Module 6 - Position, Course & Speed
Measuring Bearings on the Chart
What you will learn
Transfer a line accurately to a chart compass rose or plotter, read the intended true direction and plot an observed bearing as a labelled position line.
Chart bearing work is a controlled transfer: align the instrument with the line, preserve that direction, read the correct true graduation and check that you have not read the reciprocal.
A parallel ruler, rolling ruler or navigational plotter transfers the direction of a charted line to a compass rose or meridian. The instrument should remain aligned with the intended line while it is moved; a small slip can create a significant cross-track error over a long leg.
Read the true graduation indicated by the chart or plotter and state the answer with three figures and a T suffix. Do not assume one ring is always correct from its position alone: inspect the labels and arrow direction on the instrument or rose.
A line has two directions separated by 180°. Before recording the answer, ask which way the vessel, bearing or route leg is intended to point. A line from A to B may be 068°T while the same line from B to A is 248°T.
For a corrected true bearing observed from the vessel to a charted object, plot the reciprocal from the known object. One bearing creates only a line of position; it does not create a complete fix until combined with sufficient independent position evidence.
Worked example
A true bearing of 318°T is observed from the vessel to a tower. Find the direction to plot from the tower.
- 1318° is at least 180°, so subtract 180°.
- 2318° − 180° = 138°T.
- 3Draw 138°T from the charted tower and label the line with the observation time.
Sense check: From the tower the line must point back towards the vessel's approximate south-eastern sector, not farther north-west.
Measure a planned leg without accepting the reciprocal
A route line runs from waypoint A in the south-west to waypoint B in the north-east. Your plotter offers 052° and 232° on the same line.
1. Set the direction
Place the plotter along A-to-B and identify the arrow or reading that points towards waypoint B.
2. Read true
Use the true chart graduation and record 052°T for A-to-B.
3. Check the picture
A north-easterly line should lie between 000° and 090°; 232°T is the reciprocal B-to-A direction.
Sense check: The numeric bearing must agree with the visible quadrant of travel before it is converted for the compass or entered in a plan.
Common mistake or limitation
- Reading the reciprocal because the instrument is aligned but the intended direction was never identified.
- Reading an unlabelled magnetic graduation when the chartwork answer should be true.
- Treating one plotted bearing as an exact fix rather than one line of position with observational uncertainty.
Recap
- Preserve the line direction while transferring it to the rose or plotter scale.
- Read and label a true three-figure bearing.
- Use the visible direction of travel to reject the reciprocal.
- One object bearing creates a line of position, not automatically a fix.
Optional quick check
Section 4 of 6
A route line visibly points south-east from A to B. Which paired reading can be the A-to-B bearing?
Sources and factual review
Continue studying Position, Course & Speed
This public lesson is part of Revision Module 6. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.