Dead reckoning estimates where you are from a known starting point, called a fix, plus your speed, heading, and elapsed time: distance equals speed times time. The phrase is not short for deduced reckoning, a claim debunked by the record, since dead reckoning appears in 1613 and deduced reckoning not until 1931. The word dead likely refers to an object stationary in the water used as a baseline. The method’s enemy is cumulative error, where each estimate builds on the last like a photocopy of a photocopy, worsened by drift from currents and wind. Nature and ancient engineers both mastered it: foraging ants use path integration to walk straight home after zigzagging, and China’s south pointing chariot used differential gears to hold its pointer south without a magnetic compass.

Before the marine chronometer, dead reckoning was the only way to cross open ocean, as Christopher Columbus and John Cabot did. Crews marked heading and speed with pegs on a traverse board and measured speed with a chip log, counting knots in a rope against a sandglass, the origin of knots as a unit. Aviators faced wind as well: John Alcock and Arthur Brown in 1919 and Charles Lindbergh in 1927 relied on it, and pilots still learn the wind triangle for visual flight rules. Today cars fall back on wheel and steering sensors in tunnels, blended with GPS by a Kalman filter, and online games use the same math to predict other players’ positions between server updates.

• The Oxford English Dictionary traces dead reckoning to 1613.
• A south pointing chariot turning left spins its right wheel faster, and gears counter-rotate the pointer to compensate.
• Flight instructors may cover a student’s screens to force navigation by compass, clock, and math.
• Pedestrian dead reckoning in phones uses the accelerometer as a pedometer and the magnetometer as a compass.
• Games smooth corrections with projective velocity blending and cubic Bezier splines so players do not appear to teleport.