# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DaqsRRvyH5g
- **Gondola URL:** https://gondola.cc/posts/67635461-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/47da5edb5f.jpg
- **Posted:** 2026-07-11T21:06:43.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

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-seconds, and one arc-second of latitude is about 31 metres on the ground. The monuments drifted accordingly.

• The treaties made the error permanent. Rather than resurvey, the 1908 and 1925 treaties declared the monumented line to be the boundary. The legal border is not 49° north. The legal border is wherever a Victorian surveyor drove an iron post.

Everything else follows from that decision:

• The Slash: a six-metre vista cleared through forest along roughly 2,200 km of boundary, maintained continuously by the International Boundary Commission. Vegetation is cut on a rolling cycle so the line stays visible from the air.

• The Northwest Angle: a fragment of Minnesota stranded north of the 49th because the 1783 negotiators trusted the Mitchell Map, which placed the Mississippi headwaters somewhere they do not exist. Reachable by road only through Manitoba.

A border is not geography. It is an instrument reading, frozen by law.

#stemantics #geodesy #surveying #cartography #engineering

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## Tags

stemantics, cartography, geodesy, engineering, surveying

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