Point pair registration - GCPs

[Sch]

Hello,

I use this registration method to my study area. I have got two point clouds and would like to create one point cloud of them. I put GCPs on both area, the X and Y coordinates are the same values according to real coordinate system.
My questions are, if I put 3 GCPs, why get I better RMS result and if I appoint 5 GCPs, why get I worse RMS value?
As well as how compute the software the exact Z coordinates? What's the reference plain?

Thanks a lot for your answers.

[daniel]

The RMS displayed by this tool is only computed on the selected pairs. It's totally possible that the 3 first pairs match in a better way than the following pairs. You can see next to each point (pair) its contribution to the global error.

However, it's generally a better idea to get much more than 3 pairs, as the registration is not very robust with only a few pairs. And with the error contribution, you can detect if a particular pair is particularly bad and remove it.