If there is a crowd of fishermen busily pulling in fish, it would be foolish to do anything but join them. But, on an empty, unfamiliar lake, it takes more skill to successfully locate fish. An angler must first know what species inhabit the lake, then decide which to pursue. The angler must know the quarry's cover and water-depth requirements and know where to find those conditions on the lake. Maps showing depths at greater intervals are less useful.
Some maps also show underwater structures such as old creek or river channels and inundated roads.
Even without a map, an angler can get a good idea of what's under the ice by studying the surrounding terrain. The topography of the shore immediately adjacent to the lake probably extends into the water. A gently sloping plain probably indicates a shallow flat offshore. Rough hills at the shoreline probably continue below the waterline. A deep draw or canyon probably extends some distance underwater.