Dead Reckoning

What is Dead Reckoning?

Dead reckoning (DR) is the practice of estimating your current position by starting from a known position and projecting forward using your course, speed, and elapsed time — without reference to external observations like visual bearings, GPS, or depth. It answers: if I've been sailing 045° at 5 knots for 2 hours from my last known position, where should I be now?

The name almost certainly derives from 'ded' (deduced) reckoning — estimating by logical deduction from known data. DR is the baseline of all navigation: every GPS position, every visual fix, every depth confirmation either agrees with your DR or corrects it. Understanding DR is what makes a navigator capable of recognizing when something is wrong.

A DR position is not a fix. It is an estimate. It does not account for current, wind-driven leeway, compass error, speed measurement error, or any variation between where the boat was aimed and where it actually went. A DR position becomes less reliable the longer it's been since the last fix — but even a rough DR is vastly better than having no idea at all.

Chart showing a DR track plotted from a known fix, with courses and speeds labeled, DR positions marked at intervals, and an estimated position marked — A DR track: from the last fix, each course change and time interval is plotted to estimate current position

A dead reckoning position differs from a fix because:

DR requires a GPS signal; a fix does not DR is an estimate based on course, speed, and time — a fix is confirmed from two or more independent LOPs DR is more accurate than a visual fix in good conditions A fix requires a minimum of three observations; DR requires only one

Dead reckoning most likely derives from which phrase?

Deadly reckoning — navigation by sense of direction alone Deduced reckoning — position estimated by logical deduction from known data Dead star reckoning — navigation by stars Deck reckoning — calculated from the ship's log on deck

Plotting a DR Track

A DR track is kept on the chart by plotting each leg of the voyage from the last confirmed fix. At each course change, mark the DR position with a half-circle on the chart and note the time. Along each track line, note the course and speed. At regular time intervals — every 30 minutes to an hour in pilotage waters — mark an estimated position (EP) as well.

The convention is: a fix is marked with a circle and the time. A DR position (calculated position with no fix correction) is marked with a semicircle. An EP (Estimated Position) — a DR position that has had current and leeway factored in — is marked with a square. These distinctions communicate the level of confidence in each position to anyone reading the chart.

In practice: maintain the DR plot even while using GPS. If the GPS stops working, your last good chart plot becomes your starting point for manual navigation. Sailors who update only the chartplotter — and never mark a chart — have no fallback when the electronics fail.

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Keep a log of course changes, speed, and times in a written log book. This is your evidence base for DR if electronics fail. A logbook entry every 30 minutes is good practice; every hour is the minimum for offshore passages.

On a chart, a position marked with a square represents:

A GPS fix An estimated position — DR adjusted for current and leeway A dead reckoning position based on course and speed only A danger area boundary

Why should you maintain a chart-based DR plot even when GPS is working?

GPS cannot display positions on a paper chart automatically If GPS fails, the chart plot provides an immediate starting point for manual navigation Chart plots are legally required in coastal waters DR is more accurate than GPS in restricted visibility

Error Accumulation and When to Fix

The fundamental limitation of DR is error accumulation. Every source of inaccuracy — compass deviation, log error, helmsperson steering 3° off course, undetected current — adds up over time. After 1 hour, a small error is manageable. After 12 hours, the same error may have displaced your DR position by several miles.

The practical implication: the longer since your last fix, the wider your circle of uncertainty. In fog, at night, or in any condition where fixing is difficult, treat your DR position as a center of a growing circle — not a point. Allow for this uncertainty in your margins from hazards. If the edge of your uncertainty circle approaches a charted rock, slow down and get a fix before proceeding.

Fix regularly: in coastal pilotage, every 15–30 minutes or at every waypoint. In offshore passages, every 1–2 hours with GPS and visually as possible. After any significant course change, take a new fix to confirm the new starting point. A navigator who fixes frequently has short, small DR errors. One who fixes rarely — especially on a complex coastline — builds up uncertainty until small errors become dangers.

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Never drive toward a hazard based solely on a DR position when the uncertainty circle could include that hazard. Either take a fix to confirm you're clear, or treat the hazard as if it's at the closest edge of your uncertainty — slow down, sound the horn in fog, and get a visual confirmation before proceeding.

You've been sailing 4 hours without a fix in fog, using DR. Your position uncertainty is growing. The correct response when a charted rock appears near your estimated position is:

Maintain course — your DR should be accurate enough after 4 hours Slow down, treat the rock as if it's at the nearest edge of your uncertainty zone, and take a fix before proceeding Alter course 10° and continue Increase speed to clear the area quickly

What is the single most effective way to limit DR error over a long passage?

Using a more accurate compass Taking frequent fixes to reset the DR starting point Reducing boat speed to limit distance traveled Recording wind direction and strength

DR in Modern Navigation Practice

With GPS ubiquitous, some sailors question whether DR is still relevant. It absolutely is. GPS provides position — not navigation judgment. DR is the mental framework that makes a navigator capable of: (1) recognizing when GPS is showing an impossible or implausible position, (2) navigating if GPS fails, (3) planning ahead — 'if I sail 270° at 6 knots for 3 hours, where will I be, and what hazards might I encounter?'

Sanity checking GPS: if your GPS shows a position 5 miles from your DR position after 1 hour of sailing, something is wrong. Either GPS has a problem (antenna disconnected, wrong datum selected), or there's been a massive undetected current. Investigate — don't just accept the GPS position.

DR is also the basis for contingency planning: 'if GPS fails at this position and I need to enter the harbor, what course do I steer and what do I expect to see and when?' Answering this before it happens — using DR — is what keeps sailors safe when equipment fails at the worst possible time.

Your GPS position is 4 miles from your DR position after a 1-hour coastal passage in calm conditions. What should you do?

Trust the GPS — it is more accurate than DR Trust the DR — GPS errors this large are common Investigate — either GPS has an error or there is a major undetected current Average the two positions and proceed on that basis

Before a harbor approach at night, a navigator applies DR to estimate: 'In 45 minutes I should be on the 20-foot contour with the lighthouse bearing 045°.' This is an example of:

Electronic navigation using GPS projections Contingency planning using DR to predict where the vessel should be and what it should see A navigational fix using a single LOP Using depth-only navigation in restricted visibility

Summary

Dead reckoning estimates current position from a known fix using course, speed, and elapsed time. It does not account for current, leeway, or compass error.

A DR position is an estimate — not a fix. Mark it with a semicircle; an EP (with current allowed) is marked with a square; a fix is marked with a circle.

Errors accumulate over time. Fix frequently to reset DR error. In coastal waters, fix every 15–30 minutes or at every waypoint.

Treat growing DR uncertainty as a circle — not a point. Never approach hazards within the uncertainty circle without a confirming fix.

DR remains essential in the GPS age for sanity-checking electronic positions, planning contingencies, and navigating when electronics fail.

Key Terms

Dead reckoning (DR): Estimating current position from a known fix using course, speed, and elapsed time — not a confirmed fix
DR position: A position calculated by DR — plotted with a semicircle on a chart. Less reliable than a fix
EP (Estimated Position): A DR position that has also been corrected for known or estimated current and leeway — plotted with a square
Course made good (CMG): The actual direction traveled over the ground between two fixes — may differ from intended course due to current or leeway
Error accumulation: The growth of position uncertainty over time in DR, as each small inaccuracy compounds
Uncertainty circle: The area in which the vessel could be, given accumulated DR error — grows larger as time from last fix increases
Sanity check: Comparing a GPS or electronic position with the DR position to detect implausible errors or equipment malfunction

What is Dead Reckoning?

Plotting a DR Track

Error Accumulation and When to Fix

DR in Modern Navigation Practice

Summary

Key Terms

Dead Reckoning — Quiz

From your last fix at 1400, you sail 135° at 5 knots for 3 hours. What is your DR position?

On a chart, how is a DR position marked differently from a fix?

After 6 hours of sailing without a fix in fog, your DR position uncertainty is large. A charted shoal appears at the edge of this uncertainty zone. What is the correct action?

You have GPS but also maintain a manual DR plot. Why?

Your DR calculation places you 2 miles north of a reef. Your GPS shows you 1.5 miles south of the reef. What should you do first?

References & Resources

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