Dead reckoning calculator

Dead reckoning (DR) is the process of estimating your current position based on a previously known position (a "fix"), advancing it using known or estimated speed, course, and elapsed time. Enter your last fix, true course, speed and time elapsed to get an estimated current position in latitude / longitude, distance covered (NM), and the back-bearing to your fix.

• Short distances: flat-earth approximation is adequate; mid-latitude correction applied.
• Longer passages: Mercator sailing used (cosine of mean latitude).
• Accuracy: DR position degrades quickly with current, leeway, and steering error — always cross-check against GPS, visual bearings, radar, and celestial if available.

GPS spoofing and the value of DR

GPS spoofing — feeding false satellite signals to push a ship's displayed position to a wrong location — has been extensively documented in the Black Sea, parts of the Eastern Mediterranean, and near conflict zones in the Middle East. A parallel DR plot gives the OOW an independent cross-check: if the GPS position diverges from the DR position by more than a few miles for no obvious reason (no set/drift change, no course alteration), suspect spoofing. Compare AIS echoes from nearby ships, radar fixes on charted objects, and LORAN / eLoran where available.

See also

• · Distance & bearing calculator — great-circle and rhumb-line distance between two positions.
• · ECA checker — check if a position is in an Emission Control Area or MARPOL Special Area.
• · COLREGs — Rule 7 (risk of collision), Rule 19 (restricted visibility).

Last updated 2026-04-27