Good Overlap

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leenhoy
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Good Overlap

Post by leenhoy »

Hi Guys,

May I know what is Good overlap means in laser Scanning and how to check it in the site if you had a Good overlap
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Matt Young
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Re: Good Overlap

Post by Matt Young »

In general terms it's the amount of overlap between each scan. Ideally you would want at least 20 to 25% of overlap between each scan, in order to run cloud to cloud registration algorithms. This applies to all scanners and software generally.
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Re: Good Overlap

Post by leenhoy »

Thank You Matt, but please tell me the checking points and how the overlap justified before leaving the site
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Re: Good Overlap

Post by Scott.Warren »

Heyo,

It depends on the equipment you use, as some equipment may have a screen or companion tablet that you can visually check to see that you have 20%-25% of common data between scans. If your scanner doesn't have a way to view the data you collect in the field, then it comes down to experience. I would recommend for each static scan taken, you place a mark (paint, tape, chalk, etc) and write the scan number where the scanner was setup on the ground. It helps visualize what information was collected, and helps avoid lack of overlap, or missing key features of your site.

The goal is to collect 20%-25% of the same data that was collected by 1 or more other scans, for each scan you take.

Cheers,
Scott
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Re: Good Overlap

Post by Daniel Wujanz »

Dear Lee,

I should add that just having 20 to 25% of overlap between point clouds is not enough. What you also need to compute transformation parameters by ICP-algorithms or plane-based approaches like Scantra is "geometric contrast". That means that the overlapping region has to contain geometric information that is distributed in at least three cardinal directions. Otherwise you will not be able to register these datasets.

Here's an example:
Let's say you've scanned a very long straight rectangular shaped tunnel. That means you can only use the walls, the ceiling and the floor for registration. Even though you may have an overlap of 50% or more you will still not be able to register these datasets since one, so called degree of freedom, cannot be solved - namely the translational component along the tunnel's axis. You can imagine this effect by folding two pieces of paper in a way that they form a right angle. If you now place these pieces of paper on top of each other, you can still shift them along the folded edges. In technical terms the ICP would converge in a local minimum - which yields in misaligned point clouds.

Cheers

Daniel
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Re: Good Overlap

Post by Graham »

I guess it depends on what you are trying to deliver a point cloud that registers or one where the site has been adequately captured.
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Re: Good Overlap

Post by Daniel Wujanz »

Overlap usually refers to the common area covered by two or more scans while completeness refers to... well the level of completeness (or adequately captured as you called it) ; )
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Re: Good Overlap

Post by smacl »

Daniel Wujanz wrote: Fri Sep 14, 2018 10:17 am Overlap usually refers to the common area covered by two or more scans while completeness refers to... well the level of completeness (or adequately captured as you called it) ; )
Good explanation on the need for distinct geometry in overlapping area, though only a requirement if you're registering using a feature matching algorithm. i.e. not a problem if you have targets which would hopefully usually be the case in tunnels.
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Re: Good Overlap

Post by Daniel Wujanz »

True that, Shane! However, it would be rather unusual to report the overlap on the targets' surface.

If you're really into using targets you probably would report which targets are visible from a certain viewpoint in order to ensure that you're able to register the lot later on.

Cheers

Daniel
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