Last week I wrote a faintly ranty piece listing 13 things about the map of the US that wind me right up. A lot of people on Twitter pointed out that I’d missed a good one. So…

It’s 1783. The American War of Independence is coming to a close, everyone is knackered, and among the many pressing questions facing those drawing up the peace treaty is that of the new nation’s northern boundary: where United States territory stops, and British North America begins. This challenge is made all the more difficult by the fact that, west of the Great Lakes, everyone’s knowledge of the region’s geography gets just a little bit hazy.

But, the negotiators do what they must, and the Treaty of Paris states that the boundary will run:

“…through the Lake of the Woods to the northwestern-most point thereof, and from thence on a due west course to the river Mississippi.”

Back in 1783, the land west of the Mississippi was French territory, so this seemed to settle the matter: find the northwestern-most point of the lake, draw a line west until you hit the river, and then you’re sorted.

How you calculate the northwesternmost point of a lake: it involves drawing a lot of lines from north east to south west. Image: Strafpelaton 2/Wikimedia Commons.

There was just one problem: despite what the maps of that era showed, west of the Lake of the Woods, there was no River Mississippi. The then unknown source of the river was Lake Itasca, in the north of Minnesota about 180km south of the Lake of the Woods.

The red pin marks the source of the Mississippi. Note that it is a surprisingly long way from Canada. Image: Google.

Later treaties would correct this somewhat, by placing the boundary to the west at the 49th parallel (that’s the long straight line at the top of maps of the US). To the east, though, it was already established, so the stuff about the northwestern-most point of the Lake in the Woods stayed: the two bits of the border were linked by a straight line running north-south.

The result was a border that looks like this:

Image: Google.

Spot anything odd about that border?

Image: Google.

See it now? Here’s another map, from Wikipedia.

Image: The Illusional Ministry/Wikimedia Commons.

That looks weirdly like the top right corner of the map is actually an inset, a larger version of something else on the map. It’s not: it’s just a part of Canada (Manitoba, to be specific) with square boundaries.

And, because those boundaries are so straight, a couple of bits of land have been cut off, and ended up in the United States.

The larger is the Northwest Angle: home to around 120 Minnesotans, who can’t reach the rest of the state without either crossing a lake or going through Canada. It’s only 319km2, which isn’t very big; but that’s still around twice the size of Washington DC, so it’s not that tiny either.

The other cut off is Elm Point, a small cape which isn’t home to anyone at all, but is nonetheless considered US territory. Both of these, incidentally, are “practical exclaves” of the US: portions of territory that can’t be reached by land without going through someone else’s country. (They’re not enclaves because Canada doesn’t completely surround them.)

None of this really matters in practical terms since the US-Canada border is one of the most permeable in the world. There’s not talk of building a wall here (wonder why) though you are, technically, meant to phone customs as you cross the border to make your declaration.

But it does teach an important life lesson: don’t define boundaries on the basis of a river without checking where that river actually goes first.

For more stories of silly boundaries, click here.

Jonn Elledge is the editor of CityMetric. He is on Twitter, far too much, as @jonnelledge.

Want more of this stuff? Follow CityMetric on Twitter or Facebook.