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!!!