Compass Orientation Distance

Compass Orientation and Magnetic Declination

A magnetic compass swings toward the magnetic north pole rather than the geographic (true) one. At any given spot, the angle between those two directions is the magnetic declination, sometimes called magnetic variation. Converting a magnetic bearing into a true bearing is straightforward: true = magnetic + declination_east, and you subtract when the declination is west. The catch shows up over distance. A 5° error you never corrected for will leave you roughly 440 m off after just 5 km.

Worldwide, declination spans roughly −25° to +25°. Inside Brazil it currently sits between about −19° in the far west and +12° on the northeast coast. The number isn’t fixed: it drifts a few minutes of arc each year as the flow in Earth’s core shifts, which is why it gets refreshed annually through the WMM (World Magnetic Model) maintained by NOAA/NCEI and the British Geological Survey, with IBGE publishing the official Brazilian isogonic charts. Phones and GPS receivers handle the correction for you. Plain map-and-compass work does not, so the burden is on you.

Applications

Hiking and orienteering, search-and-rescue, topographic surveying, aviation flight planning (runways carry numbers based on magnetic heading and get renumbered once declination has drifted far enough), naval navigation, geological field work, and military land navigation training all lean on it.

FAQ

How do I find the declination for my location? The NOAA WMM online calculator works, as does the IBGE isogonic chart for Brazil. Any modern topo map will tell you too: look in the legend, where the declination appears alongside its annual rate of change and the year it was measured.

East or west, which sign do I add? There’s an old mnemonic for this: “east is least, west is best.” With east declination you subtract from the magnetic bearing to reach true; with west declination you add. Put another way, true = magnetic + E, where west counts as negative.

What can deflect a compass reading? Plenty of everyday metal: a knife, a belt buckle, a car body, rebar. Add high-voltage power lines, the magnets buried in phones and headphones, and the odd magnetic mineral deposit. Keep the compass at least 50 cm away from anything ferrous.