> The dam wall will have very accurately surveyed points
It's very likely that there will be a fixed surveying infrastructure in place, intended for use with something like a Leica TotalStation. There may even be retroreflectors mounted on the dam to assist. So, somewhere, there is likely to be a fixed position for mounting a TotalStation in a repeatable (to sub-cm accuracy) position. It's the relative movement of dam to surroundings that's important, not the absolute position of the dam...
I was on the Tube at Paddington station the other day (Circle line, I think), and whiled away the wait watching the fixed surveying (a TotalStation) doing its stuff, going around the fixed retroreflectors, taking constant measurements. I assume there must have been some problem with movement that they're monitoring.