What resolution to use when doing structural comparison

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cedrickb
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What resolution to use when doing structural comparison

Post by cedrickb »

Hi,

We have a garage (100m²) with cracks on the ground and a Faro s70 and I want to do cloud comparison to see the evolution of the cracks.

What settings do you recommend when scanning with Faro s70?

Thanks in advance for your help.

Cedrickb
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Justin Richards
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Re: What resolution to use when doing structural comparison

Post by Justin Richards »

I would probably use 1/4 2x
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Re: What resolution to use when doing structural comparison

Post by ProCro »

There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
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Re: What resolution to use when doing structural comparison

Post by Leandre Robitaille »

ProCro wrote: Thu Jul 28, 2022 9:01 pm There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
Yes. 1/4 is already at the minimum density for good sphere detection and propper accuracy.
1/5 is just a little short for sphere detection at proper range and should be used with knowing the limits you want to work with.

As for cracks evolution I dont think a 3d scanner is the tool you want to use. There are many alternatives with more sensitive accuracy you can use for crack monitoring
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Re: What resolution to use when doing structural comparison

Post by VXGrid »

ProCro wrote: Thu Jul 28, 2022 9:01 pm There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
Leandre Robitaille wrote: Fri Jul 29, 2022 2:27 am
ProCro wrote: Thu Jul 28, 2022 9:01 pm There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
Yes. 1/4 is already at the minimum density for good sphere detection and propper accuracy.
1/5 is just a little short for sphere detection at proper range and should be used with knowing the limits you want to work with.
[...]
Based on the sphere detection algorithm in our software I did some theoretical distances where spheres should automatically get extracted, using a 145 mm diameter sphere.
With 1/5 resolution they should get detected at 23 m, 1/4 should get detected at 29 m, guess the same numbers are roundabout valid for other software packages.
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Re: What resolution to use when doing structural comparison

Post by SAttaya »

VXGrid wrote: Fri Jul 29, 2022 11:24 am
ProCro wrote: Thu Jul 28, 2022 9:01 pm There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
Leandre Robitaille wrote: Fri Jul 29, 2022 2:27 am
ProCro wrote: Thu Jul 28, 2022 9:01 pm There is really small difference between 1/4, 2x (6.1mm/10m), and 1/5, 2x (7.7mm/10m). Is it worth extra time?
Yes. 1/4 is already at the minimum density for good sphere detection and proper accuracy.
1/5 is just a little short for sphere detection at proper range and should be used with knowing the limits you want to work with.
[...]
Based on the sphere detection algorithm in our software I did some theoretical distances where spheres should automatically get extracted, using a 145 mm diameter sphere.
With 1/5 resolution they should get detected at 23 m, 1/4 should get detected at 29 m, guess the same numbers are roundabout valid for other software packages.
Martin,
I'm interested in how you came up with the 29m @ 1/4 resolution:
* what is it for 10m, 15m, 20m & 25m

Any chance you'd like to describe your sphere center point algorithm?
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Re: What resolution to use when doing structural comparison

Post by VXGrid »

SAttaya wrote: Fri Jul 29, 2022 6:34 pm Martin,
I'm interested in how you came up with the 29m @ 1/4 resolution:
* what is it for 10m, 15m, 20m & 25m

Any chance you'd like to describe your sphere center point algorithm?
So this is not totally sound:
The first assumption is how many points do we need to have on the sphere to automatically extract it. I used the value of around 50 points here, because then you can take a 7x7 pixel grid aka 49 points. More is of course better, less should work as well, the 50 works well, I have seen it doing great with a 5x5 patch as well.

Then we have the scanner resolution, 1/8 is around 5000 columns to 2125 rows (this may vary as well), taken from a FARO Focus S350.
There we have a point spacing of around 0.001256 m in 1 meter distance to the scanner (I didn't check if the point spacing varies in between rows and columns, I used the columns, because I thought rows might be denser, since this is the rotation direction of the laser, and columns is the direction of the complete device).

Now more simplifications:
Sphere diameter of 0.145 m is a circle area of 0.01651 m².
Pi * 0.145^2 / 4

Treating this area not as a circle, but as a square, we have a square side length of 0.1285 m
sqrt(circle area)

Now one side of the square needs to equal 7 points, or to put it this way: On the rectangle we need every 0.01835 m one single point
square side / 7

Lastly we need to consider the point spacing to get the distance
single point / point spacing in 1 meter distance
14,608 m

--------------------------------

Things which make this theoretical value worse:
  • Considering point spacing and area size, a square and a circle won't "collect" the same amount of points
  • Treating a sphere as a circle and discarding the depths
  • Sampling issues
  • The need to discard bound points due to ghost effects

As I have written, for automatic extraction, I mean if we do manual extraction a lot less points are feasable to get "a" sphere center. The more redundant points on the sphere, the better the sphere middle point and sigma estimation of course.
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Re: What resolution to use when doing structural comparison

Post by cedrickb »

thank you all for your inputs!
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