Stupid question…but when a piece of land gets resold or divided up and resurveyed, do they do it with modern means, GPS coordinates and all that jazz? Or are you bound by the archaic methods?
Like Badger said, a good surveyor will likely use modern equipment while still applying his expertise and time-tested methods. Some of those old surveyors were commendably accurate, for their day. You have to figure that when a guy is dragging chains through the woods and following creek/river lines with a compass in his hand and calculating arpents or whatever....if he winds up close to reality, that's a good job.
Badger would know, but I think modern guys use all kinds of cool stuff. Lasers, and such (not the sci-fi kind that can kill you).
I've never seen a modern field-note description that didn't wind up calculating acreage at least slightly differently than old surveys. You would expect that to a certain degree with modern equipment. The other thing is those creeks and rivers move a little bit over time. The water changes your boundaries as it remakes the property. The legal side of it (in Texas, at least) is not necessarily clear-cut. You might assume that if the edge of a property was originally a water line, then the property line remains consistent with that body of water. And sometimes that will be the ruling in a disputed matter. But other times it may be found that the property line extends to the best estimate of where it used to be, the judge ruling something along the lines of saying that just because some land eroded, that doesn't mean a guy owns less of the Earth than he used to, or the guy on the other side owns more.
It's also not legally set in stone which set of surveyor's notes is going to hold sway. Formal legal descriptions tend to be based on surveyors' field notes. The trick is, you can't just change your legal description because a tract got resurveyed and now it's said to comprise 5 more (or less) acres. One thing we were never allowed to do was make legal judgements. I could complete the title, gather the data, put reports together and point out discrepancies, but I could not change a legal description. Frequently, that kind of stuff went to lawyers to sort out amongst varying parties.
Bear in mind, I'm thinking about this stuff in terms of calculating the total acreage of a piece of land for the purposes of knowing how much money someone should be paid for a mineral lease. Suppose a piece of land has long been legally described in deeds and wills as 300 acres. But then say the most recent survey--and the first in 50 years--when plotted out, contains only 295 acres. Does the guy legally own 300 acres or 295 acres? Do I pay him only for 295 acres or do I have to pay him for the extra 5? That kind of thing tended to ultimately sorted out by lawyers, even if we immediately lock up a lease and then later have to amend it. For me, all the recorded surveys on a property ultimately were to help me make sure I'm dealing with the piece of land I'm supposed to be dealing with. If Badger tells me this tract is 296.0532 acres, all I can really do with his updated, modern expertise, is note it and use it as confidence in my chain of title.
So if a modern survey is 25 acres off from an old survey, Badger is probably asking "How in the world did these people survey this land? On meth?" I'm asking "Which one of these do I pay on?" Badger knows the answer to "How many acres is this, definitely? Often times lawyers have to tell me the answer to "How many acres do I actually pay for?"
and, crap.....I just re-read your comment and realized I typed all this as an answer to something you didn't really ask. Oh well, posting anyway.