Agree also. Neg when you have something to protect (keel, skag, motor) below where the xducer is mounted. Positive when xducer is below like the waterline etc and wanting to know actual depth. Keel offset on Garmin. I do remember this adjustment making my 500 series HB go nuts. HB kept telling me it was unique to me, but BPS and Gander said they had a ton of returns for the same thing.