Adding data from a 'before' cloud to add to an 'after' cloud

[bluepup]

Hi all, I have two point clouds of terrain: one before an event and one after an event (an event which changes the terrain elevation in a localized area). The problem is, for after the event, I only have a small area of data available (the area which changed elevation). I would like to somehow pull the background data from the point cloud I have before the event (includes the surrounding area) and add it to the data after the event, so that I have an 'after' cloud which includes the surrounding area. I think this would require somehow detecting any x-y area that is not in the 'after' cloud (but is in the 'before' cloud) and adding all the data from the 'before' cloud into it. I do not want the elevation of the 'before' cloud in the area of elevation change to influence the 'after' cloud. Any help would be much appreciated.

[daniel]

You could use the M3C2 plugin to compute distances with purely vertical normals (second tab of the plugin). This will flag the points with points above or below in the other clouds with valid distances, while the others will remain 'NaN'.

From there, you can use the 'Edit > Scalar fields > Filter by value' tool, leave the default values (should encompass all 'valid' distances) and use the 'Split' option. You'll get the points with invalid distances in a separate cloud. You can then merge it with the points after the event.

[bluepup]

Thanks Daniel. This seems close, however I am not sure this will quite work because wouldn't it rely on both the two cloud having the exact same x-y coordinates for each measured point where they overlap? Or perhaps there is a way to add a small tolerance to capture nearby points?

[daniel]

M3C2 uses 'cylinders' with a custom radius (the 'projection scale'). This way, you can capture a point which is not exactly above the source point, but also several points (which gives even more robust results if the cloud is a bit noisy, etc.).