Dead reckoning (DR)
A DR position uses only your course steered and distance run through the water, from your last known position. It ignores the tidal stream and leeway, so it's where you'd be in still water with no wind pushing you sideways.
To plot it, convert your heading to true, draw a line along it from your last position, and mark off the distance from the log.
Estimated position (EP)
An EP adds the two things that move you away from your DR:
- Leeway: the wind pushes the boat downwind of the course she's steering. Plot your water track a few degrees downwind of your heading, by your estimate of the leeway.
- Tidal stream: from the end of the water track, draw the set (direction) and drift (distance) of the tide over the same period. Work them out from a tidal diamond or atlas.
The end of the tidal stream line is your EP. The line from your start to the EP is your ground track: where you've actually gone over the seabed.
The chartwork conventions
- One arrowhead: the water track, your course and distance through the water.
- Two arrowheads: the ground track, your course and distance over the ground.
- Three arrowheads: the tidal stream.
- An EP is usually marked with a triangle and the time; a fix with a circle and the time.
DR, EP and course to steer
An EP looks back: where have I got to? A course to steer looks forward: what heading gets me where I want to go? They use the same triangle of water track, tide and ground track, in a different order. See course to steer.
How far to trust it
An EP is only as good as your heading, log, leeway estimate and tidal stream. The errors grow the longer you go without a fix, so get a fix whenever you can and start a new EP from it. See position fixing.