Offset Mesh/surface?

[Will Pruitt]

We are flying low altitude UAV LiDAR gathering points with a Velodyne HD32 sensor. Our goal is to size trees 4.7' above the ground. Are you aware of a workflow using CloudCompare that can make this possible? We considered offsetting the ground surface/grid to use as a cutting edge with some sort of trimming tool but have not found a method of doing this. We have LOTS of point clouds to play with! Are you up to the challenge?

[daniel]

Well, there's a way to extract the ground (the qCSF plugin), optionally to mesh the result, then to offset the points based on the ground as you suggested. And of course you can compute the height and make some segmentation based on that... but that will be done on the offset-ed cloud, and I don't see how to revert the offset currently...

[Will Pruitt]

I am new to CloudCompare so I could use some help navigating all the tools. Could you help me understand how to compute the height and make segmentations of a cloud? I know about the segmentation tool but I was wondering if there was a trimming tool that would trim everything above a fixed height? I could move the cloud by the z axis but I do not know if I can insert a fixed height to offset the cloud. I appreciate any help CloudCompare seems to be a powerful tool if I could learn to utilize it better.

[daniel]

To compute the height, you have different options depending on the data. You can create a vertical plane (at z=0) and then compute the distances between the point cloud and this plane.

Another option is simply to convert the Z coordinates to a scalar field (Edit > Scalar fields > Export coordinate(s) to SF).

Thanks to this second option, you can also directly trim the points based on their altitude (Edit > Scalar fields > Filter by value).

And if you want to offset the cloud (of a fixed quantity?), then you can simply use the 'Edit > Apply transformation' tool).

[Will Pruitt]

I tried the workflow suggested and I am having a few issues. I attached an image of my problem. Basically when I trim the point cloud to a fixed height it does not follow the terrain of the ground. I was wondering if I could use the mesh as a trimming edge so that I would trim everything above a certain height from the ground. I would like to be able to have everything in the pointcloud from the ground to 4.7' above the ground following the terrain. The only way I have found so far is using the segmentation tool and manually trimming but I would not be able to trim at a fixed elevation using that method. Thanks very much this has been very helpful.