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Magnetic variation – where does the compass really point?

A compass needle does not point to the geographic North Pole but to magnetic north. The angle between the two is the magnetic variation, also called magnetic declination. It depends on where you are and changes over the years. NauticTools2 calculates it for any position and any date from 1900 to 2030.

• Position
• Date
• Variation east/west
• Annual change
• IGRF model

What is magnetic variation?

There are two “norths”: true north points to the geographic North Pole, which charts and meridians refer to. Magnetic north is the direction an undisturbed compass needle points – it follows the field lines of the earth's magnetic field.

The angle between them is the variation. If the needle points east of true north, the variation is east and counts positive; if it points west of true north, it is west and negative.

When do you need it?

• Converting courses and bearings: Every course taken from the chart is true, every compass course is magnetic (plus deviation). Without the variation you cannot turn a chart course into a course to steer.
• Old charts and textbook exercises: The variation on a chart applies to a certain year. NauticTools2 gives you the exact value for today – or for the year of an old exercise.
• Checking the compass: To find the deviation of a steering compass you need the variation at your current position.

Calculation and the chart value

NauticTools2 calculates the variation with the IGRF (International Geomagnetic Reference Field), the official reference model of the earth's magnetic field. The international association IAGA publishes a new edition every five years; it describes the field as a series of spherical harmonics, so there is no hand formula for it. The model in the app covers the years 1900 to 2030.

Without a calculator, you update the variation printed on the chart. The compass rose gives a value with a reference year and the annual change:

1. Read the chart

   Variation, reference year and annual change from the compass rose.

2. Count the years

   Number of years from the reference year to today.

3. Update

   Add annual change times years to the chart value – mind the signs: east positive, west negative.

Worked examples

The second example shows why old chart values need care: the annual change is not constant itself. At Heligoland it has sped up from 6′ to about 10′ per year since 1990.

Things to watch out for

• Signs: East positive, west negative – throughout NauticTools2. Mixing them up is one of the most common mistakes in course conversion. The old rhyme helps: “Variation east, compass least; variation west, compass best.”
• Local magnetic anomalies: In some sea areas, ore deposits in the seabed make the variation differ noticeably from the model. Such areas are marked on the chart as “local magnetic disturbance”.
• Variation is not deviation: Variation comes from the earth's magnetic field, deviation from your own boat (engine, steel, electronics). Together they make up the compass error.
• Model limits: The IGRF is a very good approximation, but it does not replace official charts in areas with known anomalies.

How it works in NauticTools2

1. Choose the date

   Today's date is filled in. For old exercises, simply enter the year of the exercise.

2. Enter the position

   Latitude and longitude by hand, from a saved waypoint or – if available – from GPS. A position you enter can be saved as a waypoint straight away.

3. Calculate

   NauticTools2 shows the variation and its annual change.

4. Use as reference value

   “Apply” makes the value the default for all other tools, such as course and bearing conversion.

Frequently asked questions

Are variation and declination the same?

Yes. “Magnetic declination” is the term used in geophysics, while navigators usually say “variation”. Both mean the same angle. Do not confuse it with the declination of a celestial body in celestial navigation.

How accurate is the calculation?

In open waters the IGRF is usually within half a degree of the actual value – about as precise as a boat compass can be read.

Why does the variation change?

The earth's magnetic field is generated by currents in the liquid outer core and changes constantly. The magnetic North Pole moves by dozens of kilometres a year.

Related tools

• Course Conversion – from chart course to course to steer and back
• Bearing Conversion – convert a compass bearing into a true bearing
• Compass Check by the Sun – find compass error and deviation with the sun