The primary mirror for the 25-m Giant Magellan Telescope is made of seven circular segments, each of 8.4-m diameter. The lack of axisymmetry and the steep aspheric departure present significant technical challenges for the metrology. These segments will be measured interferometrically using a complex system of mirrors and holograms to give a null test with high spatial resolution. While analysis predicts this system will meet requirements, an additional set of measurements will be used to corroborate the principal interferometric measurement. The set of tests, including these alternate surface measurements, assures that all aspects of the mirror surface are measured completely and redundantly. The corroboration tests discussed in this paper are: Direct surface profile using metrology system based on a laser tracker, measuring low order shape errors Shear testing with full aperture interferometer, separating small scale errors in the null test from those in the mirror Slope testing with scanning pentaprism, measuring low order shape errors and sampling small scale errors.