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

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.