Module 6 - Position, Course & Speed
Measuring Bearings on the Chart
What you will learn
Transfer a charted direction without instrument slip, select the intended true bearing rather than its reciprocal and preserve the observational uncertainty in the resulting line.
Bearing work is a controlled transfer and error check. The plotted line must agree with the visible direction, its north reference and the object-to-vessel geometry.
Use a parallel ruler, rolling ruler or navigational plotter to carry the direction of a line to a labelled meridian or compass rose. Keep the instrument aligned while it moves. A small angular slip creates a growing lateral error with distance.
Read the true graduation indicated by the chart or instrument and state three figures plus T when context could be ambiguous. Do not assume the outer ring is always true; inspect its labels, arrows and the chart note.
A line has two directions separated by 180°. First identify the intended direction of travel or observation, then use the visible quadrant as a gross-error check. For a corrected true object bearing taken from the vessel, plot the reciprocal from the known object.
One bearing creates one position line, not an exact fix. Time, object identification, observer motion, compass correction and instrument transfer all contribute uncertainty. Use sufficient independent geometry before the line controls a close-hazard decision.
Worked example
A corrected true bearing of 318°T is observed from the vessel to a charted tower.
- 1Because 318° is at least 180°, subtract 180°.
- 2318° − 180° = 138°T.
- 3Plot 138°T from the tower towards the vessel's expected south-eastern sector.
- 4Label the observation time; seek another independent line or range before calling it a fix.
Sense check: 138°T and 318°T differ by exactly 180° and describe the same straight line in opposite directions.
Reject a plausible reciprocal
A route from waypoint A to B visibly runs north-east, but the plotter scale offers 052° and 232° on the same line.
1. Name the direction
Identify A-to-B before reading the number.
2. Use the quadrant
North-east lies between 000° and 090°, so 052°T fits and 232°T is B-to-A.
3. Reverse to check
052° + 180° = 232°T, confirming the pair.
Sense check: The chart picture is an independent gross-error check on a number that otherwise looks perfectly formatted.
Common mistake or limitation
- Reading the reciprocal because the intended direction was not named first.
- Assuming a compass-rose ring or plotter scale is true from its position rather than its label.
- Treating one plotted bearing as a complete, exact fix.
Recap
- Transfer direction without losing alignment.
- Label a true three-figure bearing.
- Check the number against the visible quadrant.
- Preserve time and uncertainty on an observed position line.
Optional quick check
Section 5 of 10
A line from A to B visibly points south-east. Which member of the reciprocal pair can be A-to-B?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- BIPM SI Brochure, ninth edition
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
Continue studying Position, Course & Speed
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