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Dead Reckoning

In navigation, Dead Reckoning is the process of calculating the current position by using a previously determined position, or fix, and then incorporating estimations of speed, heading direction, and course over the elapsed time. By means of the recorded heading, speed, and time, the original fix can then be advanced, mathematically or directly on the chart. Aboard ship a Dead Reckoning plot is important for evaluating position information and planning or predicting the movement of the vessel.

Dead Reckoning positions are calculated at predetermined intervals, and are maintained between fixes. The duration of the interval varies. Factors including one's speed made good and the nature of heading and other course changes, and the navigator's judgement determine when Dead Reckoning positions are calculated.

Before the 18th-century development of the marine chronometer by John Harrison and the lunar distance method, Dead Reckoning was the primary method of determining longitude available to mariners such as Christopher Columbus and John Cabot on their trans-Atlantic voyages. Tools such as the traverse board were developed to enable even illiterate crew members to collect the data needed for Dead Reckoning.

The technique of Dead Reckoning was basically a European method of wayfinding. Polynesian navigators, used quite different wayfinding techniques. The origin of the term Dead Reckoning is not clear, however, it seems that it was not originally used to abbreviate "deduced reckoning," nor is it a misspelling of the term "ded reckoning."

The process of Dead Reckoning is inherently subject to cumulative errors.

While Dead Reckoning can give the best available information on the present position with little math or analysis, it is subject to significant errors of approximation. For precise positional information, both speed and direction must be accurately known at all times during the voyage. Most notably, Dead Reckoning does not account for directional drift due to leeway, current or steering errors. These drift errors tend to compound themselves over greater distances, making Dead Reckoning an inaccurate method of navigation for longer journeys.
The accuracy of Dead Reckoning can be increased significantly by using other, more reliable methods to get a new fix part way through the journey. For example, if one was navigating in poor visibility, then Dead Reckoning could be used to get close enough to the known position of a sea mark to be able to see it, before sailing to the sea mark itself — giving a new precisely known start point — and then setting off again.

Required instruments for Dead Reckoning

Speed can be determined by many methods. Before modern instrumentation, it was determined aboard using a ship log. More modern methods include pit log referencing engine speed (e.g. in rpm) against a table of total displacement. A naval vessel uses a device called a pit sword (rodmeter), which uses two sensors on a metal rod to measure the electromagnetic variance caused by the ship moving through water. This change is then converted to ship's speed.

Distance is determined by multiplying the speed and the time. This initial position can then be adjusted resulting in an estimated position by taking into account the current (known as set and drift in marine navigation). If there is no positional information available, a new Dead Reckoning plot may start from an estimated position. In this case subsequent Dead Reckoning positions will have taken into account estimated set and drift.

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