Compass Corrections and Course Plotting
Relating true, magnetic and compass directions, applying variation and deviation, and recording a course unambiguously.
How to prepare for Day Skipper Theory
Learn the navigation foundations first, combine tide and weather into berth-to-berth plans, then rehearse collision avoidance, electronic cross-checks, seamanship and emergency decisions.
Core concepts
Concept 1
Variation relates true and magnetic north and changes with place and time; deviation relates magnetic and compass direction and can vary with vessel heading and onboard influences.
Exam cue: Write T, M or C beside every direction before doing corrections.
Concept 2
A navigation solution must state whether each course or bearing is true, magnetic or compass and apply corrections in a consistent direction.
Exam cue: Use current chart information and the vessel's deviation data rather than a memorised correction.
Concept 3
A compass course is an instruction to steer, while course over ground describes actual movement and may differ because of tide, leeway and steering error.
Exam cue: Check the final course against the plotted track and expected environmental effects.
Risk pitfalls and guardrails
Applying variation or deviation twice or with the wrong sign.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Using variation from an old chart without updating it to the relevant date.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Treating heading, course through water and course over ground as interchangeable.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Memory anchors
True-Magnetic-Compass
Keep each north reference labelled throughout a calculation.
Variation
The angular difference between true north and magnetic north.
Deviation
The vessel-specific compass error caused by onboard magnetic influences.
Course to Steer
The direction instructed at the helm after relevant corrections and allowances.
COG
Course over ground is the vessel's actual track direction over the seabed.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What does variation relate?
What does deviation relate?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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