Georeferencing with only 2 points

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jaredmatthews
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Georeferencing with only 2 points

Post by jaredmatthews »

I have scaned 3 exisiting site reference stations and got the coordinates for them from the existing topo drawings and I am trying to transform my pointcloud data to the same coordinates.

When I try georeference them in RW, one of the coordinates is throwing out the coordinates by about 5m.
Capture1.JPG
If I have only 2 coordinates selected then my error is about 0.9mm.
Capture2.JPG
The thing is, I can not apply the transformation with only 2 points selected.

Does anyone know of a way to transform using only 2 points?
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Re: Georeferencing with only 2 points

Post by Attila »

Try swapping the x and y coordinates.
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Re: Georeferencing with only 2 points

Post by jaredmatthews »

The coordinates are in the right order.
That particular station had just had bad readings taken.

I have an idea to set one station to the right X,Y,Z coordinates and then manually (and visually) rotate the cloud to line up the 2nd station, but not sure if this is possible in RW.

Maybe its a case of doing it in TBC?
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Re: Georeferencing with only 2 points

Post by Least »

if your happy with the registration, could you not simply modify the 3rd point coordinates to suit?
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Re: Georeferencing with only 2 points

Post by smacl »

Least wrote: Mon May 17, 2021 9:34 am if your happy with the registration, could you not simply modify the 3rd point coordinates to suit?
A bit dangerous this as it assumes the error is entirely on the point with the high residual. Before doing this, I'd check the errors using the other combinations of two points. If the errors are similarly small, it suggests you are trying to create a 7 parameter transformation with dis-similar triangles. My approach, if at all possible, would be to re-observe the control. If you do use just two points you're effectively dropping from a 7 parameter (3 shifts, 3 rotations, 1 scale factor) to 5 parameter (3 shifts, 1 rotation (xy axis), 1 scale factor). Not sure if Realworks supports this, but for my money the 5 parameter would be a better solution than making up values for the 3rd station, as doing so could throw in unwanted rotations in xz and yz axes. Where time is available and site has the option, it is always worth collecting at least one additional redundant control point in these situations.
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Re: Georeferencing with only 2 points

Post by landmeterbeuckx »

smacl wrote: Mon May 17, 2021 11:53 am
Least wrote: Mon May 17, 2021 9:34 am if your happy with the registration, could you not simply modify the 3rd point coordinates to suit?
A bit dangerous this as it assumes the error is entirely on the point with the high residual. Before doing this, I'd check the errors using the other combinations of two points. If the errors are similarly small, it suggests you are trying to create a 7 parameter transformation with dis-similar triangles. My approach, if at all possible, would be to re-observe the control. If you do use just two points you're effectively dropping from a 7 parameter (3 shifts, 3 rotations, 1 scale factor) to 5 parameter (3 shifts, 1 rotation (xy axis), 1 scale factor). Not sure if Realworks supports this, but for my money the 5 parameter would be a better solution than making up values for the 3rd station, as doing so could throw in unwanted rotations in xz and yz axes. Where time is available and site has the option, it is always worth collecting at least one additional redundant control point in these situations.
If possible i'd suggest to go back on site and measure some points with a TS which can easily be determined in the pointcloud. Adding points like Shane says is a dangerous workflow. The moment you have a slight rotation between the 2 known, your whole system will become unusable.
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Re: Georeferencing with only 2 points

Post by Least »

smacl wrote: Mon May 17, 2021 11:53 am
Least wrote: Mon May 17, 2021 9:34 am if your happy with the registration, could you not simply modify the 3rd point coordinates to suit?
A bit dangerous this as it assumes the error is entirely on the point with the high residual. Before doing this, I'd check the errors using the other combinations of two points. If the errors are similarly small, it suggests you are trying to create a 7 parameter transformation with dis-similar triangles. My approach, if at all possible, would be to re-observe the control. If you do use just two points you're effectively dropping from a 7 parameter (3 shifts, 3 rotations, 1 scale factor) to 5 parameter (3 shifts, 1 rotation (xy axis), 1 scale factor). Not sure if Realworks supports this, but for my money the 5 parameter would be a better solution than making up values for the 3rd station, as doing so could throw in unwanted rotations in xz and yz axes. Where time is available and site has the option, it is always worth collecting at least one additional redundant control point in these situations.
Yes of course something has obviously gone wrong somewhere. He asked how to use the control references with only 2 points, so I was merely suggesting a work around, that is all. Whether its wise or not is down to the original poster to determine. We do not know what instrument was used or any details to make any call like that.

Personally I wouldn't be using somebody elses control without any checks.
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Re: Georeferencing with only 2 points

Post by v.serkov »

I would suggest checking your coordinate system settings. I think I need to expose NEL instead XYZ.
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Re: Georeferencing with only 2 points

Post by geedaa83 »

Hi

i suggest that you open a clean project and import your registered point cloud and try to repeat georeferenced steps again.

in case error still there try to counter check your coordinates.

in case error still present try to move your project to one of correct coordinates then start again georeferenced steps.

finally if you still face problem try to export whole project as LAS or E57 then import it again inside RW and repeat georeferenced steps.

** if you would like that i have look on data and help you so you are welcome just share what you have and i will do my best ([email protected]) **
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Re: Georeferencing with only 2 points

Post by jtw.layout »

Yes you can.

I know this is an old post... but I would like to answer this question for the next person that runs into this problem.
Yes its better to have more points but its not always possible to go out and shoot them again.
Depending on the workflow for your scans the tolerance may be inches or fractions of an inch.
Rather than tell the original poster why they shouldn't do something I'll give a work-around I used in a similar situation that kept me from having to drive 100 miles to a jobsite to shave an extra 0.01' off my residuals.

I had 2 good targets that were shot well with a total station. my third target's data was obscured in the scan. I needed to georeference an odd shaped project so i couldn't just square it up and "shift project" to one of the good coordinates. So I simply "extracted" my two square paper targets. then extracted the first one again. so now realworks thinks I have 3 targets even though 1 is a duplicate. viola. now the apply button becomes accessible in realworks georeferencing. the duplicate target adds nothing to the accuracy of my setup but for my needs this was going to work fine.
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