I'm up north, and I have a whack of GPS equipment on board that tells me which way is which, so the lowly whiskey compass at the top of the windshield doesn't get much regard. Not that I disregard it. A compass is a very cool device, incredibly low tech, but working all over the planet without power. I can imagine an advanced technological society that had lost the knowledge of compass navigation. They could have built so many structures, electronic devices and power transmission lines that compasses didn't work most places, and have so much signage and technological assistance that no one used them anyway.
The Earth's magnetic field lines, with which a compass aligns, encircle the planet emerging from and disappearing into the poles. My compass is a hemispheric shell suspended by a point in liquid (the "whiskey") and free to rotate and tilt to align with the local magnetic field. I can't use it to roll out on a heading because the turning errors are enormous at this latitude, and even in level unaccelerated flight the tilt of the bowl is such that it's not easy to read.
The HSI is a kind of stabilized compass, as it gets its information from a magnetic compass, but right now mine isn't working. It hasn't been removed from the aircraft, because we're planning to go south to Edmonton soon. There's an avionics shop with an excellent reputation there, and the AME wants them to examine the HSI in situ. That would allow them to spot if it's something simple and removes the risk and cost associated with removing and shipping it, especially as we're still orbiting the black hole of postal and shipping services. In the absence of an HSI, I'm setting the copilot side directional gyro; habit and original training makes me look at the compass to do this, even though I have far more information available from the GPS and INS.
I'm tracking 270 degrees true, and the compass reads 260 degrees, which is obviously a magnetic reading. The chart says the local magnetic variation is 23 degrees east of true. The compass correction card says for 270 degrees steer 272, and is reasonably consistent from south through west, and it's dated last month. The computer says I'm crabbing four degrees to the north. How do I put that together to make any sense?
270 true minus 23E suggests we're tracking 247 degrees magnetic, but we're crabbing four degrees north, which makes a heading of 251 magnetic, for which I should steer 253 magnetic. So there are seven degrees missing. I'm willing to believe it's the result of all the extra kit that wasn't running when the compass was swung, because we stow some of the valuable non-aviation equipment before giving the airplane over for extended maintenance. Either that or the universe is wrong.
Eastbound on the exact reciprocal track of 090 true, the compass reads about 035, but I didn't write all the associated numbers down, so I can't play with them.