The US–Canada border runs 8,891 km, and almost none of it is actually straight. The 49th parallel looks like a ruled line on a map. It is not a line at all — it is a chain of 912 monuments joined by chords, and three separate problems bend it. • Geometry works against you. A parallel of latitude is not a geodesic. The shortest path between two points on the 49th bows poleward, so every straight segment between two monuments sits slightly north of true 49°. The offset scales as roughly L²·tanφ ⁄ 8R. At 3 km monument spacing near 49° latitude, that is about 20 cm of Canadian ground handed over per span. Trivial alone; across the plains, a systematic bias baked into the definition. • The survey was human. Between 1872 and 1876, British and American parties fixed latitude astronomically using zenith telescopes levelled against a plumb bob. But a plumb bob does not hang normal to the ellipsoid — it hangs toward mass. Near the Rockies, the deflection of the vertical tilts it by arc-second...