finding systematic misalignment in points from PTX files, when angles/range calculated.

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Phill
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by Phill »

Simon

The scanstation2 was one of the first scanners to have dualaxis compensation (from the Leica range anyway). Could it be that the misalignment is caused by the scanner correcting its levelness for example in a vibrating or slightly moving environment. As the compensator corrects the horizontal angle based on dislevellment in the T axis it changes the coordinates (angle), but the grid remains the same, hence a change in the coordinates of the pixel. There would also be a different amount of correction depending on how far an object was away

Doesnt explain why it would change with the intensity, but could be worth looking into perhaps?

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

> The scanstation2 was one of the first scanners to have dualaxis compensation (from the Leica range anyway). Could it be that the misalignment is caused by the scanner correcting its levelness for example in a vibrating or slightly moving environment. As the compensator corrects the horizontal angle based on dislevellment in the T axis it changes the coordinates (angle), but the grid remains the same, hence a change in the coordinates of the pixel. There would also be a different amount of correction depending on how far an object was away

that sounds interesting, i'll check it out. although AFAICT the environments seen stable.

> Doesnt explain why it would change with the intensity, but could be worth looking into perhaps?

i'm not looking at the 'intensity' values, if that's what you mean, the greyness of the squares in the images is range, darker is closer.

my understanding of the 'intensity' values is that they can be used to give a estimate of error in range, (say, to prevent registration software from selecting low accuracy points. ) i did take a quick look at them but didn't see any correlation with this issue.

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by stevenramsey »

The SS2 corrected the next scan line after spotting any movement so unlikely to cause that pattern. Would have to see it in its native environment straight after import
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by Scott.Warren »

psiplace wrote:
Fri Mar 20, 2020 12:22 am
my understanding of the 'intensity' values is that they can be used to give a estimate of error in range, (say, to prevent registration software from selecting low accuracy points. ) i did take a quick look at them but didn't see any correlation with this issue.
Intensity values are an arbitrary measure of the amount of energy received by the scanner from a particular measurement.
They do not reflect accuracy, but how well that surface was able to reflect the laser signal. This takes into account things like range, material type/Albedo, and incidence angle.

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

> unlikely to cause that pattern

agreed

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by jedfrechette »

psiplace wrote:
Thu Mar 19, 2020 1:12 am
...its like there is a parallax...
psiplace wrote:
Thu Mar 19, 2020 5:04 pm
very roughly, an error/offset in the position of the origin, relative to the points, of about 1mm would explain this. however it becomes noticeable through projection, (magnifying the effect by the ratio of the distances) a single scan with some elements 10x closer than others, will then produce a relative offset of something of the order of 1cm. from the scans i have this is clearly much more than the absolute error of the scans. maybe x10.
Tookie wrote:
Thu Mar 19, 2020 7:39 pm
Could be ranging error made worse by close proximity to the pipe? The laser maybe landing on part of the pipe and the wall at the same time?
Apparent parallax due to range measurement uncertainty seems like a likely explanation. According to the the spec sheet, the range accuracy of a SS2 is only 8mm. Leica doesn't specify how they measure that, but it is almost certainly a best case measurement to a planar surface. At geometric or reflectance edges the uncertainty is going to be larger due to various phenomenon like mixed pixels.

When you calculate the angles are you assuming they vary continuously? Since you know what scanner was used you can get the angular step size of the encoders from the spec sheet and if you assume that the angular step is constant multiple of that value for an entire scan that would help you constrain your calculations a little more.
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

> When you calculate the angles are you assuming they vary continuously?

no, each point, on my images, is simply the angles from the scan point to the provided origin. the grid emerges from the method used by the scanner, invariably fixed horizontal and vertical angle steps (or should that be azimuthal and altitude angles)

the problem is the horizontal angles (not the vertical ones) have an offset that comes and goes, depending on range, can't see range accuracy actually effecting the 'real' angles so what are these?

> Since you know what scanner was used

only in a couple of cases. the latest one is a ptx file i found online. might well be something else judging from the wider horizontal steps.

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by jedfrechette »

psiplace wrote:
Fri Mar 20, 2020 9:21 pm
> When you calculate the angles are you assuming they vary continuously?

no, each point, on my images, is simply the angles from the scan point to the provided origin. the grid emerges from the method used by the scanner, invariably fixed horizontal and vertical angle steps (or should that be azimuthal and altitude angles)
But are the step sizes you're calculating actually fixed? From what you've shown so far it seems like they aren't?

Taking the SS2 as an example again, the spec sheet says the angular resolution is 60 μrad, I think it's fairly safe to assume that's the step size of the encoders, so you might have scans with angular steps of 60 or 120 or 240 μrad, but never one with 236.5 μrad. Even if you don't know what the specific scanner was you should still be able to estimate the step size reasonably accurately and define your grid based on that.

Taking a step back if you're trying to create solid geo, why do you need to back out the angles and ranges? Can't you just use the grids topology to define connectivity?
Jed

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

> But are the step sizes you're calculating actually fixed? From what you've shown so far it seems like they aren't?

i'm not calculating the step size because i was expecting fixed spacing, from plotting the points in spherical coordinates, if the grid spacing isn't fixed something is wrong.

the plot shows significant angular (horizontal only) offset at discontinuous range changes and i'm assuming its actually offset everywhere, by different amounts, its just you can't see it. this seems like it would throw off a calculation of step size.

>Taking the SS2 as an example again, the spec sheet says the angular resolution is 60 μrad, I think it's fairly safe to assume that's the step size of the encoders, so you might have scans with angular steps of 60 or 120 or 240 μrad, but never one with 236.5 μrad.

yes, i suppose it should be reliable to assume all scans in a ptx are from the same unit, and so that the steps in a scan could only vary in multiples of some base resolution.

that would be for the azimuthal resolution? for the vertical, i guess potentially electronically controlled, a higher resolution and possibly infinitely variable?

> Even if you don't know what the specific scanner was you should still be able to estimate the step size reasonably accurately and define your grid based on that.

it may be possible to get some average, but which point on the calculated regular grid do you assign the points to if they are significantly misaligned? also the offset depends on the range, but how it does that and if it changes between scanners i don't know.

> Taking a step back if you're trying to create solid geo, why do you need to back out the angles and ranges? Can't you just use the grids topology to define connectivity?

AFAIK its an idea that hasn't been tried before and it just requires the angles and ranges to work, anything else gets you back to already well tried/tested techniques.

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

I'd just like to say amazing help here, and apologise in advance if this turns out to some stupid thing i did.

in that vein;

is there software that plots individual scans in spherical coordinates, so i can get some independent check?

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