You want even better than optical flow. You need a SLAM system that can do loop closures (i.e. remember stuff that it has seen before). The propagated error from accelerometers and gyroscopes (that are not controlled by arms treaties is huge) blow up really quickly. Fun fact, gyroscopes and accelerometers actually do measure a change in angle and a change in velocity over a time interval (as opposed to an instantaneous angular rate and acceleration). For high precision applications you even need to worry about if the delta is relative to the frame of the inertial navigation system at the beginning or end of the time interval (shuddering at the memories).