Editing Dead reckoning (section)

===Marine===
[[File:CYA Coastal Navigation With Dead Reckoning 2.jpg|thumb|Dead reckoning navigation tools in coastal navigation]]
In marine navigation a "dead" reckoning plot generally does not take into account the effect of [[Ocean current|current]]s or [[wind]]. Aboard ship a dead reckoning plot is considered important in evaluating position information and planning the movement of the vessel.<ref>{{cite web |url=http://www.irbs.com/bowditch/pdf/chapt07.pdf |title=Archived copy |access-date=17 February 2010 |url-status=dead |archive-url=https://web.archive.org/web/20060313165051/http://www.irbs.com/bowditch/pdf/chapt07.pdf |archive-date=13 March 2006 }}</ref>

Dead reckoning begins with a known position, or [[Fix (position)|fix]], which 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 speed (''e.g''. in [[revolutions per minute|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]] (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.

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 judgment determine when dead reckoning positions are calculated.

Before the 18th-century development of the [[marine chronometer]] by [[John Harrison]] and the [[Lunar distance (navigation)|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. [[Polynesian navigation]], however, uses different [[wayfinding]] techniques.