Mike - I looked the manual over several times, no hint of whether or not you can adjust this. I saw the transparency option on Big John's Lowrance 520 unit, and kind of assumed that, since the HD5 is a new model, they would have included this feature. Guess I could fire an email to their support. Other than that, I really like the unit. If you haven't done it already, be sure to update the Lowrance firmware on their site to version 3.0 - doing this gives you more contour lines and detail, even when using the Navionics chip.