A gyro may be a bit overkill if thee is control at the start and end of the tunnel. Best is control at start and end which is 2x the length of the tunnel.smacl wrote: ↑Fri Jan 21, 2022 7:50 amAgreed entirely. In additional to the traverse, longer tunnels and those where tolerances are very tight with limited external control may also require a gyroscope to be used and included as part of the network adjustment. Tunnel surveying is all about robust control.stutosney wrote: ↑Wed Jan 19, 2022 10:25 am I had the 'pleasure' of surveying 13 tunnels over 200km of rail several years ago. The longest tunnel was about 700m in length and at no point did I ever consider C2C. We ended up drilling into the tunnel wall and installing spigots which can attach a survey prism as well as a scanner target (same offsets). We set a spigot on each side of the tunnel every 25m or so, so every scan had 4 targets in (in front and behind). After scanning, we traversed through with a theodolite.
Trimble X7 - Best practice in tunnel surveying
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Re: Trimble X7 - Best practice in tunnel surveying
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Re: Trimble X7 - Best practice in tunnel surveying
Why gyro is overkill is the best solution for underground.
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Re: Trimble X7 - Best practice in tunnel surveying
A gyro is fine when you cannot get through a tunnel, when you only have a start position but no end. If it has openings at both sides a 1" gun will do just fine.Ioannis wrote: ↑Fri Jan 21, 2022 10:05 am Why gyro is overkill is the best solution for underground.
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Re: Trimble X7 - Best practice in tunnel surveying
Depends on how many instrument setups you have and what accuracy specification you are working to. Even tripod to tripod on a 0.5 second instrument you will incur an incremental error based on the number of setups you are away from a fixed base line. If you have a look at the error ellipses on a tunnel traverse (or any other link traverse) this becomes obvious. The other issue you encounter is short base lines and/or short legs in your traverse where there is significant curvature. These can lead to more serious incremental angular errors. Having some gyro observations to introduce fixed or constrained bearings is a huge help here. Having retro targets on the walls can also add useful additional redundancy. Working in rail tunnels in built up areas can be very challenging in terms of meeting control specification and being able prove that specification has been met. As with most things in the survey world, the instrument dealer may imply the solution is about buying the highest end piece of kit but having the correct working method and QA is all important.landmeterbeuckx wrote: ↑Fri Jan 21, 2022 10:10 amA gyro is fine when you cannot get through a tunnel, when you only have a start position but no end. If it has openings at both sides a 1" gun will do just fine.Ioannis wrote: ↑Fri Jan 21, 2022 10:05 am Why gyro is overkill is the best solution for underground.
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