Dead reckoning is a navigational technique dependent on using a known position and extrapolating a new position from that one by considering the speed and direction of travel. This technique was historically used by sailors and aviators, and is less commonly used today, although some navigational systems still rely on dead reckoning to some extent. The technique has a number of drawbacks, some of which can prove quite serious when there is a narrow margin for error.
In dead reckoning, the navigator starts with a known positional fix, taken with observations and other tools. This fix is noted on a chart with an indicator showing it is a firm positional fix, not an extrapolation. The next time the navigator wants to estimate position, the amount of time elapsed is considered alongside the recorded speed of travel and the course. Dead reckoning can also include adjustments for currents and winds, as these factors may take a vessel off course. A new position is entered on the chart to reflect the outcome of the calculations.
One serious problem with dead reckoning is the risk of cumulative errors. Recording positional information relative to previous positions, rather than with the use of new data, runs the risk of compounding an error. Even if the initial positional fix was correct, subsequent positions may be erroneous, and can grow worse over time. Imagine, for example, if a navigator gets the course off by three degrees, causing the position to deviate more and more by the day.
Mistakes using this navigational technique can create substantial problems. People may enter dangerous waters with shoals and icebergs without being aware of it, or they could drift into territorial waters where they are not welcome. Dead reckoning can also delay and hinder journeys considerably if people end up far from their originally planned destination. On aircraft, using this system at night or in bad weather can put people at risk of an accident, as they may drift out of their planned flight path and not be aware of it.
Some navigators joke that the derivation of this phrase comes from “dead wrong,” referencing the unreliability of this technique. Others suggest it is a derivative of “deduced reckoning,” arguing that historical records sometimes refer to it as “ded. reckoning.” Research into the origins of the phrase does not support either of these claims, although the term itself dates to at least the 1600s, when it began to be used by English navigators.