Module 10 - Chartwork

Dead Reckoning (DR) and Estimated Position (EP)

What you will learn

Advance a timed DR and EP through more than one interval while carrying forward the uncertainty in speed, steering, leeway and stream.

A DR shows where the vessel would be from movement through the water in this course convention. An EP adds estimated environmental movement. Neither is an observation of the vessel's actual position.

Start from the latest accepted position and time. Advance the course or track through the water by distance through the water to obtain the DR. In the convention used here, tidal stream and leeway remain outside that first construction so their effects are visible. Label the result and the time using the notation required by the training material.

Build the EP by applying the stated leeway and tidal-stream displacement for the same interval. The inputs may be the best available estimates, but they still contain error. An EP can be more operationally useful than an uncorrected DR without becoming a fix or a guaranteed improvement in accuracy.

For a multi-hour sequence, use the appropriate speed, heading, leeway and stream for each interval and keep the earlier uncertainty. Do not silently restart an EP as if it were a fresh fix. When a reliable new observation becomes available, compare it with the prediction, investigate any discrepancy and begin the next DR from the accepted observed position.

Worked example

From a 1200 fix, the yacht covers 5.5 M through the water in the first hour and 5.0 M in the second while the stream estimate changes between hours.

  1. 1Plot and label the 1300 DR using the first hour's through-water course and 5.5 M distance.
  2. 2Apply the first hour's leeway and tidal displacement to obtain and label the 1300 EP.
  3. 3Use the 1300 EP as the estimated start for the second interval, retaining its uncertainty rather than calling it a fix.
  4. 4Plot the second hour with its own 5.0 M run and stream estimate, then seek fresh position evidence before the danger margin becomes small.

Sense check: If the second-hour construction appears more certain than the first without a new observation, uncertainty has probably been hidden rather than removed.

Use disagreement as information

The 1500 EP lies close to a shoal, while a checked depth trend disagrees with the depth expected there.

  1. 1. Widen the position picture

    Retain the DR, EP and uncertainty instead of selecting whichever point looks safer.

  2. 2. Check the weakest assumptions

    Review log distance, heading, leeway, stream source, time and the charted-versus-observed depth relationship.

  3. 3. Protect the vessel

    Obtain independent position evidence, slow or alter as appropriate, and keep clear of the danger-side uncertainty.

Sense check: A modest plausible change to stream or speed should not move the uncertainty area across a danger without triggering action.

Common mistake or limitation

  • Calling an EP a fix hides that its environmental inputs are estimates.
  • Starting every hour from a clean point discards the accumulated uncertainty of the previous EP.

Recap

  • DR records the assumed movement through the water.
  • EP adds the estimated effects of wind and water.
  • Carry uncertainty until new accepted observations reset the position chain.

Optional quick check

Section 2 of 13

What should happen to uncertainty when a second-hour EP begins from the first-hour EP without a new fix?

Choose one answer
Sources and factual review

Reviewed 2 September 2026 - Compass Revision editorial review.

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