Like a lot of other statements on here, there is a lot of truth to that, but there are a few "as ofs", heretofore's", whereas's that have to be considered, which is why these things occasionally end up in court.
So somebody put up a fence. Hostile/adverse possession must be open, hostile (which would be negated by giving permission, as Drake says), constant, and ongoing. The fence alone isn't an absolute answer. Nor is sneaking across it in the dark of the moon every year of twol Who uses the property on the disputed side of the fence? Cuts hay/trees, uses it as an access, parks farm equipment there, maintains a food plot,etc?

Wildland surveying occasionally relies on a compass, but more often on celestial observation, if they are looking for any degree of certainty. And, surveyors have been aware of, and account for, magnetic declination for hundreds of years.

"It’s not so simple, especially when some deed descriptions reference landmarks that may no longer be there such as trees or canebrakes or streams that may have altered their course." Typically, in Alabama, rural land surveying and mapping relies on the Township and range system, and there are indeed monuments and benchmarks at defined location at least every square mile or so. A survey ties into those known points. I have looked at GA and TN surveys that reference "the well where Mr. Jones' cow fell in,.... to the big lighning struck oak, of in the bend of the branch", but that is awfully rare here. We do have to deal with changes in water courses over time.