How this instrument works
A compass needle aligns with Earth's magnetic field, which converges near a magnetic north pole that currently sits in the Arctic Ocean, hundreds of kilometers from the geographic pole around which the planet rotates and every map grid is drawn. Magnetic declination is the angle, at a specific place, between the direction the needle points and true north. Add it, with its sign, to a compass bearing and the reading becomes a bearing you can plot against a true-north chart or topographic map.
The formula is nothing more than that definition applied directly: trueBearing = magneticBearing + declination. What makes it a genuine hazard rather than a formality is the sign convention. Declination is recorded as positive when magnetic north lies east of true north and negative when it lies west, so a west declination — common across the western United States — is a negative number that gets added, which in practice means subtracted. Get the sign backwards and a bearing meant for a ridge to the northeast can point a hiker toward a ridge to the northwest instead.
Declination is not fixed. The magnetic pole drifts tens of kilometers a year, so the angle at a given location creeps by roughly a tenth of a degree annually and can shift by several degrees across a decade — the reason a value printed in the corner of an old topographic map is worth checking against a current model, such as the NOAA/USGS World Magnetic Model, before it gets trusted.
- Enter the reading off your compass dial into Compass (magnetic) bearing, in degrees clockwise from magnetic north.
- Enter Local magnetic declination (east positive) for your location — positive for east declination, negative for west; look up the current figure if you don't already know it.
- Read True bearing, the corrected heading you can plot on a topographic map or a true-north chart.
- Working the other direction? Subtract the declination from a known true bearing to get the compass heading to walk.
Worked example — 45° compass reading, 10° west declination
A hiker in the Sierra Nevada takes a compass bearing of 45 degrees toward a trail junction. The local declination for that part of the western United States is 10 degrees west, entered as −10° since west is negative. The formula gives trueBearing = 45 + (−10) = 35°, so the junction sits at 035° true — the bearing that belongs on the topographic map, not the 45° the compass dial showed.
Skip the correction and the map line is drawn 10 degrees too far clockwise. Over a five-mile leg that error puts the plotted position about 0.87 miles off to the side — five times the sine of ten degrees — enough to miss a trail junction entirely and end up bushwhacking down the wrong drainage before the mistake becomes obvious.
Questions
What exactly is magnetic declination?
It is the angle, at a specific location, between true north — the direction to the geographic pole — and magnetic north, the direction a compass needle points. It is recorded in degrees, positive when magnetic north lies east of true north and negative when it lies west, and it varies by place because the magnetic pole is not aligned with Earth's axis of rotation.
Why is west declination a negative number?
Because the sign convention measures the angle from true north to magnetic north, with clockwise counted as positive. When magnetic north sits east of true north the angle is positive; when it sits west, the angle runs the other way and comes out negative. That lets one formula, trueBearing = magneticBearing + declination, work everywhere without a separate east or west rule to memorize.
Where do I find the declination for my location?
NOAA and the British Geological Survey jointly maintain the World Magnetic Model, and both organizations' online calculators return a current declination for any coordinate. USGS topographic maps also print a declination diagram in the margin, but that value is only accurate for the year the map was surveyed and drifts with time.
How do I convert a true bearing back to a compass heading?
Reverse the formula: magneticBearing = trueBearing − declination. If a map bearing reads 035° true and the local declination is −10°, the compass heading to walk is 035 minus −10, which is 045° — matching the compass reading in the worked example above.
Does declination really change over time?
Yes. The magnetic north pole drifts on the order of tens of kilometers a year, so declination at a fixed location creeps by roughly a tenth of a degree annually and can accumulate several degrees of change across a decade. That is why the World Magnetic Model is recalculated on a five-year epoch and an old declination figure should be rechecked before use.
Is declination the same thing as compass deviation?
No. Declination is a property of Earth's magnetic field at a location, independent of any instrument. Deviation is an error specific to one compass, caused by nearby iron, steel, or electronics — a belt knife or a phone in a pocket — and has to be measured and corrected separately, on top of declination.