Module 10 - Chartwork

Clearing Bearings and Danger Angles

What you will learn

Construct and use a clearing limit with its safe side, bearing reference, uncertainty and action point explicitly labelled.

A clearing bearing is useful only after the chart geometry establishes which side is safe. It is a monitoring limit, not an exact position or a universal less-than/greater-than rule.

During passage planning, draw a boundary that leaves the required margin from a charted danger. From an unmistakable charted object, derive the bearing of that boundary and label the safe side. Record whether the monitored bearing is true, magnetic or compass and apply the appropriate corrections once. Under way, act before uncertainty and observation delay can carry the vessel across the limit.

The numerical rule depends on object position, vessel movement and which side of the boundary is safe. 'Never exceed the bearing' is not portable from one plot to another. A bearing also provides direction rather than range, so use depth, distance, position or a second line when the geometry needs more than a directional limit.

Horizontal and vertical danger angles can provide related proximity limits when the charted objects, height reference and geometry are suitable. A larger observed angle commonly indicates a nearer position for the planned configuration, but the safe comparison and instrument corrections must be established on the specific chart. These aids retain feature-identification, height, refraction, vessel-motion and measurement uncertainty.

Worked example

A planned coastal leg passes west of an isolated rock. A lighthouse north of the leg is selected as the clearing object.

  1. 1Draw the required safety boundary around the rock using the charted danger and the vessel's margin policy.
  2. 2Draw the limiting line from the lighthouse tangent to that boundary and measure its true bearing.
  3. 3Use the chart geometry to label explicitly which observed-bearing side keeps the vessel west and clear.
  4. 4Convert the limit to the instrument reference used on deck and note the action before the limit is reached.
  5. 5Monitor position and depth as independent evidence; abandon the leg if the object cannot be identified or the limit cannot be observed reliably.

Sense check: Moving a test vessel point on the chart towards the rock should move the simulated observed bearing towards the labelled danger side.

Turn a chart line into an onboard decision

The pilotage note says 'lighthouse bearing 042' but does not identify the bearing reference, safe side or required response.

  1. 1. Repair the note

    Add the object identity, 042°T, °M or °C, the safe side and the plotted source of the limit.

  2. 2. Add an action margin

    Choose a warning and turn-away point that allows for bearing error, steering response, speed and sea room.

  3. 3. Name loss of evidence

    State the alternate action if visibility, object identity or compass reliability prevents the limit being monitored.

Sense check: A crew member should know both what value to watch and what to do before it becomes unsafe.

Common mistake or limitation

  • The safe side cannot be inferred from the number alone; it comes from the plotted geometry.
  • A clearing bearing is not an exact fix and should not consume the whole planned danger margin.

Recap

  • Construct the limit from the danger and selected object.
  • Label reference, safe side and action point.
  • Retain independent evidence and a response if the limit cannot be monitored.

Optional quick check

Section 11 of 13

Why is 'keep the bearing below 042°' incomplete as a reusable clearing-bearing rule?

Choose one answer
Sources and factual review

Reviewed 2 September 2026 - Compass Revision editorial review.

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This public lesson is part of Revision Module 10. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.

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