To certify compliance with the exceptional surface figure requirements of the laser interferometer gravity observatory, Hughes Danbury Optical Systems developed a unique high-accuracy metrology capability consisting of a custom 12 inch phase-measuring interferometer and an absolute calibration algorithm. The interferometer was specifically designed to minimize systematic retrace, phase- modulation, and ghost reflection errors. The calibration algorithm extends existing three-flat methods by employing circular harmonic transforms. Using this capability, three flats were calibrated over a 200 mm clear aperture to an accuracy of better than (lambda) /1000 rms. Comparisons with published algorithms are presented.