Scanning in loops - why?

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fobos8
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Re: Scanning in loops - why?

Post by fobos8 »

Thanks Martin and Lieven - that's really helpful
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Re: Scanning in loops - why?

Post by Daniel Wujanz »

Dear Andrew,

a good analogy to scanning geometry is mechanics. The more connections (redundancy) and the stronger (more accurate) they are, the more stable will be your network.

I've added a small network of a construction site. Circles highlight scan positions and arrows registrations among them. The fill colours depict the accumulated error as a consequence of the network configuration. The tinted arrows represent the remaining misclosures after a block adjustment (which tells you how much the registration parameters contradict and NOT some whacky distance between point clouds ; ) ).

I've computed the network in two configurations. In the first one, I've considered station 8381018 to be error free (which is called geodetic datum) and chose an open setting. Consequently, the 3D-error accumulates up to 4.5 mm (in station 8381039 in the lower left corner). This means that this station is shacking by 4.5 mm with respect to the reference station.

Once I add another registration (arrow in the blue circle) I receive a closed configuration and the largest error comes down to 3.2 mm. Note that this is small network. Without sufficient redundancy and / or control errors in the centimeter range can occur quite quickly. The problem is - most people don't even know : (

All the best

Daniel Wujanz
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Re: Scanning in loops - why?

Post by landmeterbeuckx »

Daniel Wujanz wrote: Wed Jun 24, 2020 5:51 pm Dear Andrew,

a good analogy to scanning geometry is mechanics. The more connections (redundancy) and the stronger (more accurate) they are, the more stable will be your network.

I've added a small network of a construction site. Circles highlight scan positions and arrows registrations among them. The fill colours depict the accumulated error as a consequence of the network configuration. The tinted arrows represent the remaining misclosures after a block adjustment (which tells you how much the registration parameters contradict and NOT some whacky distance between point clouds ; ) ).

I've computed the network in two configurations. In the first one, I've considered station 8381018 to be error free (which is called geodetic datum) and chose an open setting. Consequently, the 3D-error accumulates up to 4.5 mm (in station 8381039 in the lower left corner). This means that this station is shacking by 4.5 mm with respect to the reference station.

Once I add another registration (arrow in the blue circle) I receive a closed configuration and the largest error comes down to 3.2 mm. Note that this is small network. Without sufficient redundancy and / or control errors in the centimeter range can occur quite quickly. The problem is - most people don't even know : (

All the best

Daniel Wujanz
Great stuff Daniel, the problem is that's difficult to close a loop at most buildings or sites. You're explination is also as reference for traditional surveying.
What can be done is at least do some rtk measurements with like 100 epochs at the start and whgen you leave the site and scan those points.
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Re: Scanning in loops - why?

Post by fobos8 »

Daniel Wujanz wrote: Wed Jun 24, 2020 5:51 pm Dear Andrew,

a good analogy to scanning geometry is mechanics. The more connections (redundancy) and the stronger (more accurate) they are, the more stable will be your network.

I've added a small network of a construction site. Circles highlight scan positions and arrows registrations among them. The fill colours depict the accumulated error as a consequence of the network configuration. The tinted arrows represent the remaining misclosures after a block adjustment (which tells you how much the registration parameters contradict and NOT some whacky distance between point clouds ; ) ).

I've computed the network in two configurations. In the first one, I've considered station 8381018 to be error free (which is called geodetic datum) and chose an open setting. Consequently, the 3D-error accumulates up to 4.5 mm (in station 8381039 in the lower left corner). This means that this station is shacking by 4.5 mm with respect to the reference station.

Once I add another registration (arrow in the blue circle) I receive a closed configuration and the largest error comes down to 3.2 mm. Note that this is small network. Without sufficient redundancy and / or control errors in the centimeter range can occur quite quickly. The problem is - most people don't even know : (

All the best

Daniel Wujanz
Thanks Daniel and Leiven. Contrary to the impression I gave in my original post I have been scanning in loops. I generally scan the externals in a loop and put up targets so that when I scan the internals and I cannot close the loop of a cluster I have points in my external loop that can be scanned from windows. My external loop always involves scanning points referenced by a total station traverse. I put confidence in my external control loop.

I don't think that Faro Scene when using C2C has any concept of "closing". With C2C I don't think you can pick fixed points in scans and get an adjustment for the intervening scans (similar to a Traverse adjustment).

From my experience of using Scene linking clusters to clusters (when using C2C) targets (or known control points) can only be used for manual quality control as opposed to error adjustment of scans within a cluster.

Regards, Andrew
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Re: Scanning in loops - why?

Post by landmeterbeuckx »

Scene doesn't do Least Squares Adjustment (LSA). If you use an add-on like Scantra you could do this (Ask Daniel).

I adjust my scans in Riscan with LSA, it's a more surveying approach.

Lots to find about this topic on this forum and the web if interested.
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Re: Scanning in loops - why?

Post by MagicBrou »

landmeterbeuckx wrote: Wed Jun 24, 2020 6:34 pm
Daniel Wujanz wrote: Wed Jun 24, 2020 5:51 pm Dear Andrew,

a good analogy to scanning geometry is mechanics. The more connections (redundancy) and the stronger (more accurate) they are, the more stable will be your network.

I've added a small network of a construction site. Circles highlight scan positions and arrows registrations among them. The fill colours depict the accumulated error as a consequence of the network configuration. The tinted arrows represent the remaining misclosures after a block adjustment (which tells you how much the registration parameters contradict and NOT some whacky distance between point clouds ; ) ).

I've computed the network in two configurations. In the first one, I've considered station 8381018 to be error free (which is called geodetic datum) and chose an open setting. Consequently, the 3D-error accumulates up to 4.5 mm (in station 8381039 in the lower left corner). This means that this station is shacking by 4.5 mm with respect to the reference station.

Once I add another registration (arrow in the blue circle) I receive a closed configuration and the largest error comes down to 3.2 mm. Note that this is small network. Without sufficient redundancy and / or control errors in the centimeter range can occur quite quickly. The problem is - most people don't even know : (

All the best

Daniel Wujanz
Great stuff Daniel, the problem is that's difficult to close a loop at most buildings or sites. You're explination is also as reference for traditional surveying.
What can be done is at least do some rtk measurements with like 100 epochs at the start and whgen you leave the site and scan those points.

your thought is economical, but to the detriment of your end result. If you measure betweens buildings you have lots of multipath effects and even with the best conditions you get 1,5cm accuracy of ur measured points (end the height accuracy is even worse up to 2cm). Adding the digitization error to get the center of your targets, you will end up at 2cm and your overall Result gets in some way "distorted"

I would rather use a total station if you have one :-).

can you define in Riscan the relations between scans before doing the LSA or is it a "magic-button-software"?
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Re: Scanning in loops - why?

Post by landmeterbeuckx »

MagicBrou wrote: Thu Jun 25, 2020 9:13 am
landmeterbeuckx wrote: Wed Jun 24, 2020 6:34 pm
Daniel Wujanz wrote: Wed Jun 24, 2020 5:51 pm Dear Andrew,

a good analogy to scanning geometry is mechanics. The more connections (redundancy) and the stronger (more accurate) they are, the more stable will be your network.

I've added a small network of a construction site. Circles highlight scan positions and arrows registrations among them. The fill colours depict the accumulated error as a consequence of the network configuration. The tinted arrows represent the remaining misclosures after a block adjustment (which tells you how much the registration parameters contradict and NOT some whacky distance between point clouds ; ) ).

I've computed the network in two configurations. In the first one, I've considered station 8381018 to be error free (which is called geodetic datum) and chose an open setting. Consequently, the 3D-error accumulates up to 4.5 mm (in station 8381039 in the lower left corner). This means that this station is shacking by 4.5 mm with respect to the reference station.

Once I add another registration (arrow in the blue circle) I receive a closed configuration and the largest error comes down to 3.2 mm. Note that this is small network. Without sufficient redundancy and / or control errors in the centimeter range can occur quite quickly. The problem is - most people don't even know : (

All the best

Daniel Wujanz
Great stuff Daniel, the problem is that's difficult to close a loop at most buildings or sites. You're explination is also as reference for traditional surveying.
What can be done is at least do some rtk measurements with like 100 epochs at the start and whgen you leave the site and scan those points.

your thought is economical, but to the detriment of your end result. If you measure betweens buildings you have lots of multipath effects and even with the best conditions you get 1,5cm accuracy of ur measured points (end the height accuracy is even worse up to 2cm). Adding the digitization error to get the center of your targets, you will end up at 2cm and your overall Result gets in some way "distorted"

I would rather use a total station if you have one :-).

can you define in Riscan the relations between scans before doing the LSA or is it a "magic-button-software"?
Of course RTK only when there is clear line of sight. I would never use it if 1/4 like on corners of buildings would be not visible. These new tilt rtk's where you measure a corner of a house holding the rod oblique makes me creep.

The drift when scanning with a Riegl is minimal due to the combination of sensors (RTK, imu, compass,..) The Rtk points are for checking if everything is as it should be. I know of course about the difference in height. It is a means of control.

I really don't think many projects are to be delivered within 5mm total controlled points, most scanner operators don't know anything about geodetic surveying or how to close a traverse.

When doing construction work scanning, everything is tied to measured points with TS and digital level if necessary.

Riegls Riscan makes detailed reports with station deviations etc. This can be delivered as an add-on toi the client but mostly they don't understand so it is kept in office.
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Re: Scanning in loops - why?

Post by MagicBrou »

landmeterbeuckx wrote: Thu Jun 25, 2020 9:38 am
MagicBrou wrote: Thu Jun 25, 2020 9:13 am
landmeterbeuckx wrote: Wed Jun 24, 2020 6:34 pm
Great stuff Daniel, the problem is that's difficult to close a loop at most buildings or sites. You're explination is also as reference for traditional surveying.
What can be done is at least do some rtk measurements with like 100 epochs at the start and whgen you leave the site and scan those points.

your thought is economical, but to the detriment of your end result. If you measure betweens buildings you have lots of multipath effects and even with the best conditions you get 1,5cm accuracy of ur measured points (end the height accuracy is even worse up to 2cm). Adding the digitization error to get the center of your targets, you will end up at 2cm and your overall Result gets in some way "distorted"

I would rather use a total station if you have one :-).

can you define in Riscan the relations between scans before doing the LSA or is it a "magic-button-software"?
Of course RTK only when there is clear line of sight. I would never use it if 1/4 like on corners of buildings would be not visible. These new tilt rtk's where you measure a corner of a house holding the rod oblique makes me creep.

The drift when scanning with a Riegl is minimal due to the combination of sensors (RTK, imu, compass,..) The Rtk points are for checking if everything is as it should be. I know of course about the difference in height. It is a means of control.

I really don't think many projects are to be delivered within 5mm total controlled points, most scanner operators don't know anything about geodetic surveying or how to close a traverse.

When doing construction work scanning, everything is tied to measured points with TS and digital level if necessary.

Riegls Riscan makes detailed reports with station deviations etc. This can be delivered as an add-on toi the client but mostly they don't understand so it is kept in office.
We use the Trimble R12 but I will never use the tilt correction :D. I guess sometimes the manufacturers lost their ideas, so they implement crap like this :roll:

Okay for Check control this is okay or for topographic land surveying :-)!

Yes, we surveyors are a minority in the Laserscanning business. In Germany there are many carpenters or architects with no geodetic background, offering Laserscanning services and thats why so many bad things can happen with the magic-button-solution. Scantra is the only Solution right now, that gives me confidence before I send my results to the customers (no advertising :P )
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