Alignment for heavily eroded surfaces

Feel free to ask any question here
Pyroclastic
Posts: 3
Joined: Fri Sep 08, 2017 12:46 pm

Alignment for heavily eroded surfaces

Postby Pyroclastic » Fri Sep 08, 2017 1:06 pm

Hello All,
Two questions, if you will indulge me...

1)I am working on a project to try to quantify the rate of erosion at a lithified footprint site in Tanzania. I have imagery from 2010 and 2017, but there are no permanent ground control points. I have tried using the point picking and the ICP alignments based on areas that seem to have the least erosion but the alignments ultimately try to split the difference between positive and negative change. Since there has been far more erosion than deposition, this skews the result. Is there a better method of alignment I should be trying?
I2_Change2.jpg
I2_Change2.jpg (199.92 KiB) Viewed 409 times


2) I also have used the M3C2 algorithm to estimate change, but all my points come back red (insignificant change). In the attached image, you can see the change is real, but something in my parameters is not scaled correctly perhaps. Parameter and normal settings attached as well.
Normals.JPG
Normals.JPG (52.13 KiB) Viewed 409 times
parameters.JPG
parameters.JPG (54 KiB) Viewed 409 times


Any help you can provide is greatly appreciated.

Thanks!

daniel
Site Admin
Posts: 3326
Joined: Wed Oct 13, 2010 7:34 am
Location: Grenoble, France
Contact:

Re: Alignment for heavily eroded surfaces

Postby daniel » Sat Sep 09, 2017 12:40 pm

1) The ICP alignment tool will indeed try to minimize the distances between the two clouds (and you'll get therefore distances centered about 0)... apart if you set the 'overlap' parameter correctly. If you set it to 10% or 20%, then only the closest points will be used for registration.

Of course, if you rely only on the point-pair registration tool, then you'll avoid this effect completely. This tool is much more reliable when dealing with heavily modified objects.

2) Isn't the registration error already quite high? (relatively to the scales, etc.). Can you try without it?

And don't hesitate to send me your dataset (cloudcompare[at]danielgm.net), it will be easier for me to help you.
Daniel, CloudCompare admin

Pyroclastic
Posts: 3
Joined: Fri Sep 08, 2017 12:46 pm

Re: Alignment for heavily eroded surfaces

Postby Pyroclastic » Mon Sep 11, 2017 12:38 pm

Hi Daniel,
Thanks so much for getting back to me.

I'll keep trying to align the prints manually to get an optimized alignment. In this case, do I just use the RMS from point picking as the "registration error" in M3C2?

I ran a couple of M3C2 tests and the data are all colored red for insignificant change even when I do not check "Registration error" or if I use .0000001. Perhaps the texture of the footprint itself is being interpreted as surface roughness?

I will send you my dataset directly.

Thanks!
Brian

daniel
Site Admin
Posts: 3326
Joined: Wed Oct 13, 2010 7:34 am
Location: Grenoble, France
Contact:

Re: Alignment for heavily eroded surfaces

Postby daniel » Mon Sep 11, 2017 5:34 pm

I just realized: '1' for the 'significant change' scalar field means that all points are considered as being a 'significant change' actually ;D.

I did a quick and dirty registration (ICP with 30% of overlap - I didn't know if you had already registered the clouds). It gave me a 0.00045 RMS. And then I used the 'auto guess params' button in M3C2 (still quick & dirty) with vertical normals.

And the result seems quite consistant:
m3c2_test.JPG
m3c2_test.JPG (89.73 KiB) Viewed 375 times


m3c2_uncertainty.JPG
m3c2_uncertainty.JPG (82.79 KiB) Viewed 375 times


For the records, the projection diameter was 0.000708, the normals one was 0.001416 and the depth was 0.020349.
Daniel, CloudCompare admin

Pyroclastic
Posts: 3
Joined: Fri Sep 08, 2017 12:46 pm

Re: Alignment for heavily eroded surfaces

Postby Pyroclastic » Tue Sep 12, 2017 12:07 pm

Hi Daniel,
This is great news. Thanks for taking the time to run the clouds and for the clarification. I think my primary confusion came from the fact that under the "significant change" tab, all pixels were either colored red or grayed out. In Lauge's paper and on the wiki page, red is identified as non-significant change and gray means no matched points. I am thrilled to see that this analysis will work.

Thanks so much for the help!
Brian


Return to “Questions”

Who is online

Users browsing this forum: Google [Bot] and 2 guests