Editor’s note: This piece is the second of two parts. Part one, which explains data tolerances and discusses the communication issues that make registration difficult, can be found here.
There are a few techniques that I’ve found help a quite a bit with registration.
1 – First Things First
It’s never a bad idea–especially for a new scanning firm–to survey in the targets on the first few projects to verify your registration workflow. However, over time and with some established quality control processes in place, you can start trusting registration software more and more. You just get to help those clever algorithms figure things out a little bit. (It should be noted that for large scan projects, using survey control is always a good practice. Although, depending on the coordinate system, some 3D laser scanning registration packages may not round large numbers well and therefore produce mixed results or even partially corrupted data. More on that in later blogs.)
In order to register scans together, the algorithms must first have at least three reference points that correlate between scans. Some call this a “constellation” of reference markers. That means, if we have two scans that we are going to register together, the first scan must include three geometric references (which could be a sphere or checkerboard target, a spot on a wall, a cylindrical object, a plane on a floor, etc) that the second scan can also see – even if they are from different vantage points. The software then calculates the geometric arrangement of these references and attempts to match a similar arrangement in other scans.