a reference plane setting section for setting a virtual reference plane as a reference defining {HEAD ~ a geometric placement} of an arc welding torch relative to {TAIL ~ a workpiece} during a welding operation, based on said plurality of position and orientation data obtained in said data obtaining section 
relative to
a reference plane setting section for setting a virtual reference plane as a reference defining { HEAD ~ a geometric placement } of an arc welding torch relative to { TAIL ~ a workpiece } during a welding operation , based on said plurality of position and orientation data obtained in said data obtaining section
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH