What is magnetic variation?
Magnetic variation (also called magnetic declination) is the angular difference between true north and magnetic north at a given location. A compass doesn't point to the geographic North Pole. It points to the magnetic North Pole, which is somewhere in northern Canada and moves around over time.
If the magnetic north pole is to the east of true north from where you're standing, the variation is east (positive). If it's to the west, the variation is west (negative).
Why aviation uses magnetic headings
All headings in aviation are magnetic. When a controller says "fly heading two seven zero," that's 270 degrees magnetic, not true. When a pilot reads a compass, it shows magnetic heading. VOR radials are magnetic. Runway numbers are magnetic.
The reason is practical: compasses are magnetic instruments. Before GPS, pilots navigated primarily with compasses and VOR receivers. Building the entire system around magnetic references made everything consistent. A VOR radial of 270 degrees and a compass heading of 270 degrees are the same thing.
True north would be more "correct" from a navigation standpoint, but the system was built around magnetic instruments and now everything depends on it.
How much does it vary?
Magnetic variation changes dramatically depending on where you are:
| Location | Approximate variation |
|---|---|
| US East Coast | 10-15 degrees west |
| US West Coast | 12-16 degrees east |
| Central US | 0-5 degrees east |
| UK | 0-2 degrees west |
| Northern Canada | 15-30 degrees west |
| Near magnetic poles | 40+ degrees (compass nearly useless) |
In the middle of the US, variation is close to zero, so true and magnetic headings are nearly the same. On the coasts, there's a meaningful difference. At high latitudes near the magnetic poles, variation can be extreme, and magnetic compasses become unreliable.
Change over time
The magnetic poles drift. The magnetic North Pole has been moving from northern Canada toward Siberia at about 30-40 miles per year. This means magnetic variation at any given location slowly changes.
This is why runway numbers occasionally change. When the magnetic heading of a runway drifts enough that rounding changes, the runway gets renumbered. A runway aligned to a magnetic heading of 274 degrees is called runway 27 (heading divided by 10, rounded). If variation shifts and the magnetic heading becomes 265, it might get redesignated as runway 26.
It doesn't happen often - maybe once every few decades for a given runway. But it does happen. For example, London Stansted changed runway 23/05 to runway 22/04 in 2023.
Runway numbers
Runway numbers are the magnetic heading divided by 10, rounded to the nearest whole number. Runway 27 points roughly 270 degrees magnetic. Runway 09 points roughly 090 degrees. The two ends of the same strip of pavement are always 18 apart (180 degrees): runway 09/27, 14/32, 01/19.
When the number is the same for parallel runways, they get a letter suffix: L (left), C (center), R (right). JFK has runways 04L/22R, 04R/22L, 13L/31R, and 13R/31L.
Agonic line
The agonic line is where magnetic variation is zero - true north and magnetic north are the same. In North America, it currently runs roughly through the Mississippi River valley, from Louisiana up through Wisconsin. On this line, a compass points to true north.
East of the agonic line, variation is west. West of the agonic line, variation is east. The memory aid is "east is least, west is best" - east variation means magnetic heading is less than true heading, west variation means magnetic heading is more.
In radarcontrol.io
The sim uses simplified navigation where headings are effectively true headings. Real-world magnetic variation is not modeled. When you assign heading 270, the aircraft turns to 270 degrees without any magnetic offset applied.
For practical purposes, this makes very little difference to the gameplay. The headings you assign produce the expected ground track, and runway alignments work correctly. Runway numbers in the sim match real-world runway designations - if the real airport has runway 28L, the sim has runway 28L. The magnetic variation that produced that number just isn't part of the simulation math.
In the real world, controllers work with magnetic headings intuitively. They don't convert between true and magnetic in their heads - everything on the scope, every procedure, every instruction is already in magnetic. The sim gives you that same experience, just without the underlying magnetic model.
Command reference | Play Washington Center
Related: What is a flight level? | What are airways? | How runway selection works
Play Denver Center - manage traffic across the Rockies where variation swings east.