[mojoB]

RTK GPS is quite common for levelling applications. It's closer to 25mm in the vertical when a local (~10km) base station is used. Lasers are used but have line-of-sight and atmospheric distortion issues.

This [1] is a good overview of how RTK works.

[1] http://www.novatel.com/an-introduction-to-gnss/chapter-4-adv...