not the basic track is clear of navigational hazards, as well as deciding what navigational aids are available and when they are visible. By examining the DR track, all dangers and surprise are eliminated for the voyage. If the navigator finds a DR track is going to lead into shoal waters or danger, the DR track can be reevaluated in time to prevent any hazard to the ship.

When plotting the DR track, be sure that all DR tracks and distances are accurately measured. Being neat is a must to avoid confusion and error. Overlong lines and unnecessarily written information cause errors. Completeness of the DR track is necessary to show course, times, and positions. Standardization of labeling ensures neatness and clarity for any person using that plot.

The course is the intended horizontal direction of travel. This DR track starts from a known position and is plotted as follows:

Above and parallel to the course line, place a capital C and three digits to indicate the true course (C 007° ). It is customary to label courses to the nearest whole degree. Under the course line and below the direction label, place a capital S and two digits for the speed. Since the course is given in degrees true and speed in knots.DR positions are marked along the track line at specific time intervals depending upon where the ship is being navigated. In confined areas such as rivers and bays, DR plots can be plotted for every 15 minutes or half hour. Running along the coast in less restrictive waters, the DR plots can be put in every hour, when sailing great distances over open waters, they can be plotted every 4 hours. DR plots are put in wherever a course or speed change occurs. A new DR track is plotted from a established fix. Even though an estimated position is shown, you do not begin a new DR track from this point.

Time of fixes and estimated positions are placed horizontally while the times of dead reckoning positions are placed other than horizontally.