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Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon known speed,elapsed time, and course. In marine navigation a DR plot generally does not take in to account the effect of currents or wind. While this method of navigation is no longer considered primary in, aircraft, automobiles, rail engines, and construction site engines (tunnels), it is still often used as a backup in case of failure of the electronic navigation systems. Aboard ship a DR plot is considered important in evaluating position information and planning the movement of the vessel.[[1]]

In studies of animal navigation, dead reckoning is more commonly (though not exclusively) known as path integration, and animals use it to estimate their current location based on the movements they made since their last known location.

Dead reckoning (DR) begins with a known position, or fix. This fix is then advanced, mathematically or directly on the chart, by means of recorded heading, speed, and time. Speed can be determined by many methods. Before modern instrumentation, it was determined aboard ship using a chip log. More modern methods include, pit log referencing engine rpm against a table of total displacement (for ships) or referencing one's indicated airspeed fed by the pressure from a Pitot tube. This measurement is converted to an equivalent airspeed based upon known atmospheric conditions and measured errors in the indicated airspeed system. A naval vessel uses a device called a Pit Sword. A Pit sword (rodmeter) utilizes 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 a estimated position (EP) by taking into account the current (known as set and drift in marine navigation) or the winds (for aerial navigation). In air navigation, this tool is referred to as a wind triangle.

DR 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 judgment determine when DR positions are calculated. Generally speaking, DR positions are calculated at least once every 300 miles and when making combined turns totaling more than 30 degrees from the initial heading out of the last DR position.

Before the development of the chronometer, dead reckoning was the primary method of determining longitude available to mariners such as Christopher Columbus and John Cabot on their trans-Atlantic voyages.

There is some controversy about the derivation of the phrase. It is popularly thought to come from deduced reckoning and is sometimes given in modern sources as an alternatively spelled ded reckoning. However, according to the Oxford English Dictionary, the phrase dead reckoning dates from Elizabethan times (1605-1615).

The popular etymology from deduced is not documented in the Oxford English Dictionary or any other historical dictionary. Dead reckoning is navigation without stellar observation. With stellar observation, you are "live", working with the stars and the movement of the planet. With logs, compasses, clocks, but no sky, you are working "dead".

Dead reckoning is also a method used in networked computer games and simulations to reduce lag caused by network latency and bandwidth issues. Programs do this by predicting the future state of an entity based on its current state (such as predicting the path of a fighter jet based on its velocity and position). Then the program only sends updated information about the entity's current state if it is not close enough to the predicted state. Other programs in the network use the same prediction algorithm to fill in the gaps between entity updates.

• US Coast Guard Auxiliary: "Dead reckoning"
• Bowditch Online: "Dead reckoning"
• Straight Dope: Is "dead reckoning" short for "deduced reckoning"?
• Jesse Aronson: "Dead Reckoning: Latency Hiding for Networked Games" (requires free registration to Gamasutra.com)
• Paper about Helmet-Mounted Pedestrian Dead-Reckoning System (2006)