Tuesday, January 8, 2008


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, time, and course. While traditional methods of dead reckoning are no longer considered primary in most applications, modern inertial navigation systems, which also depend upon dead reckoning, are used more today.

In marine navigation a DR plot generally does not take in to account the effect of currents or wind. Aboard ship a DR plot is considered important in evaluating position information and planning the movement of the vessel. 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. Distance is determined by multiplying the speed and the time. This initial position can then be adjusted resulting in an estimated position (EP) by taking into account the current (known as set and drift in marine navigation). If there is no positional information available, a new DR plot may start from an estimated position. In this case subsequent DR positions will have taken into account estimated set and drift.

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.
Before the development of the marine 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.