Bearings and the compass

Directions in degrees

A bearing is a direction, measured in degrees clockwise from north, from 000° all the way round to 359°. East is 090°, south is 180° and west is 270°. On a charter you use bearings for three things: to say where something is (“the harbour entrance bears 045°”), to check where you are, and to stay clear of danger on the way into a harbour (Lesson 8).

• Always three figures. North-east is 045°, never 45°. Every bearing then has the same shape, so a slip stands out, and nobody mistakes it for a speed or a depth.
• Say each figure. “Zero four five”, not “forty-five”, whether you are calling it to the helm or on the radio.
• The reciprocal is the opposite direction: add 180° if the bearing is under 180°, otherwise take 180° off. If a lighthouse bears 045° from you, you bear 225° from it.

True north and magnetic north

The chart is drawn to true north, the direction of the geographic North Pole: straight up the chart, along the meridians. A compass points to magnetic north, which is somewhere else and moves slowly over the years. The angle between the two at your position is the variation. It is printed in the middle of the compass rose on the chart, with the year it applies to and how much it changes each year; the pilot book and the plotter usually give it too.

To convert, remember which figure is bigger. With westerly variation the magnetic figure is the bigger one: variation west, magnetic best. With easterly variation it is the smaller one: variation east, magnetic least.

The steering compass

The steering compass stands in front of the wheel. Its card is a magnet with a scale on it, and it keeps pointing to magnetic north while the boat turns around it. The lubber line, a mark on the front of the bowl, lines up with the bow, so the figure against it is your heading, in degrees magnetic.

Steel, the engine, speakers and electronics close to the compass can pull it a degree or two off; that error is called deviation. A charter yacht's compass is usually well placed, and for charter navigation you can ignore it. Never leave a phone, a winch handle or a handheld radio beside the compass, though: those can pull it a long way.

The hand-bearing compass

A hand-bearing compass is a small compass with a sight, held up to the eye. You look at a landmark through it and read its bearing straight off the card. Most charter yachts carry one, often forgotten in the chart table drawer. To take a bearing:

1. Choose a charted landmark you are sure of: a lighthouse, a church tower, a headland, a radio mast. Avoid buoys for anything important; they swing round their moorings.
2. Stand clear of the steering compass, the engine, the speakers and your own phone, and brace yourself against the boat.
3. Hold it level at eye height, put the landmark on the lubber line, and wait for the card to stop swinging.
4. Read it and write it down at once, with the time. Then convert it to true before it goes on the chart.

One bearing: a position line

A single bearing gives a position line: you are somewhere along it, but it cannot say where. That is still useful: if the line passes clear of every danger, you know you are safe along it, and Lesson 8 turns this into a clearing bearing. Try it below: you are hidden somewhere on the practice chart.

Two or three bearings: a fix

Take bearings of two landmarks well round from each other and draw both lines: where they cross is your fix. Two lines always cross somewhere, even if one is wrong, so a third bearing is the check. Three good lines meet in a small triangle called a cocked hat; the fix is its middle, marked on the chart with a circle and the time.

• Spread them out. Two lines should cross at 60° to 90°; three should be roughly 60° apart (or 120°). Lines that cross at a shallow angle let a small error slide the fix a long way.
• Near before far. A one-degree error moves the line about a sixtieth of the distance: 30 metres at one mile, nearly 200 metres at six.
• Beam last. Take the object nearest the bow or stern first; the one on the beam changes bearing fastest, so take it last.
• A big hat means a mistake. Usually a misidentified landmark or a misread figure. Retake the bearings; until then, assume you are at the corner nearest danger.

Transits: two objects in line

When two charted objects appear exactly one behind the other, they are in transit, and you are on the straight line through them. A transit needs no compass, so it has no variation, deviation or reading error: it is the most accurate position line you can get. Any two fixed, charted things will do: a lighthouse and a hilltop, a church and a mast, the edge of one headland against another.

Harbours build transits on purpose: two leading marks or leading lights, the rear one higher, that line up along the safe way in. Lesson 8 uses the Port Kerran leading lights to bring you into the harbour. A transit is also the quickest check of the steering compass: steer along one and compare the compass with the charted bearing, converted to magnetic.

When the plotter and your eyes disagree

Now and then the plotter shows the boat where your eyes say it cannot be: a headland on the wrong side, a buoy where the screen has none, the boat drawn on land. The screen is usually right about the GPS position, but the chart under it may be old or badly surveyed, the display may have frozen after losing its signal, or you may be zoomed in far beyond the chart's real detail. A quick bearing tells you which to trust.

1. Pick a landmark you are sure of, close and well charted.
2. Ask the plotter what its bearing should be: put the cursor on it, or read it off the chart from the plotter's position, and convert to magnetic if the plotter shows true.
3. Take the real bearing with the hand-bearing compass.
4. Compare. Within a few degrees: carry on. More than that: slow down, watch the depth, keep clear of anything close and navigate by what you can see until you know why.