I have tried to to use the map offset.  I know there is a way to set the depth offset.  If the lake water is lower or higher than when the map was made then the depth lines on the map are not correct. I set my unit to highlight 20ft and the water is only 16ft.  I have a navonics card for my area.   I set the offset like it says but noght seems to change and the depth lines same the same.  maybe I am doing something wrong.   please advise!!!