Survey control is transferred down the tunnel from surface. How it gets down there depends on whether you launch from a shaft or pit, but once survey control is

Survey control is transferred down the tunnel from surface. How it gets down there depends on whether you launch from a shaft or pit, but once survey control is established in the hole it is straight forward to locate the TBM using standard survey techniques (eg - survey prisms). The TBM has a reflector that can be shot with a survey instrument to accurately place the machine head and the TBM itself is equipped with gyroscopic or accelerometers for backup control if survey is lost during mining.