But technically, it's correct if the land contours are ignored since gravity is assumed to be from the point where the plumb bob is suspended to the Earth's center.

Ever talked with anyone who worked on huge buildings? The distances can be so large the vertical members won't be parallel, due to the curvature of the Earth. They're plumb at that point, but not aligned with the others.

I didn't come up with that- I heard it from someone I knew who worked in construction.