From the Oxford English Dictionary, “dead reckoning” means “The estimation of a ship’s position from the distance run by the log and the courses steered by the compass, with corrections for current, leeway, etc., but without astronomical observations.” The explanation applies to both ships and aircraft.
Wikipedia tells me that, “Dead reckoning is subject to cumulative errors. Advances in navigational aids that give accurate information on position, in particular satellite navigation using the Global Positioning System, have made simple dead reckoning by humans obsolete for most purposes. However, inertial navigation systems, which provide very accurate directional information, use dead reckoning and are very widely applied.”
By analogy with their navigational use, the words “dead reckoning” are also used to mean the process of estimating the value of any variable quantity by using an earlier value and adding whatever changes have occurred in the meantime. Often, this usage implies that the changes are not known accurately. The earlier value and the changes may be measured or calculated quantities.
A persistent theory says that the phrase should be, “ded reckoning” meaning “deduced reckoning” but this is quite wrong. As with many untenable theories, there’s an element of plausibility along with a great deal of enthusiasm to conclude that two and two equal five.
The OED states that the phrase originated in the early 1600s. The word “dead” is used as an adjective in the sense of “unrelieved; unbroken; absolute; complete; exact” just the same as it’s used in “dead center,” and “dead ahead.” Or, perhaps, “you’re dead if you don’t reckon right.”
In studies of animal navigation, dead reckoning is more commonly known as path integration. Animals use it to estimate their current location based on their movements from their last known location. Animals such as ants, rodents, and geese have been shown to track their locations continuously relative to a starting point and to return to it, an important skill for foragers with a fixed home.
There are several books with the title Dead Reckoning, which made me think a movie might come along soon. The film industry has already done it. The blurb says, “A new three-hour PBS series follows war crime investigators and prosecutors as they pursue some of the world’s most notorious war criminals—notably Adolf Eichmann, Saddam Hussein, Radovan Karadzic, Charles Taylor, and Efraín Ríos Montt. The principles, legal doctrines and tactics that emerged from those pursuits now inform the effort to expose, prosecute, and punish present day human rights violators whose depredations have left millions dead and displaced. It is a tale of daring escapades, political obstruction, broken promises, and triumphs and failures.” It premiered March 28th, 2017.
Which indicates that I’m behind the times, as usual. Or, perhaps, dead in the water.