Can I stop a point cloud from flipping?

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richthegoose
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Can I stop a point cloud from flipping?

Post by richthegoose » Sun Feb 05, 2012 6:16 pm

Hi,

So I have a crash scene that was surveyed with GPS, I have a geolocated scan of the area from a later date, and I have cars from crash that were scanned at a later date. I am trying to re-build the scene.

I am using RiScan Pro and trying to course register the cars onto the road surface by picking points that corrispond to the 4 corners of the car (from the GPS survey) on the road surface and picking the same 4 corners from the road surface beneath the scanned cars.

The trouble is it's 50/50 whether the car sits nicely on top of the road or flips over and sits upside down beneath it! Unfortunately I only have 4 points to 4 points on a planar surface - I just need some way of telling it to maintain Z to within a few degrees.

Any ideas?

Rich

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stevenramsey
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Re: Can I stop a point cloud from flipping?

Post by stevenramsey » Sun Feb 05, 2012 8:39 pm

Add named targets in the correct corner which will override the potential of getting the bundle adjustment wrong. Your upside down scans we're just a better mathematical fit. A scanner with a compensator wouldn't do this but I'm sure you can force riscan to think the data is upright it will depend on how accurate you want to reproduce the scene and can you stand in court and prove it is close enough.
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joe.3dlm
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Re: Can I stop a point cloud from flipping?

Post by joe.3dlm » Mon Feb 06, 2012 9:40 am

Thanks Steven for the input.

Rich, you can't name the pickpoints using RiScan's coarse registration, so you will have to fit them another way.
I'd suggest creating tiepoints in the "scene" scan and also in the "car" scan, with matching names, and then using the Scan Position tie-point list to find a match. This gives more control over the best-fit than the Coarse Registration tool, as you can match by name, or by linking tie-points together manually, if the automatic best-fit doesn't find a match.

As you have a list of GPS coordinates for the corners of the vehicle, you can import this list into the tie-point list of the scene scan's scan position:
- expand the ScanPos and find the TPL SOCS (TPL = tie-point list, SOCS = scanner's own coordinate system)
- right-click the TPL SOCS and import the GPS coordinate list (use the file format .uda, which I'll explain at the end of the post)

Now, you need to create the same list of tie-points in the "car" scan's Scan Position:
- open the car scan (a 3D view is best for this)
- right-click on the scan and select New object > Tiepoint (note the red pickpoint that identifies the point where you clicked)
- identify the first corner where you want to create a tie-point
- hold SHIFT, and click the point where you want to create the tie-point
- give the tiepoint a name, to match the GPS point you imported in the previous step
- rotate the scan around to ensure you have clicked the correct point
- click "Create tiepoint"
- repeat this for the other three corners, then close the dialogue box
- open the TPL SOCS for this scan position, and you will see the tie-points you have created. Check the names are correct and that they match the names from the GPS list you imported

Now, you need to find a correspondence between the two tie-point lists:
- in the "car" scan's TPL SOCS, find the icon for "Find corresponding points" (it's third from the right, and has a red dot, with an arrow pointing to a blue dot)
- The dialogue box has a pick, where you can choose what you want to match to
- Untick "Project coordinate system", and tick just the "scene" scan position where you imported the GPS points
- Under "Mode", select "by name"
- Under "Parameters", use a tolerance of 0.1, and a Minimum N value of 4 (this will force it to find a match with 4 tie-points)
- click "Start", and wait for the results to pop up - it should take a split second
- If you get a "No match" message, try increasing the tolerance slightly (how much depends on how accurately you measured the points with GPS and then picked them from the scan)
- Once a match has been found, it will report the Std Dev error of the adjustment
- Check the scan is sitting where you expect it to be

If you are not happy with the results, delete the tie-points from the "car" TPL SOCS and pick then again!

Let me know how you get on.

Joe

UDA file format:
The UDA file format is a very simple file format used by RiSCAN PRO to import position information from (D)GPS receivers. The file format is defined as follows:

Code: Select all

NameOfPosition01,X,Y,Z<CR><LF>
NameOfPosition02,X,Y,Z<CR><LF>
...

richthegoose
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Joined: Sun Feb 06, 2011 11:22 am
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Country: UK
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Re: Can I stop a point cloud from flipping?

Post by richthegoose » Tue Feb 07, 2012 3:21 pm

Thanks for the help chaps. I still have some ongoing issues that i have emailed Joe about.

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