In my naïve thinking I would just follow my compass in the south direction to find the magnetic south pole. When the compass flips around at 180° then you crossed the pole. I don't know if they understood the difference between the pole and the magnetic pole at that time.

I learnt in the Fram museum in Oslo that they did a lot of magnetic field measurements, which led to new and important scientific insights. So they navigated by compass (and sextant).

But not only. Amundsen did 'sensor fusion'. He did pace-counting and had meter-wheels fixed to sledges to measure their traveled distance. In addition to that he erected snow beacons every three miles.

The source for this is the article 'The Long Journey Home' from the magazine 'Navigation News'. But I didn't find the article itself online. I found only a blog that talks about it: https://www.naturalnavigator.com/news/2014/11/navigation-on-...

Halley published a chart of magnetic declination in 1701, so the fact that a compass doesn't point towards the geographic north pole was already understood at that time. https://en.wikipedia.org/wiki/History_of_longitude#Magnetic_...

I heard a podcast about this some time ago. They used a wheel on one of the sleds to measure distance travelled and navigated using a compass. They did rough calculations using the suns height over the horizon and time of culmination and more precise calculations using a sextant from time to time.

I suspect he had a little gizmo called magnetic compass, though that one would measure the magnetic pole, not the true pole. Difference between those 2 are 17 degrees if my memory serves me correctly. More info here: https://airplaneacademy.com/whats-the-difference-between-tru...

But they couldn't know that its 2 degrees until someone went there.