I think they just update the model to keep GPS on track. This ain't bad overview (eiven though the author has a mere BS degree):
https://www.washingtonpost.com/scie...ent-finally-caught-up/?utm_term=.bd104885fffb
Compass? It's OK, if you get the variation and deviation right. But also useless, in a way, because steering a given course is so impossible given current, leeway and a helmsman.
What matters is knowing where you are--the miracle of GPS.
I was a thousand miles off San Francisco, and my chartplotter showed a dozen or so waypoints in our wake, arching northeast from Oahu. The new course, after climbing over the North Pacific High, was to be a long dive south toward home. So I figured, there being not much else to pass the time, what if I lost all GPS?
I noted that my whisky compass, chartplotter compass and the compass on the wheel pilot had never agreed--five degrees different at least.
I got out the protractor and charted the course home. I made the usual corrections on the paper chart and drew a line. No problem, the way we all used to navigate by dead reckoning.
It was nowhere near the GPS course for Los Angeles. Huh? I plotted the course to nearer waypoints ahead, which I typically used for weather routing based on Gribs. The course to them was different, too. I began to sweat. Wait, what course was right? I think I used the iPhone compass, too--and it agreed more with the chartplotter than my chartwork.
I never did figure the differences out. I still don't know what I did wrong. I don't think the Great Circle Route issue made that much difference.
I concluded that all I really could do was head in the "right direction" and note my current position each noon. Once, that would've been a sextant sight good maybe to a mile (or five or ten or twenty). Any long view--the correct course for a target 1000 miles away--seemed unconfirmable. Of course such a "fastest" course exists. Any chartplotter will tell you, and every commercial ship follows that fuel-saving route exactly, when possible. I just couldn't plot it on my paper chart.
I'm not an expert navigator. They win races with this stuff. They solve all sorts of puzzles with skill and experience (our navigator in the Fastnet race found the Lizard in a full gale holding a hand-held RDF in a pitching cockpit when the null was 20 degrees wide).
The result of my experience was that I have zero confidence that my Globemaster compass will get me where I want to go, swung or not.
I recall Chichester approaching a dangerous passage, I think Tasmania. He took sights every hour, in and out of fog and other opaque skies and lost horizons. He was concerned less with his course than his position. And, being the great navigator Chichester was, when the passage hove into view 10 miles ahead he was exactly where he expected to be.
Man, that ain't me. Moving magnetic north or not, it was GPS that let me sleep.