Module 7 - Charts & Publications
Compass Rose on a Nautical Chart: True and Magnetic Bearings
What you will learn
Identify the chart's projection and orientation, then obtain a true direction from labelled chart information without relying on an inner-ring or outer-ring shortcut.
Mercator is common because rhumb lines plot as straight lines, but the chart states its projection. Read the graduation identified as true or use meridians; compass-rose ring order is not a universal rule.
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 |
Read a plotted route direction safely
A route line is transferred to a compass rose that has two graduations and a variation note dated several years earlier.
1. Projection and frame
Read the title and marginal information; establish the projection and the direction of the meridians.
2. True graduation
Identify the graduation labelled or aligned as true instead of choosing by ring position.
3. Read and label
Read the true course in three figures and mark it °T in the working.
4. Convert only if needed
Update variation from the chart note, then apply deviation from the vessel's current information if a compass course is required.
Sense check: A direction read from the chart is not automatically magnetic or compass; its reference must be explicit at each step.
Common mistake or limitation
- Assuming every nautical chart uses Mercator projection or has north at the page top without checking the chart.
- Using the outer ring by habit rather than identifying the true graduation.
- Applying old printed variation without its year and annual change, or treating chart variation as vessel deviation.
Recap
- Read the chart's stated projection and orientation.
- Identify the true graduation from labels or meridians, not ring position.
- Variation is time- and place-dependent; deviation is vessel-dependent.
- Label every direction T, M or C so the conversion is auditable.
Optional quick check
Section 5 of 5
A compass rose has two rings. How should you choose the true-bearing graduation?
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.
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.