runetag: Too much variation in x,y,z center, but not in distance?

[gveglobalscanning]

hi all,

we have been using runetag sw in our application. We have a turntable and a camera, and need to determine pose of the camera, when the turntable is in different rotated positions.

Things seem to work fine, but when looking into details it seems that the found center of the turntable varies too much: 2-4 mm in x, y and z. The pattern can of course be mis-centered on the turntable. That could explain the x and y variation. But the variation in height (z) is just way too much. The turntable center does not move 2-4 mm up or down while rotating. It is within <0.5 mm.
We have centered the pattern within <0.5 mm. The good news is that when calculating the distance to the center, i.e. sqrt(x2+y2+z2), this distance is within <0.2 mm (for all angles: 0-360 degrees). So actually very precise! We find (almost) all markers.

Can someone explain this or help me understand it?

kind regards
Gerard