Secret Tunnels

  • Traverse to entrance of tunnels

    Setting up the traverse kit outside the west entrance

  • Traverse through tunnels

    Trimble 5600 total station setup inside the access tunnel from the west entrance

  • Stairs to blocked access to Pillbox

    Stairs leading to blocked access to Northern Pillbox

This project was to provide a floor plan of the tunnels, on National grid and OS datum to show accurately where the tunnels were in relation to the surface detail. The main tunnel is about 400m long plus side areas, generally clean, dry and level. Air quality was reasonably good and well ventilated. There was no lighting inside so head and hand torches were needed. There was one entry/exit point and at the far end of the tunnel, a fixed grating in a residential back garden.

The survey required the basic floor layout in 3D, with levels at 5m intervals and ceiling levels at less frequent intervals. Any additional internal block wall detail was to be picked up. The tunnels were generally 1.2 - 2.4m wide and 1.8 - 2.4m high.

An initial GPS control network was established outside of the tunnel, from which a series of control stations with Hilti nails were installed in the tunnels. The main tunnel framework was surveyed using a Trimble 5600 series Total station with a mixture of reflector and reflector less observations. In the more restricted areas a Leica Disto and hand taping was used to complete the plan.

The survey data was converted to AutoCAD DWG (3D format) with added annotation. The drawings were output at 1:100 scale to multiple A0 sheets.

Background image: airlock viewed from west entrance, courtesy of secret-tunnels

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