Anglo-American Convention of 1818 in the context of "Rupert's Land"

The northwesternmost point of the Lake of the Woods was a critical landmark for the boundary between United States territory and the British possessions to the north. This point, on the shore of the Lake of the Woods, was referred to in the Treaty of Paris in 1783 and in later treaties including the Treaty of 1818. The point lies at the corner of the Northwest Angle in the U.S. state of Minnesota, and is thus the northernmost point of the contiguous United States. After Canadian Confederation, the point became the basis for the border between the Canadian provinces of Manitoba and Ontario.

The "northwesternmost point" of the lake had not yet been identified when it was referenced in treaties defining the border between the US and Britain; it was simply an easily described abstraction based on a large landmark. The best maps at the time of the original negotiation depicted the lake as a simple oval. However, although the southern portion of the lake is easily mapped, to the north it becomes a complex tangle of bays, peninsulas, and islands, with many adjacent bodies of water separated or connected by narrow isthmuses or straits. An 1822 survey crew declared the referenced point impossible to determine. In 1824, British explorer David Thompson was hired to identify it. Thompson mapped the lake and found four possibilities, but did not conclusively declare one location.

The Traverse Gap is an ancient river channel occupied by Lake Traverse, Big Stone Lake, and the valley connecting them at Browns Valley, Minnesota. It is on the border of the U.S. states of Minnesota and South Dakota. Traverse Gap has an unusual distinction for a valley: it is transected by a continental divide, and in some floods, water has flowed across that divide from one drainage basin to the other. Before the Anglo-American Convention of 1818, it marked the border between British territory in the north and U.S.—or, earlier, French—territory in the south.