Magnetic Declination Chart

A magnetic declination chart is a specialist navigation document used to determine the difference between true north and magnetic north at a given location. This information is essential when navigating with a magnetic compass, because the compass does not point toward geographic north. Magnetic declination varies according to geographical position and changes gradually over time.

What is magnetic declination?

Magnetic declination, also known in nautical terminology as magnetic variation, is the horizontal angle between true north and magnetic north. Depending on the location, the declination may be easterly or westerly.

An easterly magnetic declination means that magnetic north lies east of true north. A westerly declination means that magnetic north lies west of true north. The value is normally expressed in degrees and minutes, together with the letter E or W.

This correction is important because a bearing taken with a magnetic compass cannot be directly compared with a true bearing obtained from a nautical chart or calculated from geographical coordinates without taking magnetic declination into account. NOAA defines magnetic declination as the angle between magnetic north and true north and notes that it varies both with location and with time.

Why use a magnetic declination chart?

A magnetic declination chart provides navigators with the information required to convert between true and magnetic directions. It is particularly useful when preparing an ocean passage, checking compass bearings, working with traditional navigation methods or comparing a magnetic compass reading with a GPS or electronic navigation system.

For example, a navigator may calculate a true course between two waypoints using geographical coordinates. If the vessel is steered using a magnetic compass, the local magnetic variation must be applied to obtain the corresponding magnetic course.

The same principle applies in the opposite direction: a magnetic bearing observed with a hand-bearing compass must be corrected before it can be plotted accurately on a chart using true bearings.

A small error in magnetic correction can become significant over a long ocean passage, particularly when a vessel is navigating without frequent visual references.

Magnetic variation and its geographical distribution

Magnetic declination is not the same everywhere on Earth. The Earth's magnetic field is irregular, and magnetic north is not coincident with geographic north. The direction indicated by a magnetic compass therefore changes from one region to another.

Magnetic declination charts represent this geographical distribution using isogonic lines. These lines connect points having the same magnetic declination.

The zero-declination line, known as the agonic line, indicates areas where magnetic north and true north coincide. On either side of this line, the magnetic variation becomes easterly or westerly.

These lines are particularly useful for ocean navigation because they allow the navigator to estimate the magnetic variation over a large area rather than relying solely on the value indicated on an individual chart.

Annual change and secular variation

Magnetic declination is not a fixed value. The Earth's magnetic field changes continuously, which means that the magnetic variation at a particular location also changes over the years.

A magnetic chart therefore normally refers to a specific year and may also indicate the annual rate of change. This allows the navigator to update the published value and obtain a more appropriate estimate for the date of navigation.

This long-term evolution is generally referred to as secular variation. Modern geomagnetic models, including the World Magnetic Model, are used to calculate magnetic declination for specific geographical coordinates and dates.

For this reason, an old magnetic declination chart should not automatically be treated as providing the current magnetic variation. The reference year and annual change shown on the document must always be taken into consideration.

Compass roses and magnetic variation

Magnetic variation information is also commonly displayed directly on nautical charts through compass roses.

A compass rose may show both true north and magnetic north, together with the magnetic variation applicable to a particular year and the annual change.

This information enables the navigator to convert between true and magnetic courses when planning or conducting a passage.

A compass rose is particularly useful on a conventional paper nautical chart because the magnetic correction is presented directly in the geographical area covered by the chart. The navigator must nevertheless check the date of the information and apply any stated annual change where appropriate.

Magnetic declination and compass deviation

Magnetic declination should not be confused with compass deviation.

Magnetic declination is caused by the Earth's magnetic field and depends primarily on geographical location and date.

Compass deviation, on the other hand, is caused by magnetic influences on board the vessel. These may come from the engine, electrical equipment, steel structures, batteries, tools or other ferromagnetic objects close to the compass.

A vessel's compass therefore has to take account of both the geographical magnetic variation and its own deviation.

The distinction is fundamental in traditional navigation:

• True north is the geographical reference.

• Magnetic north is the direction indicated by the Earth's magnetic field.

• Magnetic declination or variation is the angular difference between true and magnetic north.

• Compass deviation is the error introduced by the vessel and its equipment.

• Compass heading is the direction actually indicated by the compass.

Understanding these different corrections is essential for accurate compass navigation.

Magnetic declination in ocean navigation

Magnetic variation becomes particularly important during long-distance sailing and ocean passages.

In the North Atlantic, for example, a navigator may prepare a route between Brest, Cherbourg, Southampton, La Coruña or Lisbon and ports such as Halifax, Boston, New York or Québec. The magnetic variation encountered along such a route may differ significantly from the value at the departure point.

The same applies in the South Atlantic, with routes involving Dakar, Cape Town, Recife, Salvador da Bahia, Rio de Janeiro or Buenos Aires.

In the Pacific, magnetic declination must be considered on passages between Vancouver, Seattle, San Francisco, Alaska, Hawaii, Japan, Tahiti, French Polynesia, New Zealand and Australia.

In the Indian Ocean, it may be relevant between Durban, Port Louis, Mumbai, Colombo, Singapore and Fremantle.

For ocean crossings, the navigator may therefore need to consider how magnetic variation evolves along the entire route rather than applying a single correction from the departure port.

Magnetic declination in the North Atlantic

The North Atlantic is a particularly relevant area for magnetic navigation because it includes long passages between Europe and North America.

A vessel leaving Brest, Cherbourg, Southampton or Lisbon and heading toward Halifax, Boston, New York or Québec may cover several thousand nautical miles. The magnetic variation at the beginning of the voyage is therefore not necessarily representative of the variation encountered later.

Magnetic variation charts covering the North Atlantic allow the navigator to visualise the geographical distribution of magnetic declination and its annual evolution.

They can complement conventional nautical charts, routeing charts and other documents used when preparing an ocean passage.

Magnetic declination in the Pacific and Indian Ocean

The same principle applies to long passages across the Pacific and Indian Ocean.

The Pacific covers an enormous geographical area, from the west coast of North America to Japan, Australia and New Zealand. Routes through Hawaii, French Polynesia and the South Pacific can also cross areas where magnetic variation changes progressively.

In the Indian Ocean, routes between southern Africa, Mauritius, India, Sri Lanka, Southeast Asia and Australia may also require careful consideration of magnetic variation.

For offshore sailors, professional navigators and ocean-racing crews, magnetic declination remains useful even when satellite navigation is available, because the magnetic compass remains an independent navigation instrument.

Magnetic declination and modern navigation

Modern navigation systems use GNSS, electronic charts, ECDIS and digital heading sensors. These systems can calculate or display magnetic and true headings using digital magnetic models.

The World Magnetic Model is widely used for navigation and heading-reference applications. Current geomagnetic tools can calculate magnetic declination according to both geographical coordinates and date.

However, electronic navigation does not make an understanding of magnetic variation unnecessary.

A magnetic compass remains an important independent navigation instrument, particularly for offshore and ocean navigation. Understanding the relationship between true, magnetic and compass directions also helps the navigator identify inconsistencies between different navigation instruments.

Magnetic declination in polar regions

Magnetic navigation becomes increasingly complex at high latitudes.

Near the magnetic poles, the horizontal component of the Earth's magnetic field becomes weaker and compass reliability can deteriorate. Modern magnetic models therefore identify areas where compass accuracy may be significantly reduced.

For voyages toward Greenland, the Arctic Ocean, Antarctica or other high-latitude regions, magnetic declination should therefore be considered together with the limitations of the magnetic compass itself.

In these areas, navigators generally rely on several independent navigation references rather than using magnetic heading alone.

Magnetic declination charts from major hydrographic publishers

Magnetic variation charts have historically been produced by hydrographic and maritime authorities and specialist nautical publishers.

Admiralty charts published by the United Kingdom Hydrographic Office include magnetic variation information and dedicated magnetic variation charts for ocean areas. SHOM, NOAA and other hydrographic organisations also provide magnetic information for navigation.

These documents can be used alongside conventional nautical charts, routeing charts and electronic navigation systems.

When selecting a magnetic declination chart, it is important to check:

• The geographical area covered.

• The reference year.

• The magnetic declination values.

• The annual rate of change.

• The chart scale.

• The publisher or hydrographic authority.

• Whether the chart is intended for coastal or ocean navigation.

• Whether the information is appropriate for the intended date of navigation.

Choosing a magnetic declination chart

The most appropriate chart depends primarily on the navigation area and the type of voyage.

For coastal sailing, magnetic variation information shown on the relevant nautical chart may be sufficient. For ocean crossings, a dedicated magnetic variation chart covering the entire passage can provide a broader view of the geographical and temporal evolution of magnetic declination.

For traditional navigation, celestial navigation, ocean racing or long-distance cruising, the magnetic declination chart can also provide a useful independent reference.

The document should always be interpreted together with the date of publication and the annual change indicated on the chart.

Conclusion

A magnetic declination chart is an important navigation document for understanding the relationship between true north and magnetic north. Magnetic variation changes with both geographical position and time, which means that the navigator must consider the reference year and, where appropriate, the annual rate of change.

Although GPS, GNSS, ECDIS and modern electronic navigation systems have transformed marine navigation, the magnetic compass remains an important independent reference. Knowing how to interpret magnetic declination, compass roses and isogonic lines therefore remains a fundamental navigation skill.

For coastal sailing as well as long ocean passages across the Atlantic, Pacific or Indian Ocean, a magnetic declination chart provides valuable information for converting true and magnetic courses and for understanding the behaviour of the compass along the route.

Frequently Asked Questions

What is a magnetic declination chart?

A magnetic declination chart shows the angular difference between true north and magnetic north over a geographical area. It generally indicates magnetic variation in degrees east or west and may also show the annual rate of change.

What is the difference between magnetic declination and magnetic variation?

In nautical terminology, the two terms are generally used for the same phenomenon: the angular difference between true north and magnetic north. “Magnetic variation” is particularly common in maritime charting terminology.

Does magnetic declination change over time?

Yes. The Earth's magnetic field changes continuously, so magnetic declination varies gradually with time as well as with geographical position.

What are isogonic lines?

Isogonic lines are lines connecting points with the same magnetic declination. They are used on magnetic charts to represent the geographical distribution of magnetic variation.

What is an agonic line?

An agonic line connects locations where magnetic declination is zero, meaning that magnetic north and true north coincide.

Is magnetic declination the same as compass deviation?

No. Magnetic declination is caused by the Earth's magnetic field and varies with geographical position and time. Compass deviation is caused by magnetic influences originating on or near the vessel.

Why is magnetic declination important during an ocean crossing?

Because the magnetic variation can change significantly along a long route. A correction that is appropriate at the departure port may not be appropriate thousands of nautical miles farther along the route.

Do GPS and ECDIS make magnetic declination unnecessary?

No. Modern systems can calculate and display magnetic information using geomagnetic models, but understanding magnetic variation remains important for independent navigation and for the correct use of a magnetic compass.

Is magnetic declination reliable near the poles?

Magnetic navigation becomes more difficult near the magnetic poles because the horizontal component of the Earth's magnetic field becomes weaker. Compass accuracy can therefore be significantly degraded in some polar regions.