We have performed a round-robin study of surface irregularity measurements of a free-form toroidal window. The measurement tools were a Leitz scanning CMM at Optimax Systems, Inc., an UltraSurf, a non-contact measuring system at OptiPro Systems, a Zeiss scanning CMM at OptiPro Systems, a F25 micro-CMM at Carl Zeiss Industrial Metrology, and an ASI(Q)™ at QED Technologies. Each instrument resulted in a 2.5D surface error map. The measurements were compared with multiple analysis settings. The different analysis settings removed some low frequency height errors, which varied amongst the measurements. This highlights the need for more study to determine the reasons for the differences in the low frequency errors. With the low frequency errors removed, the measurements compared very well, to within 0.2 μm rms.