I don't think fish can read a ruler..... Not being a smart a**, but I think I can guestimate good enough to be within a few inches of a fish. Crappie tend to stack vertically, even if suspended. So if some are at 6 foot, I bet others are at 5 and 7. Also why I run two hooks when trolling, and almost always troll with different depths (again guestimates) on each pole. Same reason I use a shotgun when bird hunting. This ain't rocket science, guys. (or geography/geometry)