Large-scale refractive index variations may look much like rigid-body motion and they can be corrected with the same recipe. Corrections for smaller-scale index variations may be optimized with a different recipe, for example a weighted average of the reference arms with the weighting depending on proximity. If the variations are on such a small scale that there is little correlation between the effects at the SMR and the nearest reference arm, there is no point in applying a correction, and the way to improve the measurement is to average over a longer time. We are investigating the spatial and temporal correlation of these effects. We will also experiment with different corrections and try to determine the best one(s) empirically.7.1SMR positioning systemA measurement with the Laser Tracker Plus system consists of something on the order of 100 samples, fairly uniformly distributed over the surface. The system includes a mechanism that moves the SMR over the surface of the mirror safely under computer control. This allows mirrors up to 8.4 m diameter to be measured without people being on the mirror or its support platform. It minimizes changing loads that would cause rigid-body motion.