True, magnetic and compass heading
There are three different norths in aviation and three headings to match. Getting between them needs variation and deviation applied in the right direction, which is where the mistakes happen.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
True heading is measured from the geographic north pole, and it is what charts and great-circle calculations use.
Magnetic heading is measured from magnetic north. The difference between the two is variation, which depends on where in the world you are. Runway designators and instrument plates are magnetic.
Compass heading is what the wet compass actually reads. The difference from magnetic is deviation, caused by metal and electrics in your own aircraft, and it changes with heading, which is why there is a correction card next to the instrument.
When this is useful
- Turning a true course from a chart or a great-circle calculation into something you can fly.
- Working out the true wind direction when the tower has given you a magnetic one.
- Any written examination question on navigation, where sign errors cost more marks than anything else.
How it is calculated
The old saying is "east is least, west is best". Going from true to magnetic you subtract easterly variation and add westerly variation. Going from magnetic to compass you do the same thing with deviation.
Rather than ask you to get the sign right, this calculator takes the size of the variation and deviation as positive numbers and asks you to pick east or west from a list. It then shows its working so you can see which way each correction was applied.
magnetic = true + westerly variation, compass = magnetic + westerly deviation
Worked example
- Known heading
- 090 true
- Variation
- 10 degrees west
- Deviation
- 2 degrees east
Magnetic heading is 100 degrees and the compass heading to fly is 098.
Check: West is best, so westerly variation is added going from true to magnetic: 090 plus 10 gives 100. East is least, so easterly deviation is subtracted going from magnetic to compass: 100 minus 2 gives 098. Both steps match the standard mnemonic.
Limitations
- Variation changes slowly over the years and differs across a route. Take it from a current chart for the area you are flying in, not from memory.
- Deviation comes from your specific aircraft and is different on every heading. Without a compass correction card, leave it at zero and treat the compass heading as approximate.
- None of this accounts for wind. Heading here is where the nose points, so apply the wind correction angle separately.
Common mistakes
- Applying variation the wrong way. This single error is responsible for more navigation mistakes than any other, which is why this page asks for east or west explicitly.
- Using the variation from your departure airfield across a long route where it changes.
- Confusing deviation with drift. Deviation is an instrument error, drift is caused by wind.
Sources
- Pilot's Handbook of Aeronautical Knowledge, navigation chapter , US Federal Aviation Administration
- World Magnetic Model , NOAA National Centers for Environmental Information