A fix is two or more position lines that cross: bearings, transits, ranges, depths. Each has its errors, the compass two of its own. Check one source against another.
Do I apply deviation to a bearing taken with a hand-bearing compass?
No, not if you use it in the open, clear of steel, engine and speakers: a hand-bearing compass then needs variation only. A bearing over the steering compass also takes the deviation for the ship's head.
Bearings are plotted in true. A hand-bearing compass used in the open, clear of steel, engine and speakers, needs variation only: add east, subtract west. Take it from the nearest rose, updated to this year.
A bearing sighted over the steering compass also takes deviation: that of the ship's head, not of the bearing, interpolated in the card. The card serves that compass only; an error found for the hand compass on a transit holds at that spot and heading.
How do I check my compass deviation using a transit?
Steer along a charted transit, convert its true bearing to magnetic and compare with the compass: reading high means deviation west, low means east. Offshore, use the tabled bearing of the rising or setting sun.
Deviation is the boat's own magnetism: engine, keel bolts, wiring. It changes with the heading and with heel. Anything magnetic left near the compass adds more: a phone, a steel tool, a kedge, a live cable. Aluminium does not.
Measure it by steering along a charted transit. With the marks astern you are on its reciprocal; offshore, use the tabled bearing of the rising or setting sun.
Compass reads high: deviation west. Low: east.
The same error on every heading means a compass mounted out of line. Swinging repeats the check all round to make the card; redo it after fitting gear nearby. A fluxgate corrects itself after slow circles but needs power: the magnetic compass is the backup.
In what order should I take the bearings for a three-point fix?
The marks ahead and astern first, as their bearings change slowly, and the mark abeam last, noting time and log with it. Choose marks about 60° apart; the three lines leave a cocked hat.
Choose marks that are fixed, sharp and charted exactly: a lighthouse, not a buoy or a sloping headland. Two lines cut best near 90°, three about 60° apart; marks ahead and astern give almost one line. A transit needs no compass: cross it with a bearing at right angles.
Take the marks ahead and astern first, their bearings change slowly, and the mark abeam last; note time and log with that bearing. Three lines leave a cocked hat.
A large cocked hat means a mistake: check each mark's identity and start again. A small one proves only that the bearings agree: if depth and EP both disagree, doubt it. A good fix beats the DR: restart from it, the gap being the stream left out. Fix as often as the danger demands: half a mile from rocks at 6 kn is five minutes.
How do I get a fix from a single lighthouse with a running fix?
Take two bearings of it, noting time and log, and transfer the first line along the ground track run between them, leeway and stream included. Where the transferred line cuts the second bearing is the fix.
With one mark, take a bearing and note time and log; later take another. From any point on the first line lay off the ground track run between them, leeway and stream included, and redraw the first line through its end. Where this transferred line cuts the second bearing is the fix.
Whenever the angle on the bow doubles, distance off at the second bearing equals the run, over the ground if there is a stream. After an alteration, add the legs and the stream end to end. One mark's line can be crossed with another's bearing. Every error of course, log and stream goes into the fix: keep the run short and prefer two marks at once.
How do I work out my distance off when a light rises over the horizon?
It is 2.08 × (√H + √h) miles, H being the light's height and h your eye's, in metres: the range at which a strong light rises. Cross that range with the light's bearing for a fix.
On a clear night a strong light rises over the horizon at 2.08 × (√H + √h) miles, H being its height and h your eye's, in metres. Cross that range with its bearing for a fix.
Name a light before using it: count the flashes and time the full period, from one group's start to the next. Occulting shows more light than dark. If two lights share a character, your EP and their ranges decide.
How do I fix my position with radar?
By ranges, not bearings: radar measures range well and bearing poorly. Cross two ranges, or a range with a visual bearing, with the VRM on the near edge of a steep target; where two arcs cross twice, the depth decides.
Radar measures range well and bearing poorly: fix by two ranges, or a range crossed with a visual bearing. Put the VRM on the near edge of a steep target; a low beach may not paint, leaving the hills behind as first echo. A low target shows only inside the radar horizon, 2.2 × (√h + √H). A head-up EBL reads from the bow: add the heading. Excess sea-clutter control hides small close echoes; rain control breaks up a shower.
The sounder gives a position line too, once the sounding is reduced to chart datum.
Two range arcs cross twice: the depth says which. In fog a line of soundings can be matched to the chart; a fog signal gives no bearing. When radar and sounder agree against the plotter, believe them and stand off.
How do I plot a GPS position quickly on a paper chart?
As a range and bearing to a waypoint charted on a known object. The bearing runs from you to the waypoint, so you lie on its reciprocal; check whether the set shows true or magnetic.
Plot a GPS position quickly as a range and bearing to a waypoint charted on a known object. The bearing runs from you to the waypoint, so you lie on its reciprocal; check whether the set shows true or magnetic.
| Symptom | Meaning |
|---|---|
| High HDOP | Poor satellite geometry |
| A wandering fix | No clear sky for the antenna |
| Boat plotted on charted rocks | Datum: apply the chart's shift |
| Lat/long and waypoint disagree | A mistyped waypoint |
| No fix · interference | DR from the last logged fix |
Questions from this topic, as the app asks them. Open one to see the answer.
You take this bearing of a headland light with the hand-bearing compass, well clear of any steel. Which bearing do you plot?
C. 309°T
Magnetic to true: subtract westerly variation — 312°M − 3°W = 309°T. Clear of steel there is no deviation.
You come on deck at night and find this. What do you do first?
C. Move the phone well away from the compass
A phone (its speaker magnet) next to the compass deflects it; remove it before you trust any heading.
You are motoring east along this coast and want a three-point fix with the hand-bearing compass. Which mark do you take last?
A. B — abeam, its bearing changes fastest
Take marks ahead or astern first — their bearings change slowly — and the one abeam last; then note the time and log.
You work up this running fix. How do you transfer the 1015 position line?
C. 5.2 M along 090°T
Move the first line by the run between bearings: log 19.4 − 14.2 = 5.2 M along the course, 090°T.
Two lighthouses on this coast could be this light. How do you tell which one it is?
A. Time its period with a stopwatch and count flashes
Identify a light by its characteristic: count the flashes, time the full period, match the chart or List of Lights.
You want a fix from these two radar targets. Which position lines do you plot?
A. The two range arcs
Radar ranges are accurate; radar bearings suffer from beam width and yaw — cross two ranges.
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.