[falcolas]

As I understand it (at least in the US), your land rights are based off of relative measurements from a static (to the land) point. They often look like medallions on the ground, anchored in place by fairly sizable stakes to ensure they don't easily move.

The positions of those medallions... not as sure, but I imagine they are in turn positioned according to other static points.

If the land cracks, well, I have no idea. That would be a legal battle no doubt.

EDIT: These fixed points are called "common points" or "points of beginning (POB)", and there's usually one per neighborhood. There are also apparently buried iron rods (survey pins) that define property lines that can be found using a metal detector or such, but they are not foolproof.

[foobarian]

Reminds me of this marker story: "Belgian farmer accidentally moves French border" [1]

[1] https://www.bbc.com/news/world-europe-56978344

[Symbiote]

The more common points are called trigonometrical points or triangulation stations.

https://en.wikipedia.org/wiki/Triangulation_station

https://en.wikipedia.org/wiki/Point_of_beginning

(I'm surprised there's no link between these two articles.)

https://en.wikipedia.org/wiki/Initial_point