Let's talk about scanner accuracy

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Let's talk about scanner accuracy

Post by Marketing - LSE »

Hi there!

Many people do not know what to expect from the accuracy specifications on the data sheets of various scanner manufacturers. What is really meant by accuracy? Can we directly compare the specifications of different manufacturers? And which factors have to be taken into account?

Our very own Eric Bergholz clarifies these and many other questions in this video. In addition, he takes a look at the accuracy specifications from the FARO Focus, Trimble X7, Leica RTC 360, the Z+F Imager scanner, as well as the Teledyne Optech Polaris and Riegl VZ long-range scanners. He also takes a look at the data sheet of the SLAM scanners GeoSLAM ZEB Horizion and NavVis VLX.


youtu.be/4vmU1DFcI1Q

Are there still questions?
Lissi Haubenreißer
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Re: Let's talk about scanner accuracy

Post by jcoco3 »

Nice explanation. Thanks for providing this. I will likely reference this video when trying to explain scanning accuracy. My most common problem in trying to explain scanning accuracy to a lay person that it is not as simple as one number. So many people just want the simple answer, but I know this is just a trait of human nature. From now on I can just send them this video which should save me some time and frustration :D
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Re: Let's talk about scanner accuracy

Post by VXGrid »

I have a question:

At 14:05 there is a part about the axis errors. (Error of sight / Aiming error).
While this holds true for total stations and theodolites or Nivs - leveling, I'd argue that this is not true for laser scanners.

In my understanding the aiming error is a product of your aim, not being identical with the target you want to measure.
Therefore you measure in two views (don't know the english word, you measure one time normally and one time you turn the telescope around for 180° in the horizontal and vertical axis, so the view direction is practically inverted twice, but aiming at the same target).

Now with laserscanning, there is no target we are aiming for.
The points we are measuring are just somewhere, we just like to have the distance to them and the position is determined by the two direction angles.
We don't care where the points are measured, because the point we like to measure is determined by context, not by single point (meaning, if I'd like to measure the middle point of a checkerboard target, the reflectivity value will change between the black and white markings, which we will see and use to say: This is the middle point, I'd like to measure).

Or is my assumption totally wrong?

I mean in the end, it does not matter, since the point spacing in 10 metre distance is already bigger than the point accuracy.
Please anybody enlighten me! :idea:
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Re: Let's talk about scanner accuracy

Post by Least »

Hi VXGrid,

You are referring to changing face, when you flip the telescope over and re-measure the same point.
This is not to correct aiming errors, you could take the average of multiple aims on the same face to minimise that; it is to cancel out a systematic error.
Changing face will cancel out any circle eccentricity error on the horizontal circle of a total station.
Circle eccentricity is where the theoretical center of the mechanical axis of the instrument does not conincide exactly with the centre of the measuring circle. Changing face means a measurement is taken on opposite sides of the horizontal circle and the mean of the two compensates for the circle eccentricity.

Cheers
p
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Re: Let's talk about scanner accuracy

Post by VXGrid »

Least wrote: Fri Jul 09, 2021 6:23 pm Hi VXGrid,

You are referring to changing face, when you flip the telescope over and re-measure the same point.
This is not to correct aiming errors, you could take the average of multiple aims on the same face to minimise that; it is to cancel out a systematic error.
Changing face will cancel out any circle eccentricity error on the horizontal circle of a total station.
Circle eccentricity is where the theoretical center of the mechanical axis of the instrument does not conincide exactly with the centre of the measuring circle. Changing face means a measurement is taken on opposite sides of the horizontal circle and the mean of the two compensates for the circle eccentricity.

Cheers
p
Thanks for clearing up the name.

So if I understand that correctly, it looks something like this:
perfectlyDrawnMeasurements.png
Measuring in two faces, we can remove this device error, by simple math.

If this is visualized correctly, we have for the laser scanner something like that:
perfectlyDrawnLaserscannersMeasurements.jpg
So the aiming error is not an influence in laser scanning, since there is nothing we are aiming to (and we don't have a telescope....).
For distance measurements we snap to the nearest point, or we take the mean of a point group.
It's like putting all measured points 1 cm to the right. If we do that for all points in a 360° scan, all that happend is that the beginning of the circle is not at 0°, but it is still a full circle.
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Re: Let's talk about scanner accuracy

Post by Least »

It's like putting all measured points 1 cm to the right. If we do that for all points in a 360° scan, all that happend is that the beginning of the circle is not at 0°, but it is still a full circle.

Except its not a constant, its a sine wave.
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Re: Let's talk about scanner accuracy

Post by VXGrid »

Least wrote: Mon Jul 12, 2021 8:45 am It's like putting all measured points 1 cm to the right. If we do that for all points in a 360° scan, all that happend is that the beginning of the circle is not at 0°, but it is still a full circle.

Except its not a constant, its a sine wave.
But does it affect laser scanning, or does it not?
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Re: Let's talk about scanner accuracy

Post by Least »

Most modern total stations have 2 sensors to simultaneously measure opposite sides of the horizontal cirlce.
Do scanners do this?
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Re: Let's talk about scanner accuracy

Post by Oxbow »

Hi,

not, the scanner mirrows turns typcally only in one direction. Only teledyne optech has an oscillating mirror.
I never got information, that there is additional sensor place inside.

I believe the vertical position is determined from the stepper motor of the mirror. I'm not sure what the Trimble X7 does. Since one version of this scanner can also do stakeouts, maybe there is some robotic technology built in, like in the total station.

But maybe same producer of scanner can give more information to this.

...Eric
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