surveyor

I don't know what your exact definitions is of "cheap", "millimeter-level", and "every few hours", but your comment does sound a bit misinformed.

What you are possibly referring to is called dimensional control in geomatics and civil engineering, and there are various measurement techniques that can be applied, including GNSS-based.

In a way, the hardest part of it is the planning phase, where you have to decide which technique is most appropriate to meet precision, accuracy, timeliness, and cost constraints. All those factors kind of fight against each other. Finding the sweet spot is non-trivial.