Oddly enough, this has a sizable impact to general aviation. Heading is typically measured using a gyro, but the heading values are calibrated to magnetic north (typically via a magnetometer or manually against a compass).
The creep of the magnetic field is noticeable in particular in the numbering of runways - they are numbered according to their heading. Some runways that have existed for a few decades are, accordingly, off by a few degrees. An older runway numbered "35" may have an actual approach heading of 340 (340 degrees from magnetic north). Not a huge problem when flying by eye, but it has the capability of causing some confusion when flying using instruments. Alternately, the runway numbers can even change.
As for GPS, this won't solve the problem either, since GPS can give you a "true" flight path using the movement over time, which with wind will not always match the actual heading of the aircraft due to wind.
This is pretty sad. For at least decades, maybe even centuries, there have existed compasses that let you rotate the "north" indicator to match the current local magnetic declination. Adjusting as you travel is no different than the pressure adjustments that pilots make for altimeters. There is no good reason we couldn't have all of the airports using geographic north. That one is far more resistant to change, with continental drift and Earth axis wobble being far slower than magnetic pole movement.
Ease and reproducibility of the measurement is more important than its absolute accuracy. Whisky-ball magnetic compasses are cheap and easy to build, and have been throughout aviation's history, and don't require access to an accurate geomagnetic survey to use.
Its interesting that you mention pressure adjustments, because:
* Above 18,000 feet in the US, you don't make pressure adjustments - because its more important to know that your neighbors in the airspace are measuring their altitude the same way you are than it is to know exactly how far off the ground you are. In other parts of the world the threshold is lower, sometimes much lower.
* Airspeed is also measured with a pressure instrument, whose accuracy is affected by the density of the air, which in turn is affected by temperature and altitude. But the speed limits in the regulations, and the speeds ATC might tell you to fly, are all based on the uncorrected output of the pressure instrument - "indicated air speed", as its called. You only try to adjust for altitude and pressure when using the airspeed for navigation.
Magnetic declination changes quite a lot as you travel the kind of distances that aircraft travel. Pilots would have to periodically adjust their declination while flying.
Declination is also not linear or varying only by latitude/longitude, but it also varies by area due to magma flows and iron deposits. Take a look at this map of global declinations for reference: https://en.wikipedia.org/wiki/File:World_Magnetic_Declinatio...
There is a good reason. An adjustable compass like that would be much more prone to user-error. Someone may rotate it too much causing very large discrepancies and land on the wrong runway.
Same can be said of altimeter adjustments which would literally cause you to fly into the ground, but that isn't stopping us from allowing manual adjustments.
Barometric pressure can fluctuate every day, and by relatively large amounts causing errors of 500ft+ so there, the 'price' of manual adjustment is worth the gain in accuracy.
I'm not convinced the same can be said for magnetic adjustment.
If you buy maps from the USGS [0], they have the magnetic north and true north indicated on them so you can adjust your compass. I didn't realize how rapidly my maps got out of date.
For naviagtion, charts are updated via [0] Notices to Mariners .
It is of course the responsibility of the mariner to mark/attach the notice to the chart.
Runway numbers roughly corresponding to heading is really just a numbering convenience and doesn’t even always happen. Many US airports are renumbering their 02/20 runways to something else to avoid pilot confusion, even though those new numbers don’t are even more off the actual heading.
Out at 57D they had 18/36 and 9/27. A few months ago they changed them all and now it's 19/1 and 10/28. On the radio they are called out by digits so "one", "one nine", and "one zero". Yeah, they should have just left them all off by 10 degrees. Fortunately the wind is usually out of the west.
The creep of the magnetic field is noticeable in particular in the numbering of runways - they are numbered according to their heading. Some runways that have existed for a few decades are, accordingly, off by a few degrees. An older runway numbered "35" may have an actual approach heading of 340 (340 degrees from magnetic north). Not a huge problem when flying by eye, but it has the capability of causing some confusion when flying using instruments. Alternately, the runway numbers can even change.
As for GPS, this won't solve the problem either, since GPS can give you a "true" flight path using the movement over time, which with wind will not always match the actual heading of the aircraft due to wind.