Trimble X7 - Best practice in tunnel surveying

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GPTag
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Trimble X7 - Best practice in tunnel surveying

Post by GPTag »

I am looking for the best practice for tunnel surveying with the Trimble X7. The conditions do not allow SX10/12 (too long recording time) or MX90 (not accurate and dense enough point cloud) recordings in the field.

Does anyone have experience with tunnel shots with the Trimble X7? What is the best way to proceed?
The C2C registration worries me a little. Is it possible to calculate a georeferencing per scan station (with the "precise point" mode)?

I am grateful for any input.

Best regards
GPTag
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Re: Trimble X7 - Best practice in tunnel surveying

Post by kpob46 »

How long is the tunnel that you have to survey? I would not rely on C2C for registration within a tunnel. Certainly use spherical targets, if you can, for registration.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by benoit.lachapelle »

Yes, the C2C in tunnel isn't a good idea. I would go with artifical targets, spheres to connect scan to scans and some checkerboard here and there to georeference it and lock the point cloud to the control network.

In that case, you can use Perspective/Fieldlink just to do a coarse registration with cloud to cloud, but than upload the full project in Trimble Realworks to do a final registration with targets and controls.

cheers,
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Re: Trimble X7 - Best practice in tunnel surveying

Post by GPTag »

kpob46 wrote: Tue Jan 11, 2022 7:26 pm How long is the tunnel that you have to survey? I would not rely on C2C for registration within a tunnel. Certainly use spherical targets, if you can, for registration.
The tunnel is about 500m long.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by landmeterbeuckx »

For tunnel surveying always, i mean always have targets measured in with a total station.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by GPTag »

Thank you for your feedback.
I will work with spheres in the field and Trimble Realswork in the office.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by rollorigby »

One thing to bear in mind, if the tunnel is curved then the wall on the inside of the curve will have a much tighter angle of incidence meaning reduced point density and often very large shadows from very small objects. Being in a tunnel you can't set up further back, so the only solution is reduce scan spacings.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by stutosney »

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.
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Re: Trimble X7 - Best practice in tunnel surveying

Post by Attila »

Sorry for the poor English. I now have a project similar to tunnel measurement. I'm monitoring a castle wall on a steep hillside. 170m long homogeneous repetitive surface. I think the X7 is suitable for tunnel scanning with the c2c method. Every 8-10 meters should be scan station point. Registration can be used if the average erorr is less than 2 mm and the overlap is about 70% or higher. If it fails, register again and again or go back halfway, scan the new one and register for the penultimate one. As a check I measured 3 precision points on each scan station if possible. I used felt-tip pen crosses, not spheres. I also made sure the laser angle of incidence was greater than 60 degrees and no further than 25 meters. I named points as follows: 1129 geodetic point from the first scan station: "a129", from the second scan station "b129" ... See the report in the attachment. I'm curious about your opinions.
Várfal2022 01 13_2022_01_20_19_46_19_Registration Report.pdf
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Re: Trimble X7 - Best practice in tunnel surveying

Post by smacl »

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.
Agreed 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.
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