Maintaining the DR Plot

What the rule requires

Four events oblige you to put a new DR position on the chart: a change of course, a change of speed, the taking of a fix, and the passing of an hour with none of the other three having occurred. In pilot waters the interval closes up and positions go on more often than hourly Bowditch Ch. 8 §801.

The list is taught almost everywhere as hour, course, speed, fix, and that is the version to carry into the exam room. Its one weakness is that it reads like a schedule of four equal appointments. The hourly entry is a floor, not an interval you are entitled to run out: the other three fire whenever they fire, and the hour only governs a leg on which nothing happened.

What goes into the DR is as heavily tested as when it goes in. The DR plot projects position from the last fix using course steered and distance run through the water. Current and leeway are excluded by definition. That exclusion is the operative distinction on most DR questions — the moment you apply an estimated current, you are no longer producing a DR position.

Each leg is drawn from the last fix, not from wherever the previous watch left the pencil. Reset the plot to each new fix as it comes in.

Labelling is standardised so that a relieving watchstander can read the plot without asking. The course line carries the true course above it, written as C 090, and the speed below it, written as S 8. Every position gets a 24-hour time. The symbols are a dot inside a circle for a fix, a dot with a semicircle for a DR position, and a dot inside a square for an estimated position Bowditch Ch. 8 §802.

The reason the plot is maintained at all is that DR accuracy decays with time run, so the DR must be confirmed by external fixes whenever they can be had Bowditch Ch. 1 §102. Exam options are frequently built around that decay: the position that has been carried longest is the one least entitled to be trusted.

You take departure at 0700 on course 135°T at 10 knots. At 0740 you increase to 12 knots. You alter to 160°T at 0755. How many DR positions between 0700 and 0800, and at what times?

Three: 0740 for the speed change, 0755 for the course change, and 0800 for the hour. The 0800 DR is required because the hour has come round; the fact that you plotted fifteen minutes earlier for the alteration does not excuse it. No DR is due at 0700 itself — that is a fix, plotted as a dot in a circle, and it is the origin the DR runs from .

Telling it apart — fix, DR position, estimated position

The single criterion is how much information outside your own course and speed has gone into the position.

• Fix — nothing projected. It is an external observation of where the vessel is, and it resets the DR plot; the DR runs forward from it Bowditch Ch. 8 §801. Symbol: dot in a circle. Most often misfiled when a candidate keeps running the old DR track past a good fix instead of restarting at it.
• DR position — course steered and distance run through the water, and nothing else. Symbol: dot with a semicircle. Most often misfiled when set and drift have been worked into it, which makes it an EP that has been drawn with the wrong symbol.
• Estimated position — the DR position corrected for the estimated set and drift of the current Bowditch Ch. 8 §803. Symbol: dot in a square. Most often misfiled as a fix, because it is the best position available between fixes and candidates promote it accordingly. It is still an estimate, and it does not become the origin of the next leg.

Set and drift are two halves of one vector and the exam separates candidates on which is which. Set is the direction toward which the current flows. Drift is its speed.

Working a question

The problem below is the standard shape: a departure, a leg or two, an estimated current, and a demand for a position at a stated time.

You take a visual fix at 0800 off a charted point. Ordered course 090°T, speed 8 knots. At 0930 you alter to 060°T, speed unchanged. Current is estimated as set 180°T, drift 1.5 knots. Required: the 1000 position.

1. Identify the origin. The 0800 fix is the last fix, so the DR runs from it and from nothing earlier. Draw the course line from the fix, C 090 above the line and S 8 below Bowditch Ch. 8 §802.
2. Plot the 0900 DR. No course or speed change has occurred, so the hourly trigger is what puts this position on the chart. One hour at 8 knots is 8.0 nautical miles: measure 8.0 nm from the fix along 090°T, dot with a semicircle, labelled 0900.
3. Plot the 0930 DR before you alter. The course change is itself a trigger, and the position is plotted at the time of alteration — not projected ahead to some later point on the new course. Half an hour at 8 knots is 4.0 nm, putting the 0930 DR 12.0 nm from the 0800 fix along 090°T.
4. Start the new leg from that 0930 DR position, labelled C 060 above and S 8 below. Had the order also changed speed, the label below the line would change with it and every subsequent distance run would be computed on the new figure.
5. Plot the 1000 DR. From 0930 to 1000 is half an hour, so 4.0 nm along 060°T. This is your DR position for 1000, and it contains no current whatever Bowditch Ch. 8 §801.
6. Apply the current to get the EP. The current has been acting since the 0800 fix, so the elapsed time is 2 hours and the displacement is 2 × 1.5 = 3.0 nm in the direction of set. From the 1000 DR, lay off 3.0 nm toward 180°T. The end of that vector is the 1000 EP, a dot inside a square, labelled 1000 Bowditch Ch. 8 §803.
7. Decide what you report and what you keep. With only the current estimated, the EP is the best position you have. The DR track stays exactly where it was drawn — the EP is a correction to a position, not a new origin. Take a fix at 1030 and the plot resets to that fix.

If the question asks for the 1000 DR position, stop at step 5. If it asks for the 1000 estimated position, step 6 is the answer. Reading which of the two is wanted is worth more marks on this topic than any part of the arithmetic.

Same problem, but the current is estimated as set 180°T, drift 1.5 knots and you are asked for the 0900 EP rather than the 1000 EP. What changes?

Elapsed time from the 0800 fix is 1 hour, so the current vector is 1.5 nm, not 3.0 nm, laid off toward 180°T from the 0900 DR position. The DR position it is worked from is the one for the same time as the EP you are asked for — pairing a 0900 EP with a 1000 DR is a plotting error, not a rounding error .

Where candidates lose the point

Working the current into the DR track and then answering that the result is the DR position. This is the single most productive distractor on the topic, because the arithmetic is right and only the label is wrong. The DR ignores current and leeway; a position that has current in it is an EP and belongs in a square Bowditch Ch. 8 §801.

Reversing set. A candidate who reads set as the direction the current comes from lays the vector off on the reciprocal and lands twice the drift distance away from the correct answer — in the worked example, 6.0 nm out. Set is the direction toward which the current flows Bowditch Ch. 8 §803.

Answering that a DR position is due hourly and treating that as the complete rule, so the alteration at 0930 goes unplotted. Options are written to reward exactly this: one distractor will give the count of hourly positions only.

Labelling the course magnetic. The line carries the true course Bowditch Ch. 8 §802. A plot labelled with compass or magnetic headings and no annotation is unreadable to the next watchstander, and questions on labelling turn on the word "true".

Running the next leg from the EP because it is the better position. The DR is projected from the last fix; the EP is an estimate laid off to one side of the DR and has no standing as an origin.

Promoting an old DR to a fix because nothing has gone wrong for several hours. DR accuracy degrades with time run and must be confirmed by external fixes whenever they are available Bowditch Ch. 1 §102. The longer the DR has run unconfirmed, the weaker it is.

Check yourself

You have a 1200 fix, course 270°T, speed 10 knots, and an estimated current of set 000°T, drift 2 knots. At 1300 you are asked for your position for a radio report. Which do you pass, and how did you get it?

The 1300 EP. Plot the 1300 DR first: 10.0 nm from the 1200 fix along 270°T. Then lay off the current from that DR position — one hour at 2 knots, so 2.0 nm toward 000°T. The end of the vector is the 1300 EP, a dot in a square. With the current estimated and no fix available, the EP is the best position you have Bowditch Ch. 8 §803.

Your relief takes over and finds a dot inside a square labelled 1600 on the chart. What has the previous watch told him, and what may he not do with it?

That it is an estimated position: a 1600 DR position corrected for estimated set and drift. He may not treat it as a fix, and he may not run the next course line from it. The DR continues from the last fix Bowditch Ch. 8 §801, and the symbol for a fix is a dot inside a circle Bowditch Ch. 8 §802.

Between 0400 and 0500 you hold one course and one speed and take no fix. Is any DR position required?

Yes — one, at 0500, on the hourly requirement. A leg on which nothing happens still gets its hourly position .

You obtain a good visual fix at 1420 and it falls 1.8 nm south of your 1420 DR position. What do you do with the DR track?

Reset it. The new leg is laid off from the 1420 fix; the earlier DR track is not adjusted or re-drawn, and the discrepancy is not carried forward as an offset. The DR is projected from the last fix , and confirming the DR by external fix is the reason it is maintained Bowditch Ch. 1 §102.

You are inbound in a narrow buoyed channel with a fix available every few minutes. Does hourly DR plotting satisfy the requirement?

No. In pilot waters DR positions are plotted more often than hourly . Frequent alterations in a channel also fire the course-change trigger repeatedly, and each fix taken requires a DR position at the same time.

A question gives you a course line labelled `C 145` above and `S 6` below, with a position labelled 0930 on it. If you are asked for the 1030 DR position, what do you measure and along what line?

6.0 nm along 145°T from the 0930 position. The figure above the line is the true course and the figure below is the speed in knots , and one hour at 6 knots is 6.0 nm run through the water with no current applied.

At 0600 you take departure and at 0700 the master orders a reduction from 12 knots to 6 knots, holding course. A candidate answers that no DR position is required because the course has not changed. Where is the error?

A speed change is a trigger in its own right . The 0700 DR position is required — and here it is required twice over, since the hour has also come round. Distance run on the next leg is computed on 6 knots, and the label below the line is changed to S 6 .