Dead Reckoning
Dead reckoning navigates from a known position by tracking direction and distance traveled. Start at the harbor. Sail southwest at 8 knots for 6 hours. You’re 48 nautical miles southwest of where you began. Update your position; repeat. The method predates instruments — sailors used it for millennia, estimating speed by watching foam pass the hull.
The term likely derives from “deduced reckoning” — position deduced from accumulated vectors. Each measurement introduces error. Sail for days and the errors compound. Medieval navigators couldn’t know their longitude within hundreds of miles. Many ships wrecked on unexpected coastlines.
Every mobile organism performs dead reckoning. Desert ants (Cataglyphis fortis) forage in meandering paths but return home in straight lines, even when hundreds of meters out in featureless terrain. Researchers displaced ants mid-return; they walked the precise direction and distance they would have needed from their original position. The path integration was accurate within centimeters.
This works through step counting and sun position. The ant’s neural system integrates these signals continuously, maintaining an internal home vector. When experimenters surgically lengthened or shortened ant legs, the ants over- or under-shot home by exactly the predicted amount. The odometer is in the steps.
Humans use dead reckoning constantly without conscious awareness. Walk to the kitchen in the dark and you know roughly where the counter is. Close your eyes, spin, and point back to the door — you can probably get close. The vestibular system and proprioception feed an implicit position-tracking system.
The limits appear in unfamiliar territory. Walk into a new building and you lose orientation quickly. The internal compass needs calibration from landmarks, from stored maps. Dead reckoning bridges between known points; it can’t replace them entirely. The error accumulation demands periodic correction.
This is why explorers cared obsessively about fixing position. Even with dead reckoning, you need occasional certain fixes — a sighting of Polaris, a known landmark, later a radio beacon or satellite signal. Pure dead reckoning drifts. Correction anchors.
Go Deeper
Books
- Desert Navigator: The Journey of an Ant by Rüdiger Wehner — A lifetime of research on Cataglyphis, distilled into a lavishly illustrated masterpiece. The definitive account of ant navigation.
- We, the Navigators by David Lewis — Polynesian wayfinding techniques, including dead reckoning across thousands of miles of open ocean.
- The Natural Navigator by Tristan Gooley — Practical introduction to human wayfinding without instruments.
Essays
- Müller & Wehner’s 1988 paper “Path Integration in Desert Ants, Cataglyphis fortis” in PNAS — The seminal experiment with leg-length manipulation.
- Wehner’s 2003 review “Desert ant navigation: how miniature brains solve complex tasks” in Journal of Comparative Physiology A.
Related: [[navigation]], [[cognitive-maps]], [[polynesian-wayfinding]]