Magnetic North vs True North: Understanding Declination

If you have ever used a compass alongside a map, you have likely encountered one of the most fundamental — and most misunderstood — concepts in land navigation: magnetic declination. It is the angular difference between where your compass needle points (magnetic north) and the actual direction of the Earth's rotational axis (true north). Ignoring this difference can lead you hundreds of meters off course on a short hike, or kilometers off on a multi-day expedition. This guide will give you a thorough, practical understanding of what declination is, how it varies across the globe, and how to apply it correctly in the field.

What Is Magnetic Declination?

Magnetic declination — also called magnetic variation — is the angle on the horizontal plane between magnetic north and true north. It is measured in degrees and minutes, and it carries a critical sign convention: declination is positive (east) when magnetic north lies to the east of true north, and negative (west) when magnetic north lies to the west of true north. A declination of 10° E means your compass needle points 10 degrees to the right (east) of true north. A declination of 15° W means it points 15 degrees to the left (west).

The Earth's magnetic field is generated by the motion of molten iron in the outer core, thousands of kilometers beneath our feet. This geodynamo produces a field that is roughly dipolar — similar to a bar magnet — but with significant irregularities. The north magnetic pole, which is where the field lines point vertically downward, does not coincide with the geographic (true) north pole. As of 2025, the north magnetic pole is located in the Arctic Ocean, roughly 400 kilometers from the geographic north pole, and it moves continuously. This offset is the root cause of magnetic declination.

Declination is not a single global value. It changes depending on where you stand on the Earth's surface. In some locations, magnetic north and true north align almost perfectly — a condition called the agonic line — and declination is near zero. In other places, the difference can exceed 20 degrees. For example, in parts of the eastern United States, declination is around 15° W, while in parts of Western Europe it can be near 0° or even slightly east.

"A compass needle does not point to the North Pole. It points to the north magnetic pole, which wanders. The difference between the two is declination, and ignoring it is the most common navigation error in the backcountry."

True North, Magnetic North, and Grid North

To navigate accurately, you need to understand three distinct "norths" that appear on maps and in compass work. Confusing them is a frequent source of error.

• True North (Geographic North): The direction toward the Earth's rotational axis — the geographic North Pole. This is fixed and does not move. On a map, true north is the direction toward the top of the map (assuming the map is oriented with north up), and all lines of longitude converge at the true north pole.
• Magnetic North: The direction a compass needle points — toward the north magnetic pole. This pole moves over time due to changes in the Earth's magnetic field. Declination is the angle between true north and magnetic north at your location.
• Grid North: The direction of the vertical grid lines on a projected map (such as UTM — Universal Transverse Mercator). Because map projections flatten the curved Earth onto a flat surface, grid north may differ slightly from true north. This difference is called grid convergence or grid declination, and it is usually small (under 2–3°) but can matter for precise work.

The relationship between these three norths is often visualized as the "declination diagram" found on topographic maps. It shows three arrows: one pointing to true north, one to magnetic north, and one to grid north, with the angles between them labeled.

Most modern topographic maps include a declination diagram in the margin. Always check it before beginning navigation. The diagram will tell you the magnetic declination for the map's edition date and the annual rate of change, so you can calculate the current declination.

How Declination Varies by Location

Declination is a function of the Earth's magnetic field geometry at your specific location. The field is not a perfect dipole, so declination values can change dramatically over short distances, especially near the magnetic poles or in regions with strong crustal magnetic anomalies. The agonic line — the line of zero declination — currently runs roughly through the central United States, across the Atlantic, and through parts of Europe and Asia.

To give you a concrete sense of how declination varies, here is a table of approximate declination values for major cities around the world. These values are based on the World Magnetic Model (WMM) and are accurate to within about 0.5° for 2025–2026.

Notice the enormous variation: from 26° W in Cape Town to 15° E in Anchorage. A navigator in Cape Town must add 26° to a compass bearing to get a true bearing, while a navigator in Anchorage must subtract 15°. Getting the sign wrong — adding instead of subtracting — would introduce a 30° to 50° error, which over a 5-kilometer hike could put you nearly a kilometer off course.

How to Find Your Local Declination

There are several reliable ways to determine the magnetic declination at your specific location:

• Topographic maps: Most USGS, Ordnance Survey, and other national topographic maps include a declination diagram in the margin. It shows the magnetic declination for the map's publication date and the annual rate of change (e.g., "15°30' W (2020), annual change 6' E"). Multiply the annual change by the number of years since the map date and adjust accordingly.
• NOAA Magnetic Field Calculator: The U.S. National Oceanic and Atmospheric Administration provides an online calculator at ngdc.noaa.gov/geomag/calculators/magcalc.shtml. Enter your coordinates and date, and it returns the current declination, inclination, and field strength based on the World Magnetic Model.
• Smartphone apps: Many navigation apps (Gaia GPS, Avenza Maps, etc.) display current declination for your location. Some compass apps also show it directly.
• Compass with adjustable declination: Many modern baseplate compasses (Suunto, Silva, Brunton) have an adjustable declination scale. You set the declination once for your area, and the compass automatically corrects all subsequent bearings.

Converting Between Magnetic and True Bearings

The fundamental conversion formula is straightforward:

Where declination is positive for east and negative for west. For example, if your compass reads a magnetic bearing of 45° and your local declination is 10° E, the true bearing is 45° + 10° = 55°. If the declination is 10° W, the true bearing is 45° + (−10°) = 35°.

The reverse conversion — going from true bearing to magnetic bearing — uses the same formula rearranged:

So if your map gives you a true bearing of 120° and declination is 8° E, the magnetic bearing you should follow with your compass is 120° − 8° = 112°.

Here is a table of common conversion examples to illustrate the principle:

A helpful mnemonic is "East is least, West is best" — meaning you subtract east declination and add west declination when converting from true to magnetic. Another way to remember: if declination is east, magnetic north is to the right (east) of true north, so your compass reading will be less than the true bearing. If declination is west, magnetic north is to the left (west), so your compass reading will be more than the true bearing.

Compensating for Local Magnetic Interference

Even with the correct declination applied, your compass can be affected by local magnetic interference — a phenomenon called magnetic deviation. Unlike declination (which is a global, predictable field effect), deviation is caused by local magnetic sources and is specific to your immediate surroundings.

Common sources of magnetic deviation include:

• Ferrous metals: Belt buckles, zippers, watches, knives, trekking poles, and vehicle bodies can all deflect a compass needle. Even the iron in a smartphone can cause significant deviation if the compass is held close to it.
• Electrical currents: Power lines, generators, and electronic devices produce magnetic fields that interfere with compass readings.
• Mineral deposits: Areas with high concentrations of magnetite, iron ore, or other magnetic minerals can create local anomalies. The Kursk Magnetic Anomaly in Russia is an extreme example, but smaller anomalies exist worldwide.
• Your own body: The iron in your blood and certain clothing items can introduce small errors, usually under 2–3°.

To minimize deviation, always hold your compass away from your body, metal objects, and electronics. Take bearings on open ground, not next to your car or under power lines. If you suspect local interference, take multiple bearings from different positions and average them. Some compasses (especially those on ships and aircraft) include deviation cards — tables that list the compass error at different headings, allowing the navigator to correct for it.

Declination Change Over Time

The Earth's magnetic field is not static. The north magnetic pole has been moving at an accelerating rate — from about 15 km/year in the 1990s to over 50 km/year in recent years. This means declination values are constantly changing, and a value that was accurate a decade ago may be significantly off today.

The rate of change varies by location. In some areas, declination changes by less than 0.1° per year — negligible for most navigation purposes. In others, particularly near the magnetic poles, it can change by 0.5° or more per year. The World Magnetic Model is updated every five years (WMM2020 is the current version, with WMM2025 expected soon), and these updates can shift declination values by a degree or more in rapidly changing regions.

For most recreational navigation, using a declination value that is a few years old is acceptable — the error introduced is usually smaller than the inherent accuracy of a handheld compass (±2–5°). However, for professional surveying, military operations, or scientific work, using current WMM data is essential.

There is also a long-term phenomenon called geomagnetic secular variation — the slow, continuous change in the magnetic field over decades and centuries. On a timescale of centuries, declination can change by tens of degrees. Historical records show that declination in London was about 11° E in 1650, 0° around 1800, and 24° W in 1900 — a swing of over 35° in 250 years.

Conclusion

Magnetic declination is one of the most important — and most frequently overlooked — concepts in navigation. The angular difference between magnetic north and true north varies by location, changes over time, and can introduce significant errors if ignored. By understanding what declination is, how to find your local value, and how to apply the conversion formula correctly, you can navigate with confidence whether you are hiking in the backcountry, sailing offshore, or surveying land.

The key takeaways are simple: always check the declination for your area before navigating, apply the correct sign (east or west) when converting between magnetic and true bearings, and be aware that declination changes over time — so use current data when possible. With these habits, you will avoid the most common navigation error and stay on course.

At Live Compass, we believe that understanding the tools of navigation is just as important as using them. Whether you are a seasoned mountaineer or a weekend hiker, a solid grasp of magnetic declination will make you a more confident and capable navigator. Explore our other guides for more practical navigation knowledge, and use our online compass tools to put these concepts into practice.