Module 6 - Position, Course & Speed
True Bearings and Courses for Chartwork
What you will learn
Distinguish a bearing, course, heading and track, label their north reference, and recognise when a reciprocal direction is needed.
A bearing gives the direction of an object or line from north; a course is a direction selected for travel. Bearings shown on a nautical chart are true, but observations and steering instructions may use true, magnetic or compass references and must be labelled.
Navigational bearings are measured clockwise from north: 000° or 360° is north, 090° east, 180° south and 270° west. IHO chart specifications require charted bearings to be true and normally written with three figures, such as 005°, 045° or 230°.
A bearing is the direction of an object or line from the stated north reference. A course is the direction selected for the vessel to travel or steer. Heading is where the vessel's fore-and-aft line points at an instant; track is the path actually followed through the water or over the ground. Wind, current and steering can make those quantities differ.
Write T, M or C whenever the context could be ambiguous: true is referenced to True North, magnetic to Magnetic North and compass to the vessel's compass after its local deviation. Do not combine a compass observation with a true chart line until the required corrections have been applied and checked.
The reciprocal of a direction points along the same line but the opposite way. Add 180° to bearings below 180° and subtract 180° from bearings of 180° or more. A bearing of 045°T from the vessel to a lighthouse therefore defines a position line plotted from the lighthouse on 225°T.
Worked example
Find the reciprocal of 302°T and check it without relying on a memorised quadrant rule.
- 1Because 302° is 180° or more, subtract 180°.
- 2302° − 180° = 122°T.
- 3Reverse it again as a check: 122° + 180° = 302°T.
Sense check: Reciprocal bearings differ by exactly 180° and describe the same straight line in opposite directions.
Turn an observed bearing into a charted line
From the vessel you obtain a corrected true bearing of 045°T to a charted lighthouse and need to plot the line on which the vessel lies.
1. Confirm reference
Check that 045° is already true; a raw compass bearing needs the applicable compass corrections first.
2. Find the reciprocal
Add 180°: 045° + 180° = 225°T.
3. Plot from the known object
Draw 225°T from the lighthouse. The vessel lies somewhere on that line until another independent position line constrains it.
Sense check: The line must extend from the lighthouse back towards the vessel's approximate area; 045° from the lighthouse would point the wrong way.
Common mistake or limitation
- Using bearing, course, heading and track as synonyms, which hides wind, current and steering effects.
- Dropping the T, M or C suffix and then applying a correction to an unknown reference.
- Plotting the observed direction from the object without reversing it by 180°.
Recap
- Bearings run clockwise from north and are normally written with three figures.
- Bearing, course, heading and track describe different relationships.
- Label the north reference before converting or plotting.
- A reciprocal bearing differs by 180°.
Optional quick check
Section 2 of 6
What is the reciprocal of 278°T?
Keep revising this topic
Sources and factual review
Connect bearings to chartwork
Once true bearings are clear, practice measuring distance, plotting positions, and converting courses for compass work.