Were discrepancies from the 45th parallel in the US-Canadian border ratified?

Question

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.)

Answer 1

A 4-part "Ideas" radio program by the CBC (Canadian Broadcasting Corporation), called "On the Line: A Journal of Exploration along the Canada - U.S. Border" (presenter Marian Fraser), aired in Nov 1986 and used a series of interviews to tell the story of the border. Ep 2, "Survey and Surveillance" has a section about the Quebec/Vermont border.

Dr. Alec McEwan, [then] Canada's commissioner on the International Boundary Commission, explained how the border was established: The border between Lower Canada and New York was marked in the 1760s. That the line was off due to surveyor error was known at an early time. But Vermonters were sure it was already too far south and every arbitration attempt (1790s, 1814, 1830s) to correct the survey error was stymied.

So the boundary was not settled there until 1842. The Webster-Ashburton Treaty established the line, or kept the line where it had always been, along the old so-called Valentine Collins line, along the demarcated line, approximately a mile north of the true 45th parallel, which kept the fort [Fort Montgomery, at Rouse Point] in the United States.

Answer 2

CGPGrey covered this in his video Canada & The United States (Bizarre Borders Part 2).

You're basically correct. There's been a series of treaties about the US-Canada border. Rather than go into them in detail I'll refer you to Wikipedia and the International Boundary Commission to read through the progression of the border details.

And yes, the realities of surveying a border through thousands of miles of wilderness in the early 19th century meant the US-Canada border does not perfectly follow the line of latitude. Instead, it's defined by thousands of surveyed border markers and the lines between them. This is the official border.

This is all watched over by the International Boundary Commission / Commission de la frontière internationale formed between the US and UK (on behalf of Canada) in 1908 and reaffirmed in a 1925 treaty between US and Canada making the commission permanent. There were numerous temporary commissions brought together to resolve specific border disputes, but this one was now permanent. No need to negotiate a new treaty every time there's a border dispute, let the IBC work it out.

As a side note, this treaty organization is supposed to be beholden to the treaty, not their governments, but This American Life has an interesting episode, The Audacity Of Government about US politics about private property and executive control intervening during the Bush administration.

This line is literally carved into the land by a continuous deforested 3 meter wide area called the "Border Vista". The International Boundary Commission's web site calls it "A visible line between two friendly neighbors" while CGPGrey calls it "The No Touching Zone".

The boundary vista must be entirely free of obstruction and plainly marked for the proper enforcement of the laws of the two nations. The job of keeping the boundary vista in proper condition falls to the International Boundary Commission. The Commission was founded under the Treaty of 1908 for one specific purpose: the complete re-establishment and mapping of the boundary from the Atlantic Ocean to the Pacific Ocean. The boundary had already been defined by treaty and most of it surveyed by 1874.

Source: internationalboundarycommission.org, The Boundary

Answer 3

Basically, the surveyors were drunks. For instance, see The Canada - U.S. Border,or Carusa Street or Quirky Border Towns of North America.