In this chapter, we characterize the effects of angle bias due to refraction and angle precision due to turbulence. As a result, angle bias due to refraction can be calibrated and removed, if possible, and any residual error can be quantified in the interferometer angle error budget. Also, the effects of angle precision due to turbulence can be determined and accounted for in the same angle error budget. For turbulence it will be important to understand the spatial correlation of errors that will determine the magnitude of the impact of turbulence effects on angle precision for an interferometer.