(1) By measuring the location of a few designated landmarks, it should be possible to extrapolate tectonic-level shifts to all other points on the same plate pretty accurately.

(2) Changes on a smaller scale, e.g. due to a landslide, are probably less surprising when they cause a change in location codes. On the other hand, there might be some houses on a hill somewhere slowly sliding down, where the system doesn't work.

(I also realized that my previous comment can be read as supporting a flat earth theory, especially the part about the sphere being imaginary. I firmly believe that the earth is a bumpy ellipsoid, though.)