How do you establish survey control for tunnel boring machine guidance?

TBM guidance survey: establish surface control network connecting portal areas with high precision. Transfer control underground using gyro-theodolite for azimuth (avoiding magnetic interference) or forced centering traverses. Install targets on TBM for real-time tracking. Guidance system measures prism positions relative to as-built tunnel axis. Compute deviation from design alignment. Account for TBM articulation and ring build. Install back-sight targets as tunneling progresses. Control network extends with tunnel, checked against surface when possible. Accuracy requirements: +-25mm horizontal, +-15mm vertical for breakthrough tolerance. Automated systems provide continuous position updates to operator.