Wet spheres

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DCATR3D
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Wet spheres

Post by DCATR3D »

While scanning the other day it began to rain. Just a light drizzle. Does water on sphere targets affect the scans?
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jcoco3
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Re: Wet spheres

Post by jcoco3 »

Don, I would say yes, but does it make a sphere unusable? Would have to test to be sure. Many variables to consider. Depending on the wavelength of the laser scanner you are using you would get some different results. If its 1550nm I would expect the spheres to have many missing points where each water droplet absorbed the light, and if 905nm I we expect the light to reflect in unusual ways producing a noisier result. The distance to the spheres and scan settings would also affect the result, as well as your scan filter settings after import.

If I were to test this, I think I would setup half a dozen spheres outside in the parking lot with two spheres at the center about 3 feet apart from each other. One would be dry the other sprayed with garden hose. Do half a dozen scans around the two in the middle, and exclude them from the registration using only the outer spheres. Then go back and manually pick spheres targets on the two middle spheres from each scan. Don't update or re-register the scans, simply export those middle spheres' coordinates to examine in CAD. You might be able to just measure the distances between those sphere centers in your registration software, I just prefer CAD. I would expect that the wet sphere would report a larger average magnitude of distance between the calculated centers than the dry sphere, but depending on you setup and scanner it could be minimal.

I imagine many others here have more experience with the wet sphere scenario, but it might be helpful to make sure you are using similar equipment and software before making your final conclusions.

Edit: Its been a long time, but I would be lying if I said I never used a wet sphere in registration. In my case the registration results were good so I did not dig any deeper at the time...looking back I probably shod have at least examined the points on a wet sphere to see if more noise or voids could be observed.
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Re: Wet spheres

Post by DCATR3D »

jcoco3 Thank you for the quick reply.
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Re: Wet spheres

Post by DCATR3D »

The reason I was asking is, for this project, I will be returning to layout the rail posts with the total station. I set up spheres so I could transfer the points from TRW back into TFL. When I transferred them 1 point was misaligned with the scan. Looking closer I noticed 1 scan (#20 out of 22) was not registered correctly. in TFL. I had missed that when I exported to TDX. After TRW refined the clouds, it corrected it. Once I registered it correctly in TFL the point location matched TRW. I don't believe the wet spheres affected the scans.
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Re: Wet spheres

Post by jcoco3 »

Good, that is about what I would expect. Even with some noise or points missing on spheres there are typically so many points that it doesn't matter. If anything I think you would see a reduction in effective range as the quality of the sphere fit decays. I think it would have to be an awfully wet sphere to run into any serious problems.
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