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Bearing conversion – from compass bearing to position line

A bearing can only be plotted on the chart once it is true. Whatever you read on the steering compass, the radar or a hand bearing compass has to be corrected for deviation and variation first. NauticTools2 works through the chain in both directions – for steering compass, radar, hand bearing compass and gyro.

• RB / RRaB
• CB
• MB
• TB
• Hand bearing compass
• Gyro compass

The bearing chain

• RB / RRaB – relative bearing: the angle from your own bow to the object, clockwise. On the radar it is the relative radar bearing.
• CC – compass course (heading). It is needed to turn a relative bearing into a bearing and to look up the deviation.
• CB – compass bearing on the steering compass.
• DEV – deviation for the ship's heading.
• MB – magnetic bearing.
• Var – variation.
• TB – true bearing: the line you draw on the chart.

When do you need it?

• Position lines: Cross bearings, a bearing and distance or a running fix – every bearing must be true before it goes on the chart.
• Radar: The radar usually shows relative bearings. Only together with the heading does it become a bearing.
• Planning bearings ahead: From the chart you can see on what true bearing a lighthouse lies at the alter-course position. Working backwards tells you what you should see on the compass or the radar.

Formulas and working

1. Relative to compass bearing

   CB = CC + RB – subtract 360° if the result exceeds 360°.

2. Deviation

   MB = CB + DEV – deviation for the heading, not for the bearing.

3. Variation

   TB = MB + Var

The classic trap: Deviation depends on the ship's heading, not on the direction of the bearing. Taking a bearing of 090° while heading 190°, you need the deviation for 190°. Working back from TB to CB and RB uses the same chain with the signs reversed.

Worked examples

Hand bearing compass and gyro compass

Hand bearing compass: It is used on deck well away from iron and electrics and usually has no deviation. The chain shrinks to TB = CB + Var – no heading and no deviation needed.

Gyro compass: Deviation and variation do not apply. Instead TB = GB + GA + SEC with the gyro error GA and the speed error correction SEC. Conversely, a check bearing gives the gyro error:

Things to watch out for

• Deviation for the heading: see above – the single most important point.
• Bearing and heading at the same moment: For a relative bearing the heading at the moment of the bearing counts. If the boat yaws, the bearing moves with it.
• Signs: East positive, west negative – for deviation, variation and gyro error.
• Using a hand bearing compass: Take bearings away from shrouds, engine and loudspeakers, or the hand compass will have deviation too.

How it works in NauticTools2

1. Choose the compass

   Switch between steering compass/radar, hand bearing compass and gyro at the top.

2. Choose the known bearing

   On the left, choose whether RB, CB or TB is known. The heading can be entered as CC or TC – with TC, NauticTools2 takes the deviation from the steering table.

3. Deviation and variation

   Both are filled in automatically from the active deviation card and the settings. A value you type yourself takes priority.

4. Calculate

   The whole chain appears with full working and can be saved and printed.

Frequently asked questions

What is the difference between a relative bearing and a bearing?

A relative bearing is measured from your own bow and changes when the boat turns. A bearing is measured from north and stays the same wherever the bow points.

Why does a hand bearing compass have no deviation?

Because it is used where iron and electrics are far enough away. At the fixed position of the steering compass, the engine and the boat's electrics always have an effect.

What does RRaB mean?

Relative radar bearing: the relative bearing as read off the radar. Mathematically it is the same as a relative bearing.

Related tools

• Course Conversion – the same chain for courses
• Magnetic Declination – current variation for any position and date
• Distance by Vertical Angle – distance to the object you are bearing
• Compass Check by the Sun – find the deviation with the sun