Wavy line of ground points

[celamk]

Hi!

Since there is no straightforward way of normalizing the point cloud for further visualization in CC I decided to combine two functions:

• C2C algorithm for calculating height of the points above ground

• export data as .PLY

• import of PLY file but switching Z coordinates with results of C2C calculation

I know that process is primitive but I couldn't find out other solution...

Everything looked perfect untill I zoomed into ground points. Their line - which from the distance looked absolutely flat - displayed in a regular wavy pattern with constant distances between maximums and minimums of the waves.
I've checked the data, I've repeated the process few times on a different datasets and results are the same...

Can anybody give me a hint where the problem might be?
Or maybe You've got better workflows to normalize the cloud and display results without such hiccups?

[daniel]

So one way to ease the process is to use the 'Edit > Scalar fields > Set SF as coordinate(s)'. You can set the distances as the 'Z' coordinate this way.

And then, about the wavy pattern, do you work with large coordinates maybe? In which case it could be an accuracy issue (make sure to use a Global Shift that makes the coordinates smaller when working in CC).

[celamk]

Hi Daniel!
Thanks for such lightning quick response!

Yup, this method works perfectly. Brilliant simplicity!

The pattern remains despite few attempts. I've tried on photogrammetry and lidar datasets, it is visible on both.
While prepping the data I've also tried rasterization of the ground points (to further export cloud as a reference for C2C) with choosing different cell heights (max, medium, nimimum) but the result remains the same.

What's curious, such pattern shows up even in the extracted ground points...

[daniel]

Would you be able to provide us with a small cloud and the exact parameters you use to generate these waves? (a few screen grabs of the various steps, with the dialogs and parameters clearly visible).

You can send it to admin@cloudcompare.org

[celamk]

Sure! Will send it asap.

Thanks!

[daniel]

So for the records, the reason why this pattern appeared was because of the C2C distance used to compute the distances between the 'above ground' points, and the rasterized ground cloud. The low density and gridded pattern of the rasterized ground cloud was causing these artifacts.

One solution is to export the rasterized ground as a mesh, or another solution is to use the 2.5D Volume Calculation tool (which has the advantage of computing 'vertical' distances).