Dead Reckoning — DR Plot Maintenance

TL;DR — Dead reckoning is the determination of position by applying course steered and distance run from a known starting point; DR accuracy degrades over time and must always be confirmed by external fixes whenever possible. Bowditch Ch. 1 §102

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What the Rule Says

Marine navigation is the art and science of determining a vessel's position and conducting it safely from one point to another. Four primary methods exist: dead reckoning, piloting, celestial, and electronic. Modern voyages combine all four, with electronic navigation as the principal real-time reference and the others serving as cross-checks. Bowditch Ch. 1 §101

Dead reckoning (DR) is the determination of position by applying course steered and distance run from a known starting point. Bowditch Ch. 1 §102 That definition is precise and testable — every word carries weight:

• Course steered — the compass or true heading the vessel is actually holding through the water, not the course over ground.
• Distance run — derived from speed through the water multiplied by elapsed time.
• Known starting point — the last confirmed fix, not a previous DR position.

DR accuracy degrades over time and must be confirmed by external fixes whenever possible. Bowditch Ch. 1 §102 This degradation is not a flaw in the method; it is an inherent characteristic. The longer the interval since the last fix, the larger the circle of uncertainty surrounding the DR position.

Piloting as the Complement to DR

In confined waters, dead reckoning alone is insufficient. Piloting — navigation by reference to landmarks, aids to navigation, and soundings — becomes the primary method when electronic positioning may be obstructed by terrain or interference. Bowditch Ch. 1 §103 The exam will test whether you understand that DR and piloting are complementary, not competing, methods.

Lines of Position and Fixes

A line of position (LOP) is a line along which the vessel is known to lie at a given moment, based on a single observation. A fix is the intersection of two or more LOPs taken simultaneously. Bowditch Ch. 7 §701 The fix is what resets the DR plot — every new fix becomes the new known starting point from which the next DR track is projected.

A running fix is obtained when LOPs cannot be taken simultaneously. The first LOP is advanced along the DR track to the time of the second observation; the intersection of the advanced LOP and the second LOP is the running fix. A running fix is less accurate than a fix derived from simultaneous LOPs. Bowditch Ch. 7 §702 On the exam, expect questions that ask you to distinguish a fix from a running fix, or that ask why a running fix carries greater uncertainty.

Navigational Errors and the DR Plot

Navigational errors arise from five sources: instrument error, observation error, plotting error, computation error, and the basic assumption that the vessel has held the planned course. Cumulative errors must be checked against fixes whenever possible. Bowditch Ch. 23 §2301 This last source — the assumption of steady course — is the one most directly relevant to DR plot maintenance. Every time the helmsman wanders off course, every time speed changes without a log entry, the DR position drifts further from reality.

Set, Drift, and the Current Vector

DR plots through the water do not account for current. Set is the direction toward which the current is flowing; drift is the speed of the current. The vector sum of the intended course made good through the water and the current vector gives the actual course over ground. Bowditch Ch. 23 §2302

The vector triangle for current correction states: course steered plus the drift vector equals course made good. Inverting the triangle: from a known course made good and a known drift, you can derive the course steered required to achieve a desired track over ground. Bowditch Ch. 24 §2402 This is the mathematical backbone of current correction problems on the exam.

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Why It Matters on the Exam

OUPV and Master 100 GT written exams test DR in several recurring question types:

1. Definition questions. "Dead reckoning is best defined as…" The answer is always: applying course steered and distance run from a known starting point. Bowditch Ch. 1 §102 Distractors will substitute "course over ground," "GPS track," or "last estimated position" for the correct elements. None of those are correct.

2. DR vs. fix vs. running fix. Candidates must distinguish three position types:

• A fix — two or more simultaneous LOPs intersecting. Bowditch Ch. 7 §701
• A running fix — first LOP advanced along the DR track to the time of the second LOP; less accurate than a simultaneous fix. Bowditch Ch. 7 §702
• A DR position — projected from the last known fix using course and speed; no external observation involved. Bowditch Ch. 1 §102

3. Error accumulation. Questions will ask what causes DR accuracy to degrade. The answer encompasses all five error sources — instrument, observation, plotting, computation, and course-holding assumption — with cumulative effect checked only by external fixes. Bowditch Ch. 23 §2301

4. Current vector problems. Given a course steered, a vessel speed, a current set and drift, find the course and speed made good — or the reverse. The vector triangle is the tool: course steered plus drift vector equals course made good. Bowditch Ch. 24 §2402 These problems appear in both the chart plotting section and the general navigation section of the exam.

5. Piloting in confined waters. When asked which navigation method is primary in restricted or confined waters, the answer is piloting — by landmarks, aids to navigation, and soundings — not DR and not electronic navigation alone. Bowditch Ch. 1 §103

6. Relationship between DR and fixes. The exam may ask when a new DR track begins. The answer: every time a new fix is obtained. The fix resets the starting point; the DR track projects forward from that fix. Bowditch Ch. 1 §102 A DR position is never used as the starting point for a new DR track — only a fix qualifies.

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Common Pitfalls

Pitfall 1 — Confusing course steered with course over ground. DR uses course steered through the water and speed through the water. Current displaces the vessel from the DR track; the DR position does not account for that displacement. Bowditch Ch. 23 §2302 Candidates who conflate course steered with course over ground will set up current vector problems incorrectly every time.

Pitfall 2 — Treating a DR position as a fix. A DR position is a projection, not an observation. It carries no independent verification. Only the intersection of two or more LOPs constitutes a fix. Bowditch Ch. 7 §701 Never advance a DR position as though it were a confirmed fix.

Pitfall 3 — Treating a running fix as equivalent to a simultaneous fix. A running fix is explicitly less accurate than a fix from simultaneous LOPs because the advanced LOP carries the accumulated DR error from the interval between observations. Bowditch Ch. 7 §702 On the exam, if a question asks which position type is most reliable, the simultaneous fix ranks above the running fix, which ranks above the DR position.

Pitfall 4 — Ignoring cumulative error. Candidates sometimes assume that careful plotting eliminates DR error. It does not. Even with perfect instrument readings and perfect plotting, the assumption that the vessel held the planned course introduces error whenever the actual track deviates. Cumulative errors must be checked against fixes whenever possible. Bowditch Ch. 23 §2301

Pitfall 5 — Misidentifying set and drift. Set is the direction toward which the current flows — not the direction from which it comes (that convention applies to wind, not current). Drift is the speed of the current, expressed in knots. Bowditch Ch. 23 §2302 Reversing set and drift in a vector problem produces a position error on the opposite side of the intended track.

Pitfall 6 — Starting a new DR track from a DR position rather than a fix. When a fix is obtained, the DR track restarts from the fix. If no fix is available and only a DR position exists, the DR track continues from the last fix — it does not restart from the DR position. Bowditch Ch. 1 §102

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Quick Check

Q1 — How is dead reckoning defined?

Dead reckoning is the determination of position by applying course steered and distance run from a known starting point. Bowditch Ch. 1 §102 The known starting point must be a confirmed fix, not a previous DR position.

Q2 — What is the difference between a fix and a running fix?

A fix is the intersection of two or more LOPs taken simultaneously. Bowditch Ch. 7 §701 A running fix is obtained when simultaneous LOPs are not possible — the first LOP is advanced along the DR track to the time of the second observation, and the intersection is the running fix. A running fix is less accurate than a simultaneous fix. Bowditch Ch. 7 §702

Q3 — Why does DR accuracy degrade over time?

DR accuracy degrades because errors accumulate from instrument error, observation error, plotting error, computation error, and the assumption that the vessel has held the planned course. Bowditch Ch. 23 §2301 Additionally, current continuously displaces the vessel from the DR track without being reflected in the DR position. Bowditch Ch. 23 §2302

Q4 — What are set and drift, and how do they relate to the DR plot?

Set is the direction toward which the current is flowing; drift is the speed of the current. Bowditch Ch. 23 §2302 The DR plot reflects course steered and speed through the water only. Current displaces the vessel from the DR track. The vector sum of the course made good through the water and the current vector gives the actual course over ground. Bowditch Ch. 23 §2302

Q5 — Which navigation method is primary in confined waters, and why?

Piloting is the primary method in confined waters. It uses landmarks, aids to navigation, and soundings to determine position. Dead reckoning alone is insufficient in confined waters, and electronic positioning may be obstructed by terrain or interference. Bowditch Ch. 1 §103

Q6 — State the current vector triangle relationship.

Course steered plus the drift vector equals course made good. Inverting the triangle: from a known course made good and a known drift, derive the course steered required to achieve a desired track over ground. Bowditch Ch. 24 §2402

Q7 — When does a new DR track begin?

A new DR track begins every time a new fix is obtained. The fix becomes the new known starting point, and the DR track is projected forward from that fix using course steered and speed through the water. Bowditch Ch. 1 §102 A DR position does not qualify as a starting point for a new DR track.