Tunnel survey needs good instruments.

Total station and level are necessary for every surveyor. But GPS is not necessarily, this work can be outsourced, after all, it is only a few control points, and it is unrealistic if the construction unit is equipped with a GPS. Need to learn and understand the construction drawing, curve calculation, coordinate calculation, and use measuring instruments for construction positioning and lofting. Salary depends on your contribution, you can't say for sure. Two or three assistants are almost enough.