Module 7 - Charts & Publications
Compass Rose on a Nautical Chart: True and Magnetic Bearings
Mercator projection is widely used for navigation because it is conformal and represents rhumb lines as straight lines, but it is not the only chart projection. IHO specifications require the projection name to be shown in the title information. Check the actual chart rather than assuming every sheet is Mercator or every chart frame is a simple north-up rectangle.
A compass rose may provide true and magnetic graduations, variation for a stated year and an annual change. The safest method is to identify the graduation explicitly aligned or labelled as true, or transfer the line to a labelled meridian. Do not memorise 'outer means true' as a universal instruction: design and ring order can vary.
If magnetic information is used, update the printed variation from its stated epoch and annual change using the convention on the chart, then keep true, magnetic and compass values labelled. Variation changes over time; deviation belongs to the vessel and is not supplied by the chart rose.
| Chart item | What it tells you | Required action |
|---|---|---|
| Projection name | How the curved Earth is represented | Use the plotting and scale method suitable for that chart |
| Meridians / grid | True north-south reference | Use them to verify true direction and orientation |
| True graduation | Direction clockwise from true north | Read and label the result °T |
| Variation note | Magnetic offset at a stated epoch and annual change | Update only with the chart's stated sign convention |
| Deviation card | Vessel-specific compass error | Obtain from the vessel; it is not a chart property |
Common questions
Which ring of the compass rose is used for true bearings?
Use the graduation explicitly labelled or aligned as true, or transfer the line to a labelled meridian. Do not rely on outer-ring position alone.
What does the inner ring of a compass rose show?
A magnetic graduation, when provided, relates to the variation stated on the rose for a particular epoch. Check its label, date and annual change.
Why is the annual change printed on a compass rose?
Magnetic variation changes over time, so the annual change helps you update the printed variation before converting between true and magnetic bearings.
Optional quick check
Section 5 of 5
A compass rose has two rings. How should you choose the true-bearing graduation?
Keep revising this topic
Sources and factual review
Reviewed 5 July 2026 - Day Skipper Revision.
Practice bearing conversions
Compass work links the chart compass rose to variation, deviation, and the CDMVT conversion process.