The treaty of 1783 between the United States and Britain said that the boundary separating Quebec from the states of Vermont and New York was to be at the 45th parallel of north latitude. (The state of New York adamantly insisted that Vermont was a part of New York, and thus this line put Vermont within the U.S., although Vermont's government at the time took the position that it was not a part of the U.S., but in light of later events in 1790 and '91, that need not concern us here.) But Google Maps shows some locations along the boundary more than a quarter mile north of the 45th parallel and some as much as a half mile north of there. (I think the Haskell Free Library and Opera House, deliberately constructed to have the international boundary passing through the building, is something like a half mile north of the 45th parallel.) I am not simply trusting Google Maps; rather, I looked at locations of official border crossings and at the Haskell Free Library.
I surmise that the discrepancy resulted from the inability of 18th-century land surveyors to be more precise than that. But my question is: Did later treaties ratify the details of 18th-century measurement errors? What are the specifics of those later agreements? Were they between the U.S. and Britain, or between the U.S. and Canada, or some of each? (Here I do not have in mind the agreement rectifying the mistakes in 1783 concerning the relative locations of the Lake of the Woods and the Mississippi River; that's quite a different story.)