Long tunnel by working from both ends using tunnel boring machine, it requires stringent accuracy. A highly accurate control network needs properly set up to meet project environment.
Typically, the primary control network will connect to national and local network, distribution of control station are optimally allocated along the tunnel line. The most frequently used survey instrument are GNSS which are required with 1ppm baseline accuracy or better. For instance, if the tunnel is 50KM, the accumulated error for 1ppm GNSS system could be around 5CM. With 0.5PPM GNSS, it will be reduce to 2.5 CM on 50KM tunnel. With Stonnex S9III PLUS, it is equipped with high precision survey module and can achieve 0.1 ppm for horizontal and 0.4 ppm for vertical in static mode. In RTK mode, it can achieve 1ppm baseline accuracy.