Magnetic Declination Converter

Magnetic declination is the angle between true north and magnetic north.

The conversion:

• True Bearing = Magnetic Bearing + Declination
• Magnetic Bearing = True Bearing - Declination

East declination is positive, West declination is negative.

Example declination values (rough, mid-2020s, and drifting):

• New York, USA: about -13 deg (west declination)
• Los Angeles, USA: about +12 deg (east declination)
• Sydney, Australia: about +12 deg (east declination)
• Tokyo, Japan: about -7 deg (west declination)
• London, UK: close to zero, and it has recently crossed from west to east

London makes the point about drift better than any warning could. For more than three centuries the compass in southern England pointed west of true north. Around 2019 the agonic line, the zero-declination line, swept across Greenwich, and the correction there went from west, through nothing at all, to east. Any chart or guidebook printed before then gives London a west declination that now has the wrong sign.

Important notes:

• Declination changes slowly over time (about 0.1 deg per year in many places).
• Always use the current declination for your specific location.
• NOAA provides current declination data at ngdc.noaa.gov.

Which way do I turn? This is where people freeze in the field, and the formula alone does not help because it does not say what to do with the compass in your hand. The plain version:

• Going from a map bearing to a compass bearing, subtract east declination and add west.
• Going from a compass bearing to a map bearing, do the opposite: add east, subtract west.

The old mnemonic is “east is least, west is best”, meaning east declination is subtracted and west is added, going map to compass. Note the second half of that sentence, because half the people who remember the rhyme have forgotten which direction it applies in.

The error is not academic. At 13 degrees off, which is New York’s figure, you drift about 370 metres sideways for every 1.6 km walked. Over a day in the backcountry that is the difference between a trail junction and a hillside.

Declination is the correction that reconciles a compass with a map. The needle points to the magnetic pole, but maps are drawn to the geographic (true) pole, and the angle between them depends entirely on where you stand. Near the agonic line, where the two norths happen to align, declination is zero; far from it, it can exceed 20 degrees, enough to put you well off course over a few miles.

The catch that surprises people is that declination isn’t fixed. The magnetic pole wanders, so the correction drifts over years, which is why good topographic maps print both the declination and the date it was measured; an old map’s value can be stale. For short walks the error is negligible, but for serious backcountry navigation you set your compass’s declination adjustment to the current local value, then read bearings straight off the map without redoing the math each time.