Multiple marker bouy's. I have had them as close as 50' apart, and as far as 100 yds, depends on the contour changes(left to right) and how tight they are to it. Long somewhat straight ones need less detail where shorter more curvy ones need more bouy's closer together to get it done. Sounds tough but it's really very simple. After he first time you'll be an old hand with it. I keep 4 or 5 in the boat.