Calculator Form
Formula Used
The calculator uses signed corrections. East is positive. West is negative.
| Purpose | Formula |
|---|---|
| Adjusted variation | Adjusted Variation = Base Variation + Annual Change × (Target Year − Chart Year) |
| True to magnetic | Magnetic = True − Variation |
| Magnetic to compass | Compass = Magnetic − Deviation |
| Compass to magnetic | Magnetic = Compass + Deviation |
| Magnetic to true | True = Magnetic + Variation |
| Total compass error | Compass Error = Variation + Deviation |
| Cross track estimate | Cross Track = Distance × sin(|Compass Error|) |
How to Use This Calculator
- Select whether the entered value is true, magnetic, or compass heading.
- Enter the heading in degrees from 0 to 360.
- Add chart variation and choose east, west, or nil.
- Enter annual variation change when your chart provides it.
- Add deviation from your compass deviation card.
- Enter a leg distance if you want cross track estimation.
- Press Calculate to show the result above the form.
- Use CSV or PDF download for records.
Example Data Table
| Known Type | Heading | Variation | Annual Change | Years | Deviation | Expected Use |
|---|---|---|---|---|---|---|
| Compass | 075° | 3.5° W | 0.1° E | 6 | 2° E | Find true heading from steering compass |
| True | 120° | 4° E | 0° Nil | 0 | 1.5° W | Find compass course to steer |
| Magnetic | 210° | 2° W | 0.05° W | 4 | 0.5° E | Compare magnetic and compass values |
Magnetic Compass Correction Guide
A magnetic compass is simple, but its reading needs correction before it becomes a dependable course. The needle points toward magnetic north, not true north. Nearby metal, wiring, radios, tools, and engines can also bend the local magnetic field. This calculator combines both effects so a navigator can compare true, magnetic, and compass headings in one place.
Why correction matters
Variation is the angle between true north and magnetic north. It changes by location and by year. Charts often show a base value and an annual change. Deviation is the error caused by the vessel, vehicle, or instrument installation. It usually comes from a deviation card made during compass swinging. When these values are ignored, a small error can become a large lateral offset on a long leg.
How the tool helps
The form accepts a known true, magnetic, or compass heading. It then solves the missing headings with signed east and west corrections. East values are treated as positive. West values are treated as negative. You can also enter chart year, target year, and annual variation change. The adjusted variation is calculated before the final course conversion.
Practical navigation use
Use true headings when planning from charts or maps. Use magnetic headings when comparing with magnetic references. Use compass headings when steering with the installed compass. The calculator also reports total compass error, reciprocal course, and an estimated cross track effect over a selected distance. This helps show the real effect of a heading error during practice, passage planning, or field instruction.
Good habits
Always verify chart variation from a current source. Keep deviation tables updated after equipment changes. Remove magnets, speakers, phones, and tools from the compass area during checks. Use this calculator for planning, education, and record keeping. For critical navigation, confirm results with approved charts, instruments, and standard operating procedures.
Reading the result
The corrected values should be read with their direction labels. A positive total error means east correction. A negative total error means west correction. Round the final steering course only after reviewing the detailed values. Small decimals may matter when distances are long, speeds are high, or visibility is poor during coastal work or open water training sessions safely.
FAQs
What is magnetic variation?
Magnetic variation is the angular difference between true north and magnetic north. It depends on location and changes over time. Charts usually show the value and yearly change.
What is compass deviation?
Compass deviation is the error caused by local magnetic influences near the compass. Engines, wiring, speakers, tools, and metal structures can affect it. A deviation card records this error.
What does east correction mean?
In this calculator, east values are positive. When converting compass heading to true heading, east total compass error is added. West total compass error is subtracted.
Can I enter a true heading?
Yes. Select true heading as the known type. The calculator will compute magnetic heading and compass heading using adjusted variation and deviation.
Why is annual variation included?
Variation changes over years. If your chart gives a base year and annual change, the calculator updates the variation for your target year before solving headings.
What is total compass error?
Total compass error is the signed sum of variation and deviation. It shows the full correction between compass heading and true heading.
Is the cross track value exact?
No. It is an estimate based only on heading error and distance. Current, wind, steering changes, and instrument accuracy can change the real position.
Can I use this for critical navigation?
Use it for planning, training, and checking. For critical navigation, verify with current charts, official procedures, reliable instruments, and local navigational guidance.