Replies

Geographic North would be the same. Magnetic North would change to South but the process goes back and forth many times over a long period in terms of human lives. The field also reverses in some parts before other parts of the world. Magnetic navigation would be very tricky for a long time. But that's where GPS satellites can take up the slack.

Don't forget there is also navigation by the stars and radio navigation too. I once read a 1943 book on aeronautical navigation on how planes can use longwave beacons for navigation and in a pinch even AM radio stations. I think that's how the Japanese homed in on Pearl Harbor, the lead plane set the radio direction finder on the local AM station in Honolulu and the rest was history.

I know today we also have VOR station for aeronautical navigation and I think one of the LORAN (LOng RAnge Navigation) either A or C is still in use too for shipping. I have to check on that one, I think A was phased out. I remember as a kid picking them up just above the AM band around 1900 kc or so. The British has a similar system called Decca that was in use during World War II. You might see a return to those system if the magnetic field goes haywire.