1mm accuracy

Please post any tips or advice you have in general relating to laser scanning either whilst onsite or office based.
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Jamesrye
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1mm accuracy

Post by Jamesrye »

I've had a couple of enquiries recently where the client is asking for a laser scanning survey that provides 1mm accuracy on hard surfaces (concrete slabs). For sure, precise levelling can achieve this- but laser scanning?

I am curious to know what is the most accurate tripod mounted laser scanner on the market at the moment.

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Formula1982
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Re: 1mm accuracy

Post by Formula1982 »

Jamesrye wrote:
Thu Mar 05, 2020 9:29 am
I've had a couple of enquiries recently where the client is asking for a laser scanning survey that provides 1mm accuracy on hard surfaces (concrete slabs). For sure, precise levelling can achieve this- but laser scanning?

I am curious to know what is the most accurate tripod mounted laser scanner on the market at the moment.
If people really need that level of accuracy after being asked what they're using the data for in detail to make sure they really need it and they're not just quoting some semi-related manufacturer tolerances... I usually throw a Surphaser 400 at the problem. Not really ideal if it's a large area though. Trying to maintain accuracy to 1mm over a network if it's a larger area...much more difficult.
You could consider one of the new Hexagon scanning laser trackers. But the client may decide the 1mm accuracy is a bit more flexible after they've seen the quote prices including ever more expensive equipment.

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Daniel Wujanz
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Re: 1mm accuracy

Post by Daniel Wujanz »

Dear James,

have a look at Surphaser in this case; http://surphaser.com/.

However, the registration error is much larger than the measurement accuracy.

B.t.w. Achieving 1 mm accuracy with a total station is tough and in the league of engineering surveys. I know that people claim this on a daily basis but in my opinion this is dodgy and consequently causes harm to the entire industry.

I suggest to talk to your client and ask him how he computed the required quality measure. Another option is to add two options to your quote: the 1 mm option as well as your usually stated accuracy. As soon as clients see the price of high-quality measurements, they really start thinking.

All the best

Daniel

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Daniel Wujanz
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Re: 1mm accuracy

Post by Daniel Wujanz »

Ooups, Stephen was quicker - anyway we meant the very same ; )

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Re: 1mm accuracy

Post by Jamesrye »

The Surphaser does seem to have sub mm range capability but that's not the same as 3D accuracy.

The Surphaser has an angular error of 25" (P40 is better in this respect) which contributes 1.2mm at 10m range or 2.4mm vertical error at 20 range !

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Re: 1mm accuracy

Post by Daniel Wujanz »

Their angular precision is either 15 or 25" depending on the model. We don't know anything about your intended survey configuration. Hence, I cannot perform error-propagation in order to estimate the resulting 3D-accuracy.

Again, registration will be far beyond these numbers, so the question is how many viewpoints are required to capture these slabs.

Cheers

Daniel

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Re: 1mm accuracy

Post by jcoco3 »

Wow 1mm :o Are they moving space shuttles around on this slab? :lol:

I tend to agree with the registration error propagation being a big concern, but I think I would be more concerned about the next step, as in what exactly is being delivered? A registered point cloud? Seems like there would be too much inherent range noise to deliver something that would be precise enough to say its accurate to 1mm (or plus or minus 0.5mm).
We scanned (with Faro x330) a large 30' diameter 50mm thick steel vessel that was being subjected to intense pressure and heat in an effort to locate small non-uniform expansions on the surface. We scanned periodically over the startup period (increasing pressure and heat) and was able to finally detect 3mm after first observing the expansion when it made it to 12mm. In other words we had to back in to the 3mm post as we thought it was simply noise on the initial scans. Anything below 3mm was deemed noise due to the inaccuracy of the system including the scanner's range noise on the somewhat poor reflectivity surface, the inclination inaccuracy and the unknown motion of the structure and platform, and of course registration error propagation. It might be possible with the right scanner to get better results, but it seems like this is kind-of attacking the problem the hard way (not that I am opposed to that).

Would it not be less risky for you to scan the slab with less of an accuracy guarantee (perhaps a number you know you can hit), so they can get a more general idea of areas that might need to be filled or ground down, then also provide them with a leveling service using a laser tracker to facilitate leveling of every component after the slab is poured? If their accuracy requirements are truely that precise then I cannot imagine that the mounted components would be designed without leveling screws.


On second thought, how did they level the slabs for the space shuttles at Nasa anyway? I heard a long time ago that they were the most level/flat slabs on earth, but how did they do it and who did it? Maybe you can just call them and ask. :lol:

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Re: 1mm accuracy

Post by stutosney »

It is all relative as well, what do they want the accuracy to relate to? The control, if any is even installed, or the single points taken on the slab? As someone else mentioned, you could use a total station and in theory get 1mm accuracy, but when you factor in traversing and control, that 1mm accuracy is only relative to itself. I personally would have a chat with the client and find out what exactly they require as their expectations may come from a lack of information. Good luck, keen to hear how it works out.

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Re: 1mm accuracy

Post by landmeterbeuckx »

Mostly these people who ask this don't have a clue. Mm accuracy can only be done with 1" total stations or better (the 0,5" sokkia one). And then it still depends on factors : you're measurement will be at 1mm but if your peanut prism is slightly off level you loose.

Stupid specs, Besides cnc manufacturing no one can build this secure!

Avoid this client i would say and let some other fool deal with them :lol:
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Re: 1mm accuracy

Post by Scott »

landmeterbeuckx wrote:
Thu Mar 05, 2020 7:38 pm
Avoid this client i would say and let some other fool deal with them :lol:
+1

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