NavList:

Frank Reed you said: "The video talks about surveying with magnetic compasses. I don't believe that this is correct. Surveying townships and all that was based almost exclusively on astronomical observations for baselines and standard ground-based techniques to fill in the details."

I agree.  I can remember one of the first lecture of my MSc course was about base land surveys and base lines.  I understood about 10% at the time (I'd never heard the word baseline before), and I can remember even less now.  However, I do remember the example used was the Survey of India, which was done by triangulation all the way up to the Himalayas.  See https://upload.wikimedia.org/wikipedia/commons/4/47/Index_chart_of_the_Great_Trigonometrical_Survey_of_India.jpg (Public Domain).  Lt Everest and Radhanath Sikdar will long be remembered, but who remembers the name of the poor chap (or donkey) who was required to carry the theodolite up those mountains? 

You also asked: “So what real phenomenon are people trying to describe, if any, when they talk about spinning compass needles?” 

They’re unlikely to spin.  They might stick or become sluggish if the vertical component of the prevailing field is increased significantly, but they’re most likely to simply align themselves with the horizontal component of the prevailing magnetic field, which will be the Earth's field offset slightly by its desire to pass through the iron.  It’s a bit like walking a dog on a lead past a lamppost.  Why didn’t they just say "a magnetic compass becomes unreliable".  I suspect, because even scientifically qualified presenters must stick to script put together by experienced screenwriters who know which words make the best television entertainment. DaveP