Dead reckoning is a navigation technique that uses a known position to estimate a new position based on speed and direction. It has drawbacks, including cumulative errors, and can lead to dangerous situations. The term dates back to at least the 17th century.
Dead reckoning is a navigation technique that relies on using a known position and extrapolating a new position from that taking speed and direction of travel into account. This technique was used historically by sailors and aviators, and is used less frequently today, although some navigation systems still rely on dead counts to some extent. The technique has a number of drawbacks, some of which can be quite serious when there is a narrow margin of error.
Based on calculations, the browser starts with a known positional solution, taken with observations and other tools. This solution is observed on a graph with an indicator showing that it is a firm positional solution, not an extrapolation. The next time the navigator wants to estimate the position, the amount of time elapsed is considered along with the recorded speed of travel and the course. The dead count may also include adjustments for currents and winds, as these factors can throw a vessel off course. A new position is entered on the chart to reflect the result of the calculations.
A serious problem with dead calculation is the risk of cumulative errors. Recording positional information relative to past positions, rather than using new data, risks compounding an error. Even if the initial positional correction was correct, subsequent positions may be wrong and may worsen over time. Imagine, for example, if a navigator drifts off course by three degrees, causing the position to drift more and more each day.
Errors when using this navigation technique can create substantial problems. People may inadvertently enter dangerous waters with shoals and icebergs, or they may drift into territorial waters where they are not welcome. The death toll can also significantly delay and hinder travel if people end up far from their originally planned destination. On airplanes, using this system at night or in bad weather can put people at risk of an accident, as they may stray from their planned flight path and not be aware of it.
Some browsers joke that the derivation of this phrase comes from “totally wrong”, referring to the unreliability of this technique. Others suggest that it is a derivative of “deduced calculus”, arguing that historical records sometimes refer to it as “ded. calculation.” Research into the origins of the phrase does not support either of these claims, although the term itself dates to at least the 17th century, when English navigators began using it.
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