This came up recently in a drivers sig discussion. The question I asked was that if sensor is measuring pose data in some world-referenced frame, but is mounted on the robot with some static offset (transform) from the vehicle's origin, what should we put in the frame_id field of the message? In this case, @mikepurvis pointed out that the NavSatFix message indicates what is expected. You should put in the frame_id in which the sensor is mounted (the transform from base_link).