Offshore every heading and bearing passes through a compass you must trust; a fix is as good as its worst line.
1 · The steering compass
Lubber line, liquid, variation
How do I bring the variation on an old chart up to this year?
Multiply the rose's annual change by the years since its date and apply it: 4°10'W in 2016, changing 8'E a year, is 2°50'W in 2026.
A compass reads the heading if its lubber line is parallel to the fore-and-aft line. A bubble makes the card stick and jump: cross-check against another compass, refill ashore. After a hard turn the liquid drags the card; log the heading once it settles. Keep gear outside its compass safe distance; a loudspeaker needs the most.
Variation comes from the nearest rose or, on a small-scale sheet, the isogonic line, updated by its annual change; a long leg crosses several. Heeling error, iron below the compass tilting with the boat, is largest heading north or south: check the deviation heeled.
Variation brought up to date
Rose: 4°10'W 2016 (8'E)
2026: 10 years × 8' = 1°20' less westerly
Variation now 2°50'W
Old chart: 1°30'W 2010 (9'E)
16 years × 9' = 2°24': 0°54'E
It has passed through zero
2 · Deviation
The swing and the card
How do I check my compass deviation at sea?
On a steady heading, compare the compass with a known magnetic direction: a transit, the sun's or Polaris's tabled azimuth, or a satellite compass, never COG. A compass reading low has easterly deviation.
Deviation is the boat's own magnetism; it changes with the heading, so the card lists it heading by heading, and each compass has its own. Find it on a steady heading against a known magnetic direction: a transit, the sun's or Polaris's tabled azimuth, a satellite compass; never COG, which a current sets off the heading. A compass reading low has easterly deviation.
Deviation by azimuth: worked example
Heading steady 165°C; sun over the pin 119°C
Tabled azimuth 118°T + variation 4°W = 122°M
Compass reads 3° low: deviation 3°E on 165°C
Swing on a far peak: mean of 8 bearings 222°
That mean is magnetic; on 090°C it read 218°
Deviation 4°E there; on 270°C, 226°: 4°W
Large deviation calls for an adjuster and a new swing. A steel hull changes her magnetism after a winter bow-north, welding or hours on one heading; new wiring or instruments by the compass change it too: swing again. On steel take bearings over the steering compass. A fresh check that disagrees with the card is logged and used.
3 · Electronic heading
Fluxgate, satellite compass, COG
Can I use GPS COG instead of a heading sensor for radar overlay?
No: COG is your track over the ground, not a heading, so it is no good for radar overlay or MARPA. A satellite compass gives true heading, even stopped.
Source
Gives
Weakness
Magnetic compass
Magnetic heading, no power
Deviation
Fluxgate
Magnetic heading
Iron, cables, tilt in a turn
Satellite compass
True heading, even stopped
Jamming
GNSS COG
Track over the ground
Not a heading: no good for overlay or MARPA
Calibration circles take out a fluxgate's deviation, not an alignment error: set the offset after a transit check. Mount it low, central, clear of iron. A satellite compass needs no card and feeds autopilot, radar and AIS. The magnetic compass keeps its card and, under jamming, is the only heading left.
4 · Position lines
What each line tells you
In what order should I take bearings for a three-point fix?
Slow-changing bearings ahead and astern first, the beam one last, with the lines crossing near right angles. A cocked hat the size the errors give is honest: treat it as an area.
A position line fixes you across itself, not along it: a bearing of the light ahead checks the cross-track error, a contour across the track checks the distance run, a sounding helps where contours cross your line. Take bearings near right angles, slow-changing ones ahead and astern first, the beam one last. 3° at 20 M is 1 M.
Line
Trust
Radar range (VRM)
±0.05 M off steep charted targets
Radar bearing
±1–2°; head-up adds the heading; land paints half a beamwidth wide
Transit · racon
Weak beyond 3× the spacing · range to the flash's inner end
Compass bearing
Same deviation in each: the same cocked hat every time
A cocked hat the size the errors give is honest: treat it as an area. Horizontal sextant angles need no compass. Fix on charted lighthouses, not aero lights or flare stacks; AIS targets come from your own GNSS. A soft echo far down is plankton; soundings at equal intervals, on tracing paper slid over the chart, fix you offshore.
5 · Running fixes and bow angles
Two bearings of one mark
How do I get a fix from two bearings of the same light?
By a running fix: take a bearing, run on, take a second, and move the first line along the ground run, water legs and stream added as vectors. Where the lines cut is the fix.
With one mark: a bearing, a run, a second bearing, and the first line moved by the ground run: water legs and hours of stream added as vectors. A range circle moves by its centre, a contour likewise. The fix is as good as the run: 0.4 kn of doubt over 2.5 h is 1 M.
Running fix with stream
0100: light bears 050°T
Water track 080°T, 8.0 kn, 2 h: 16.0 M
Stream 170°T 1.0 kn, 2 h: 2.0 M
Ground run: 16.1 M along 087°T
Move the 0100 line 16.1 M along 087°T
0300: light bears 330°T; the cut is the fix
Bow angles
Distance off
22½° → 45°
At 45°: the run; abeam 0.7 × run
26½° → 45°
Abeam: the run
45° → abeam
The run
They need the ground run on a steady course: a stream along the track goes into the run; a cross-set breaks them: plot.
6 · Rising lights and landfall
Rising range and distance off
At what distance will a lighthouse rise above the horizon?
At its rising range, 2.08 × (√H + √h) miles with heights in metres, within its nominal range. The charted height is above MHWS, so at low water add the tide's fall.
A light rises at 2.08 × (√H + √h) miles, within its nominal range; its charted height is above MHWS, so add the tide's fall at low water. Seen only from the crests, it is on your horizon, at its rising range. Abnormal refraction shows it far beyond. Card each expected light: characteristic, rising range, bearing. One 15–20° off may still be yours if the EP is miles out: time it. Nothing after rising time: an obscured sector. A colour change puts you on the sector's edge.
Rising range and distance off
Light 40 m above MHWS; MHWS 5.0 m; tide 2.0 m
H = 40 + 3.0 = 43 m; height of eye 4 m
Rising range 2.08 × (√43 + √4) = 17.8 M
24 M to go at 11 kn: it rises in 34 min
Vertical sextant angle 0°20.0' of the 43 m: 1.854 × 43 ÷ 20.0 = 4.0 M off
A vertical sextant angle, light to the shoreline below it, gives 1.854 × H ÷ angle in minutes, index error applied, tide added to H.
7 · Trust and record
Believing a fix, recording it
How do I check that my GPS position is right?
Against something independent: depth, radar, lights or, offshore, the EP from the last fix. Plotter, AIS and DSC share one receiver, so their agreement proves nothing.
Plotter, AIS and DSC share one receiver: their agreement proves nothing; nor does HDOP without RAIM. Cross-check against depth, radar, lights or, offshore, the EP from the last fix. Jumps, a frozen speed or a wrong date mean spoofing: back to DR from the last non-GNSS fix. A visual fix 0.5 M from the GNSS at 20 M agrees; hand bearings beside steel share one error.
Fix hourly in open water, and before the EP's doubt, about 10% of the run, reaches half the gap to a bank. Keep a single line: plot it with its time, log the bearing and the log.
On the chart
In the log
Circle with a dot
Fix: time, position, method, log
Triangle
EP: log, compass, stream, leeway
Line with its time
One line: bearing and log
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Flybridge steering compass
22 m motor yacht
Compass 1.2 m to starboard of the centreline
Lubber line: to be checked before sailing
You are taking over a 22 m motor yacht whose flybridge compass sits 1.2 m to starboard of the centreline. What must be true for it to read her heading?
Its lubber line is parallel to the fore-and-aft line
It is moved onto the centreline before you sail
A small offset is added to every heading read
Its lubber line points straight at the stem head
Show the answer
A. Its lubber line is parallel to the fore-and-aft line
A compass reads the heading anywhere aboard if its lubber line is parallel to the keel; aiming it at the bow from off-centre builds in an error.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
