Variation in navigation: how true north and magnetic north shape your heading

Variation: The Tiny Angle That Keeps Your Course Honest

Let’s start with a simple question we’ve all asked ourselves at some point on a boat deck or a cockpit: what really is north pointing at when you’re steering a vessel or flying a plane? The straight truth is this: there are two kinds of north, and they don’t always line up. That misalignment is what navigators call variation.

True north versus magnetic north: what’s the difference?

• True north is the direction toward the geographic North Pole—the fixed point on the globe that cartographers line up with when they draw a map.

• Magnetic north is where a compass needle actually points. It’s the direction of Earth’s magnetic field at your location. And here’s the kicker: magnetic north isn’t stationary. It wanders with time and it shifts from place to place because the Earth’s core and mantle are always on the move.

In plain terms, variation is that difference—the angle between true north and magnetic north at your current spot. It’s a real thing, and it matters because your compass wants to show magnetic north, while your chart or flight plan might be written using true north. If you don’t account for that difference, your course drifts off course, sometimes more than you’d expect.

Why variation matters in navigation

Think of it like two rulers lined up side by side. One ruler is true north; the other is magnetic north. If you’re plotting a route, you want to know how far apart those rulers are at your location, so you can translate a heading from one to the other without getting lost.

For mariners and aviators, the practical consequence is simple but serious: you must adjust your headings when you switch between true and magnetic references. If you’re aiming for a true heading of 090°, but your variation at your location is 5° East, your magnetic heading—what your compass would show you—would be 085°. If you ignore that little five-degree offset, you’ll gradually drift to the east of your intended track.

A quick rule of thumb you’ll see bands around in the field is the saying East is least, West is best. It’s mnemonic but reliable: if variation is East, subtract it when converting true to magnetic (Magnetic = True - Variation). If variation is West, add it (Magnetic = True + Variation). It’s a handy mental shortcut when you’re triangulating your position in a hurry or when the chart in front of you is a touch dated.

A simple mental illustration

Imagine you’re sailing along a perfectly straight line toward a lighthouse that anchors your course. Your map uses true north, and you’re mentally thinking in those terms. Your compass, though, is whispering toward magnetic north. If the local variation is 6° West, and you want to move true 120°, you’d set your magnetic heading to 126° (because True + West variation equals Magnetic). The opposite would be true if the variation is East: True 120° becomes Magnetic 114°. It’s almost like translating a language—one that’s spoken by the Earth and one that’s spoken by your compass.

Where variation comes from and how it changes

Variation isn’t a fixed tattoo on the globe. It’s a product of Earth’s magnetic field, which is shaped by the core’s movements, crustal magnetism, and a dash of local anomalies. That means:

• The value can differ from place to place, sometimes a few degrees apart at nearby latitudes.

• It can shift over time, though not usually at the speed of weather. Still, the numbers you see on navigation charts get updated as our world’s magnetic field nudges subtly.

On a chart, you’ll often see isogonic lines—curvy lines that connect places with the same variation. If you’re near one line, you’ll notice that the numbers along your route stay relatively steady; cross a line, and the correction can change. It’s a bit like crossing between time zones, just with angles instead of hours.

How navigators actually use variation day-to-day

Here’s the practical flow you’ll see in the field, whether you’re on a boat, in a cockpit, or training to work with modern avionics:

• Start with true headings on your course plan. Your charts will typically present true north as the reference, even if you’ll be steering with a magnetic compass or a GPS that displays magnetic headings.

• Check the current variation for your location. This information is published on nautical charts and aeronautical charts, often labeled clearly with the degree and whether it’s East or West.

• Convert as needed. If you’re plotting a route using true headings and your navigation tool can only show magnetic headings (or vice versa), you adjust using the simple rule: Magnetic = True Heading minus Variation if Variation is East; Magnetic = True Heading plus Variation if Variation is West.

• Update as you move. If you travel to a different location, you’ll need to recalculate. On long voyages or flights, variation isn’t something you set once and forget; it’s a dynamic companion you keep in view.

It’s easy to imagine this as a tiny, almost invisible adjustment, but the effects can accumulate. Think about a mile every ten minutes over the course of several hours—tiny angles become noticeable drift. That’s why pilots and mariners layer in this correction as a second-nature habit, just like checking fuel or weather.

A note about what variation isn’t

It’s helpful to keep straight what variation does and doesn’t cover. Variation is about the Earth’s magnetic field and the difference from true north. It’s not the same as deviation, which is the magnetic interference caused by a vessel’s own equipment and metal—think of a ship’s hull, engines, or nearby electronics tugging at the compass. Deviation is a separate correction you’d apply to your compass readings, beyond the global variation. When you’re learning navigation, it’s useful to keep these two terms distinct in your mind, because they sit in the same family but affect readings in different ways.

A practical mindset: learning variation without getting lost in the jargon

If you’re new to the topic, here are a couple of quick mental hooks to help you remember and apply variation without overthinking:

• Variation is location-based and time-sensitive. It changes with place and, slowly, with the Earth’s magnetic field over months and years.

• It’s a bridge between true headings on your chart and the magnetic readings you actually rely on in the cockpit or on deck.

• The basic conversion rule—East is subtract, West is add—works like a compass in your head when you’re plotting or parsing a logbook entry.

Let me explain with a brief real-world vignette

Picture a small coastal aerodrome: a single runway, a chart with isogonic lines curling like gentle river bends, and a pilot glancing from the cockpit window to the cockpit instrument panel. The wind is light; the sun is setting; the radio crackles with a routine weather update. The pilot intends a true heading of 045° to skim along a preferred transect toward a coastal waypoint. With a local variation of 3° West, the actual magnetic heading in the instrument readout would be 048° (True 045° plus West 3°). It’s a tiny delta, but in the grand scheme of air traffic coordination and fuel planning, every degree counts. The pilot adjusts, keeps the flight path tidy, and saves a few minutes of drift that would otherwise compound into a correction you’d have to chase down with clever small corrections later on.

A quick nod to the technology we rely on today

Modern navigation isn’t only about paper charts and magnetic compasses. Global positioning systems and inertial navigation systems bring their own layers of precision, but many pilots and sailors still cross-check these with magnetic references. Variation is the kind of knowledge that keeps your head clear when technology is giving you data and you’re deciding how to apply it. And, honestly, there’s something satisfying about balancing old-school magnetism with digital accuracy—like listening to a vinyl record while streaming your favorite album in high fidelity.

Putting it all together: why you should care

Here’s the bottom line. Variation is the language your instruments use to tell you “the earth isn’t perfectly aligned with your compass.” It’s the difference between a course that lands you right on target and a drift that takes you off course, sometimes by a lot more than you’d expect if you forget to account for it. It’s not something to memorize and forget; it’s a continuously relevant part of navigation literacy—whether you’re plotting a coastal leg on a small boat or planning a cross-country flight across several latitudes.

If you’re curious to see how it plays out in real planning, grab a chart for a coastal region you know well and check the listed variation. Notice how the numbers change as you move along the coastline and imagine how your heading would need to adapt as you cross those isogonic lines. It’s a small exercise, but it reinforces a big idea: even the most precise compass is only as good as the angle it represents between true and magnetic north.

Closing thought: a compass’s quiet superpower

Variation isn’t flashy. It doesn’t shout like a gusty storm or flash like a beacon. But it’s the quiet adjustment that keeps your course honest. When you’re at the helm and the world feels still, variation reminds you of the planet’s grand rhythm—the magnetic field breathing softly, nudging your path ever so slightly. And if you’re ever tempted to ignore it, just remember that one good correction can save you from a long, uncomfortable detour later on.

If you want to carry this idea forward, keep it handy: the next time you look at a chart, glance at the variation value for your spot. Let that angle sit in your head as you plot. It’s a small detail, yes, but in navigation, small details often steer the big journeys.