Module 10 - Chartwork
Dead Reckoning (DR) and Estimated Position (EP)
What you will learn
Build and distinguish a dead-reckoning position and an estimated position while carrying their uncertainty forward.
A DR advances the last known position using movement through the water. An EP adds estimated environmental effects. Neither is an observation of where the vessel actually is.
Start a dead-reckoning plot from a known or accepted position. Advance the vessel using the course or track through the water and the distance run through the water. In the Day Skipper convention used here, the DR excludes tidal stream and leeway so those assumptions remain visible rather than being hidden inside the first line.
An estimated position applies the best available estimates of leeway and tidal stream to the DR construction. Because the current, wind effect, steering and log may all differ from the values used, an EP is a reasoned estimate rather than a fix. It may be more useful than the uncorrected DR, but it is not automatically more accurate than every other position source.
Label the time and use the notation required by the course material. In the plotting convention used in this product, a DR is shown with a cross and an EP with a triangle containing a dot. The label is valuable only when the inputs and uncertainty are also understood. Near danger, shorten the interval between checks and seek fresh position evidence.
Keep the two answers on the chart
After one hour, the DR is clear of a shoal but the EP lies closer because the estimated stream sets towards it.
1. Do not choose the prettier answer
Keep both positions visible so the environmental assumption and its consequence can be seen.
2. Build an uncertainty area
Allow for steering, speed, leeway and stream errors instead of treating the EP as an exact point.
3. Obtain new evidence
Use an appropriate visual, depth, radar or checked electronic observation before the danger margin becomes small.
Sense check: If a modest change to the assumed stream puts the vessel near danger, the position needs checking sooner.
Common mistake or limitation
- Calling an EP a fix hides that its key inputs are estimates.
- An EP is not guaranteed to be better than a DR when the leeway or stream estimate is poor.
Recap
- DR shows movement through the water from the accepted start.
- EP adds the estimated effects of wind and water.
- Both positions need time labels, uncertainty and later verification.
Optional quick check
Section 2 of 7
Which new information changes a DR into an EP in this course convention?
Continue studying Chartwork
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