The property line is the property line, no reasonable person would question that.
Although I do understand that in the US an old fence has the legal potential to override an original survey and establish an official property line.
@Silver has it right here. The original surveyors were going across the country literally dragging chains and doing their measuring, through unsettled country full of brush and forests and swamps. Can you IMAGINE how difficult of a job that would have been to maintain accuracy? And it is absolutely amazing to me just how accurate they got it, when I have a hard time pulling a fenceline though brush in a straight line. This is why you see "straight highways" that aren't straight. When they put them on a map, they're straight, but many times, they're anything but. That's how far off from a GPS coordinated survey the original survey was though.
The point then is, the
ORIGINAL survey is what counts... wherever they set the pins ORIGINALLY, that's what you have to go by. Typically, when the land was homesteaded, a fence, placed ON that property line, was one of the first things that had to be put up, to designate what is your property, and to actually have an "improvement" in order to receive it from the government as homestead property. That fence then WAS the property line, literally. And there "should be" some original corner markers (called monuments), or at least "quarter section markers" or something that they must reference from. The "new" surveyors simply go off of the references on the property abstract, using GPS coordinates... which may or may not be exactly where the "original survey" back in the 1800's had placed the pins.
As far as property line fences... they are SUPPOSED TO BE BUILT right on the property line (not off to one side or the other "to be safe"), to intentionally designate "what's yours" and "what's your neighbor's" property. That setting it off just a bit is a sure way to have a property line dispute in the future! Each of you is responsible for half of that fence too... even if only one of you wants a fence. At least that's Minnesota's fence law. Meet your neighbor at the property line, facing each other, at the center point of your shared property line. Shake hands there like good neighbors should... whatever is to your right side is "your fence" to pay for, install and maintain. If you can't agree on what kind of fence has to be installed... it goes to the "fence authority"... which in our area is the township supervisors. There are "minimum standards" in Minnesota law that can be applied. What they determine becomes enforceable as the law.
Current MN law says that a fence is only "required" if ONE of the property owners wants it. If neither of them wants a fence though, it can be eliminated... and often, there's no corner markers or anything left in place. And then also just as often, once the fence is removed, greed sets in, and farmers "crowd that line" with their tillage equipment... and if one is just a little pushier than the other, he'll eventually be farming a fair amount of his neighbor's ground. Had it right here with my one neighbor who insists on getting every sq. in. ... the other neighbor was in the nursing home, renting to another guy... so the "pushy neighbor" got away with alot... in the end, he had 6 rows of corn over the line for a number of years. I mentioned it to the neighbor in the nursing home once when I went to visit him... and he had a survey done and took back his land. There's markers every so often along that line now... but "less of them" as the years go by, because that neighbor still has to plow right up to those T posts (we put a 10' piece of 2" PVC over the T post, so you can see them for sure... but with this wide equipment, it's pretty easy to swipe down a post).
I WISH that the law still REQUIRED a minimum system of fencing around all rural properties... it'd eliminate alot of property line disputes. They never should have been removed. If you split off a parcel from your property, it should have to be fenced ON THE PROPERTY LINE before selling it. If I'm replacing a "marker post" (like an old rotted wooden post with a new one), I make sure it goes right back in the same hole, so nobody can claim that I moved it. Lately I've been replacing all the wood with fiberglass posts... they should last forever pretty much... or at least well past MY time here.
Good fences make good neighbors!