If your GPS can reliably determine a 3 m difference? It sounds more like false precision to me.
I would assume the OP was generalizing based on previous comments, but yes, in North America under ideal conditions and with WAAS (Wide Area Augmentation System) enabled, the most precise non-military GPSr units are accurate to ±3m (so an error range under working conditions of ~6m). Close enough (for me) to rely on my memory for the last details, but not precise enough to rely on completely.
I'm guessing that's the reason for the all the ridge flagging tape?