To plot is to mark a position, a course line or a bearing on a chart, and the plot is the resulting record of where the boat has been and where she is going. Every fix, whether from a GPS readout, three compass bearings or a transit and a depth, becomes real when it is plotted, because only then can you see it in relation to the dangers, the tide and the destination. Beginners learn to plot as an exercise in parallel rules and dividers; experienced navigators keep plotting long after the chart plotter has made it unnecessary, because a pencil position on a paper chart is the one piece of the navigation system that never needs power.
What gets plotted
A working plot carries several different kinds of mark, and the convention is to label each so that anyone coming on watch can read it:
- Fix: a position you are confident of, from GPS, visual bearings, radar or a transit. Marked with a small circle around a dot and the time, usually with the log reading beside it.
- Dead reckoning (DR) position: where course steered and distance run since the last fix put you, ignoring tide and leeway. Marked with a cross or a half-circle and the time.
- Estimated position (EP): the DR corrected for tidal stream and leeway. Marked with a triangle. The EP is your best guess when no fix is available.
- Course line: the line you intend to follow, labelled with the true or magnetic course in three figures.
Symbols vary a little between British and American practice, so agree one convention on board and stick to it.
The tools and the technique
Plotting needs a soft pencil (2B rubs out cleanly on a damp chart), a good eraser, dividers, and either parallel rules or a plotter such as a Breton or Portland type with a rotating compass rose. Transfer the bearing from the compass rose on the chart with parallel rules, or set it directly on the plotter, correcting for variation and deviation as you go. Distance comes from the latitude scale at the side of the chart – never the longitude scale – with one minute of latitude equal to one nautical mile.
The rhythm of a good plot is fix, label, project. Plot the fix, write the time and log reading beside it, then immediately project the DR forward along the course for the next hour so the next fix has something to be compared with. If the two disagree, that disagreement is information: the tide is stronger than predicted, the compass is off, the log is under-reading. A plot that is only ever a string of GPS positions teaches you nothing; a plot that keeps testing the DR against fixes teaches you the boat.
Electronic plotting
A chart plotter plots for you, continuously and to a few metres, and no sensible navigator ignores it. But it also hides its working. The cruising habit that has saved many boats is to transfer the GPS position to the paper chart at regular intervals – every hour offshore, every 15 minutes in coastal waters – and to write the time and log in the deck log alongside. If the plotter fails, or a lightning strike takes the electronics, the last pencilled fix and the log reading are all you need to pick up the DR and carry on. If the plotter is right but the chart cartography is wrong, the paper chart with its source diagram may be the thing that tells you so.
Racing and cruising perspectives
A cruising navigator plots to know where the boat is and to have a record if things go wrong; the log and the chart together are the legal and practical account of the passage. A racing navigator plots to make decisions: where the tidal stream changes, where the laylines fall, how the fleet is placed. On a modern race boat that plotting happens on a tablet with routing software, but the racing navigator who can also lay a quick fix on paper is the one still working when the tablet has gone flat halfway through a night at sea.
Frequently Asked Questions about Plot
How often should I plot a position?
Offshore, at least once an hour and at every change of course. In coastal waters every 15 to 30 minutes, and continuously by transits and marks in a harbour approach.
What is the difference between a DR and an EP?
A dead reckoning position uses only course steered and distance run. An estimated position adds the effect of tidal stream and leeway, so it is the better guess; it is usually plotted as a triangle.
Do I need to plot on paper if I have a chart plotter?
Yes, at sensible intervals. A paper plot is your backup when the electronics fail, your cross-check when the chart data is wrong, and the record of the passage that a log alone cannot provide.
Plotting is the moment when navigation stops being numbers and becomes a picture. A pencil mark on a chart, with a time beside it, ties everything else together: the tide, the dangers, the course and the decisions. Keep plotting by hand even when the screen is working, and you will always know where you are and, more importantly, how you know it.