Runways are designated by a two digit number, formed from the magnetic compass heading of the runway, rounded to the nearest 10° and dropping the last digit, so a runway pointing 243° magnetic would be designated runway 24. This number is shown on signage on all the taxiways, in information for pilots and painted on the end of the runway, in large letters, so it can be easily seen from the air.
Runways are designated this way is because aircraft operations, by agreed convention, all follow magnetic compass headings, not true headings. The reasons for this are part historical and part safety. If all else fails in an aircraft, the pilot still has an old fashioned magnetic compass to navigate with, so it makes sense for this to be the standard. In addition, some types of aircraft, sailplanes/gliders for example, often
only have a magnetic compass to navigate by, so must navigate using magnetic headings.
That leads to a problem, because over time, magnetic north moves, hence occasionally, runways need to be renumbered to take account of this. Exactly what happened at Tampa International in 2011.
Between the 11th and 13th Jan 2011, the main runway at Tampa International Airport was closed to allow all the signage and the numbers on the runway to be changed from 18R/36L to 19R/01L. The true direction of the runway is actually 002°/182°. Of course this never changes.
Here's an account of the changes made.
https://phys.org/news/2011-01-tampa-airport-runways-renumbered-due.htmlSo here we have evidence that
not only does magnetic variation/declination (essentially the difference between true north and magnetic north) exist, but it can change significantly over a few decades.
London Stanstead UK had to do the same thing, changing runway 23/05 to 22/04 in 2009, for exactly the same reason. Many other airports around the world have had to make similar changes.
In many cases the magnetic variation is small and there is only a small difference between true north and magnetic north, but there are some more extreme cases. Here are a few examples.
The green lines indicate true north, the red lines indicate the runway direction and the blue lines indicate the direction the runway would be pointing if you interpreted the runway magnetic heading as a true heading. The angle between the red and blue is the magnetic declination/variation.
These examples are all from parts of the world with relatively large variations.
If magnetic variation didn't exist, then why are all these runways pointing in the wrong direction and why did Tampa International waste a lot of money closing the main runway and changing all the signage?