The least expensive option for precision work has long been the level. Gravity has always been pretty reliable from what I've heard. For example, I've never heard of a "gravity storm".
The earth is not stiff enough to resist deformation. If you walk by a sensitive level it will notice the earth around it move under you. You think an HLV is stiffer?
You are far beyond the type of help anyone on a internet forum could possibly provide.