We are interested in buying a mapping chip (Navionics sells them, most likely there are other companies as well) that will enable us to input the current lake elevation and it will give us accurate information of the lake's makeup below the water. Does anyone use one, and does anyone have any suggestions as to what kind is best? We would certainly want to use it for Powell, but we see they have zones you can buy chips for. We would put it on our main navigation graft (a Lowrance Elite 7). Thanks for your help!