[pdal] Default 'tt' value when using filters.icp

[Joachim Meyer]

Hi PDAL-devs,

I have a few questions using `filters.icp` (https://pdal.io/stages/filters.icp.html).
First off, I am very new to aligning two point clouds using this filter and have a limited understanding of the underlying algorithm or terminology.

The options give a default value of 9e-8 for the translation threshold.
Looking at the source code, I read that if I wanted to have a different value for that, I should give the square root for a different threshold: https://github.com/PDAL/PDAL/blob/a8b0bfbefb7f8ffc467092f11cedc155f862fa6f/filters/IterativeClosestPoint.cpp#L68

Is that correct?

My use case aligns two larger point clouds (100+ million) to each other and, to improve performance, I reduced the size of those through downsampling in point density, as well as filtering to stable reference ground between the two. I save the output of that alignment and now planning to use the same matrix with `filters.transformation`. There, it says that it should be given in row-major order, and I assume that is what the output of `filters.icp` with the —metadata is giving me?

Let me know if I can contribute your answers back to the docs through a PR.


Thank you,
 Joe