Online Tool Store
· 4 min read
How to Convert Magnetic and True Bearings
Manesh Jayawardhana
CIO & Co-founder
A compass points at magnetic north. A map is drawn to true north, or to grid north. The angle between them is declination, and applying it backwards produces an error of twice the declination — which over a few kilometres puts you somewhere else entirely.
Three norths
True north is the geographic pole, where the lines of longitude converge. It’s what a map’s latitude and longitude grid is based on.
Magnetic north is where the Earth’s magnetic field points, which is not the geographic pole and moves measurably over time.
Grid north is the direction of the vertical grid lines on a projected map. Because a curved Earth is being drawn flat, grid north deviates slightly from true north — negligibly near the projection’s central meridian and increasingly at the edges.
Most topographic maps print a diagram showing all three and the angles between them, along with the date the declination was measured and its annual rate of change.
Getting the direction right
East declination means magnetic north lies east of true north:
true = magnetic + east declination
magnetic = true − east declination
West declination reverses both.
With a declination of 4.2° East, a magnetic bearing of 127° is a true bearing of 131.2°. Going the other way, a true bearing of 127° is 122.8° magnetic.
Apply the correction backwards and you get 122.8 where 131.2 was needed — out by 8.4°, twice the declination. Over five kilometres that’s roughly 700 metres off target, which in poor visibility means missing a feature entirely.
The mnemonic used in navigation courses is “east is least, west is best” — subtract east and add west when going from true to magnetic. Whichever mnemonic you use, sanity-check the result: if the correction moved the bearing the wrong way, you’ve applied it backwards.
| Converting | East declination | West declination |
|---|---|---|
| Magnetic → True | Add | Subtract |
| True → Magnetic | Subtract | Add |
Declination changes with time and place
It varies by location — substantially across a large country — and it drifts year on year as the magnetic field changes.
A map printed a decade ago states the declination at the time of survey and usually an annual rate of change. Applying the printed figure without adjusting for elapsed years introduces an error that grows steadily.
For current values, national geological or geospatial agencies publish declination calculators based on the global magnetic model, updated every few years. That’s the authoritative source, and it takes a moment.
Near the magnetic poles declination changes very rapidly with position and compass navigation becomes unreliable — a specialist problem, and worth knowing exists.
Local magnetic interference
Declination is the large-scale correction. Locally, iron and steel deflect a compass regardless.
Vehicles, fences, rebar in concrete, mobile phones, camera bodies and belt buckles all affect a needle at close range. Take bearings well away from metal and away from your own equipment — a phone in a chest pocket is enough to matter.
Some geology is magnetic too, and in a few well-documented areas compass bearings are locally unreliable. Maps of those areas usually say so.
Common mistakes to avoid
- Applying the correction in the wrong direction.
- Using a decade-old printed declination without adjusting for drift.
- Confusing grid north with true north on a projected map.
- Taking a bearing next to a vehicle or with a phone in a pocket.
- Trusting a single bearing over a long distance without checking against terrain features.
How to do it with Compass Declination Calculator
The Compass Declination Calculator converts in both directions with the sign convention explicit.
- Find the current declination for your location from an official source.
- Note whether it’s east or west — the sign determines everything.
- Convert in the right direction, and sanity-check that the result moved the way you expected.
- Take bearings away from metal, and verify against a known feature before committing.
Other navigation tools are in the tools directory.
Frequently asked questions
Which way does the correction go?
East declination is added converting magnetic to true, and subtracted going the other way. West is the reverse. Applying it backwards doubles the error.
Does declination change over time?
Yes, measurably. The magnetic poles move, so declination drifts by a fraction of a degree a year in many places. A map printed a decade ago may state a figure that’s noticeably out.
What is grid north?
The direction of the vertical grid lines on a projected map, which differs slightly from true north away from the projection’s central meridian. Maps usually print all three norths and the angles between them.
Final thought
After converting, check the bearing moved the direction you expected. That one-second sanity check catches the error that doubles rather than corrects.