In this lesson you will learn
- Explain what a position line is, and why one line on its own is not a fix
- Take an accurate bearing with a hand-bearing compass, noting the time and log
- Choose good objects, and use the 1 in 60 rule to see why near ones are better
- Convert a hand-bearing compass bearing from magnetic to true
- Plot and label a position line, and recognise other kinds: transits, depth contours and radar ranges
What is a position line?
In Lesson 6 you learnt that a bearing is the direction of an object from you, and that every direction has a reciprocal, 180° away. Put the two together and you have the most useful idea in coastal navigation.
Suppose a church spire that is marked on the chart bears 045°T from your boat. That alone doesn’t say exactly where you are. But every place from which the spire bears 045°T lies on one straight line: the line through the spire running 225°T, back out to sea. You must be somewhere on it.
A position line is a line on the chart on which you know you were, at the time you made the observation.
One position line narrows “somewhere off the island” down to “somewhere along this line”. A second line crossing it shows where along — and that crossing is a fix. This lesson is about getting one good line; Lesson 12 crosses three.
The hand-bearing compass
A hand-bearing compass is a small magnetic compass with a sight, which you hold up to your eye. Most have a prism or lens so that you see the object and the compass card at the same moment, with a fixed mark, the lubber line, to read the bearing against. Put the object on the lubber line and the figure on the card beside it is the bearing.
Like every magnetic compass it points to magnetic north, so it gives a magnetic bearing (°M), as in Lesson 9. Unlike the steering compass it isn’t fixed to the boat, so you can use it away from the boat’s magnetic influences: the engine, loudspeakers, electronics and steelwork. Used like that — and with your phone well out of the way — a hand-bearing compass is taken to have no deviation.
Taking a bearing
- Identify the object — on the chart and in front of you. Make sure they are the same thing.
- Brace yourself. Wedge yourself into the cockpit or against the shrouds so both hands are free and you are steady. Hold the compass level, at eye height.
- Sight the object and put it on the lubber line.
- Wait for the card to settle. It swings as the boat rolls and pitches. Read it when it steadies; if it never quite stops, read the middle of the swing. If in doubt, take it again.
- Note it at once: the bearing, the time and the log reading. The boat keeps moving, so the line is only true for that moment.
Choosing good objects
A position line is only as good as the object it came from. A good object is:
- Charted — on the chart, so you know exactly where it is.
- Conspicuous — easy to see from seaward. Admiralty charts often mark especially prominent landmarks “conspic” (Lesson 8).
- Unmistakable — it can’t be confused with anything else. Two spires on the shore and only one on the chart is a trap.
- Fixed — a lighthouse or spire stays put; a buoy swings round its mooring and can drag out of position.
- Near — the closer, the better, for the reason below.
| Object | Good points | Watch out for |
|---|---|---|
| Lighthouse | Charted, conspicuous, and identifiable at night by its light | Often seen from far off — a long range makes any error bigger |
| Spire, mast or tower | Tall, narrow and easy to sight exactly | Check it is the charted one, not a similar building |
| Edge of a headland | Easy to see | Only use steep-sided ones: on a gently sloping point it is hard to say which part you are seeing |
| Hilltop | High, often visible from a long way off | A rounded summit is hard to pin down exactly |
| Buoy | Charted, and often close | It moves on its mooring and can drag; use one only if nothing better is available |
And it must be in sight. Land hides anything behind it that isn’t tall enough to show over the top. The practice chart gives no heights for the land, so this course keeps it simple: the radio mast and Beacon Hill (87 m) stand above the rest of the island and can be seen over it, but lower objects — the church spire, the old tower and the lighthouses — show only over land close in front of them, within about 2 M.
Why near objects are better
No bearing taken on a small boat is perfect. A swinging card, a rolling deck and a compass not quite level all add up: an error of two or three degrees is normal, and more in rough weather. The same angular error moves the line further the further away the object is, because the line pivots about the object.
offset ≈ distance × error° ÷ 60
The 1 in 60 rule: 1° of error moves the line about 1 M for every 60 M of distance. A rough guide, good for small angles.
Worked example
How good is a bearing that may be 2° out?
- 1Object 1.5 M away: 1.5 × 2 ÷ 60 = 0.05 M — about 0.5 cables either side of the line.
- 2Object 6 M away: 6 × 2 ÷ 60 = 0.2 M — 2 cables either side.
Four times the distance, four times the error: choose the nearer object when you can.
InteractiveHow bearing errors spread
Move the boat nearer to or further from Gull Point lighthouse, and change the size of the error. The shaded wedge is everywhere the boat could really be.
Gull Point lighthouse bears 330°T from the boat. The solid line is the line you would draw; the shaded wedge is everywhere the boat could really be if the bearing is up to 2° out either way.
Where the line could really be
Up to 0.13 M (1.3 cables) either side of you
1 in 60: 4.0 × 2 ÷ 60 = 0.13 M
With ±2°, the line is good to about 0.5 cables at 1.5 M but only to 2 cables at 6 M — four times worse, because the object is four times further away.
From magnetic to true
The chart is drawn to true north, so every bearing must be converted to true before you plot it. With a hand-bearing compass there is only one correction to make: variation. The practice chart’s compass rose shows VAR 3°00'W (2025) (8'E); by 2027 that is 2°44'W, which this course rounds to 3°W for chart work (Lesson 9).
Plotting the position line
You can plot the line with the plotter in either of two ways, and they give exactly the same line:
- Using the bearing. Set the true bearing on the dial, lay the plotter’s edge through the object with the arrows pointing towards it, and turn the whole plotter until the grid is square to the meridians (Lesson 6). Draw along the edge, on the seaward side of the object.
- Using the reciprocal. Work out the reciprocal of the true bearing and draw the line from the object in that direction.
Either way, the line runs from the object out to sea and through where you are. You needn’t draw all of it: the stretch across the area where you might be is enough, and keeps the chart clear. Then label it with the time, on the 24-hour clock. It helps to note the object and the bearing beside it too.
Worked example
1015: the church spire from south-east of Port Kerran
- 1At 1015, log 12.4, the spire bears 336°M by hand-bearing compass.
- 2Convert to true: variation 3°W, magnetic best, so 336°M − 3° = 333°T.
- 3Reciprocal: 333° is 180 or more, so subtract 180: 153°T.
- 4Draw the line from the spire on 153°T, out across the sea to the south-south-east, and label it 1015.
You were somewhere on the 1015 line running 153°T from the spire.
InteractiveTake a bearing
You are at a hidden position. Choose a landmark you can see, take its bearing, convert it to true and draw the position line. Then reveal the boat — or take a second bearing first.
Position 1 of 6: you are somewhere north-west of Kerran Head.
Landmarks
1. Choose a landmark you can see. Greyed-out ones are hidden behind the island or too far away to use.
Use + / − to zoom, and drag to move around the chart.
One line is not a fix
A single position line tells you which line you are on, not where on it: you could be close inshore or miles out. To find out where, cross it with a second position line. A bearing is only one kind:
| Position line | What you need | Where it comes in |
|---|---|---|
| Bearing of a charted object | A hand-bearing compass and a good object | This lesson; three of them make a fix in Lesson 12 |
| Transit | Two charted objects seen exactly in line — very accurate, and no compass needed | Lesson 13 |
| Depth contour | The depth, corrected to a charted depth, matching a contour on the chart | Below; tidal heights in Module 4 |
| Range from radar | The distance of an object, which gives a circle round it rather than a straight line | Module 5 |
A depth contour works because, as Lesson 8 showed, the depth you measure is the charted depth plus the height of tide. Take away the height of tide and you have the charted depth; if it matches a contour, you are somewhere along that contour. Where your bearing line crosses it, you have a position.
Worked example
A bearing and a depth contour
- 1Gull Point lighthouse bears 313°M. Convert: 313°M − 3° = 310°T.
- 2Reciprocal: 310° is 180 or more, so subtract 180: 130°T. Draw the line from the lighthouse on 130°T.
- 3The depth sounder, set to show depth from the surface, reads 22.6 m and the height of tide is 2.6 m: charted depth 22.6 − 2.6 = 20 m.
- 4The line crosses the 20 m contour 1.5 M from the light.
Position 49°46.0'N 004°04.6'W.
Common mistakes
- Plotting the magnetic bearing. Forget to convert and the line is 3° out.
- Converting the wrong way. Adding westerly variation instead of subtracting puts the line 6° out.
- Drawing the bearing instead of the reciprocal. From the object, the bearing itself points away from you — inland, or off to the far side of the island. The line must come back towards you: 333°T to the spire means drawing 153°T from it.
- The wrong object. The most dangerous mistake of all, because the line looks perfectly convincing. Identify before you sight.
- No time on the line. An hour later you won’t remember when you took it, and it is useless.
- Standing by the engine, or sighting with a phone in your other hand: the compass picks up deviation you can’t correct.
Identify, brace, sight, note the time, convert to true, draw the reciprocal from the object, label it.
Key terms
- Position line
- A line on the chart on which you know you were at the time of an observation.
- Hand-bearing compass
- A small hand-held magnetic compass with a sight, for taking bearings of objects.
- Lubber line
- The fixed mark on a compass against which the reading is taken.
- Conspicuous (“conspic”)
- Easily seen from seaward; how charts mark the best landmarks.
- Magnetic bearing (°M)
- A bearing measured from magnetic north, as a compass gives it.
- Variation
- The angle between true and magnetic north: 2°44'W on the practice chart in 2027, used as 3°W.
- Reciprocal
- The opposite direction, 180° away: the direction to draw a position line from the object.
- 1 in 60 rule
- 1° of error moves a line about 1 M for every 60 M of distance.
- Transit
- Two charted objects seen in line: a very accurate position line (Lesson 13).
- Fix
- Your position, found where two or more position lines cross (Lesson 12).
Exercises
Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.
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Position-line practice
Five fresh questions each time: convert a bearing, draw it from the object, plot a line against a depth contour, spot the point on the line, and one more on technique.