Fish Finders

Question
Hey Ken, I just recently started using a Hummingbird fishfinder and I'm a bit confused. When I go out on the lake I can see structure and fish just fine. Now I know I can't look at it and then know exactly where to cast to put my lure right on the mouth of a bass, but i should be able to know where to cast to put my lure on some fallen timber or an underwater mound right? But I can't seem to do that very effectively so I need some help on that? And one other question, howcome when my boat is sitting in the same spot the lake bottom is still constantly changing up and down? Thanks and I hope you can help.

Answer
Hi Travis. Another very good question! This one is harder for me as I've never become much of an authority on depth finders, but I'll try to help. One of the least understood facts about fish finders is that you could be looking a much smaller area of the lake bottom than you think. If you're in shallow water the transducer is only "seeing" a spot of  very few feet, perhaps only two or three in eight feet of water. This is probably the reason you're having trouble hitting that piece of cover you've seen.
Scroll speed could have something to do with the constantly changing bottom you see. Remember, only the very right edge of the screen is what is current, everything to the left of that has been passed already. (This might also be a reason you cannot target what you've seen.) With wave and wind action your boat is moving more than you think, and the depth finder is reflecting that. Experiment with scroll speed until you feel you get the best representation of the bottom. This is easiest, and most accurate, if you can launch on some ultra clear water and can see the bottom while you compare what your eyes see to what the depth finder sees.
Hope this helps.