So I have read countless articles and spent hours on the water trying to get down what I am seeing on the graph. I have gotten on the lake and messed with the settings and also messed with the settings on the Eagle emulators.

The thing that I still don't really understand is how to interpret the sonar cross section. I understand that the transducer emits a cone which covers more and more of the bottom of the lake as you get in deeper water. What I still don't really understand is how that is displayed on the screen. I know that what you see on the left side of the screen is what is currently under your boat. What I don't understand is how you determine where something is in relation to the back of the boat.

So lets say that the cone is covering an area 10 feet in diameter on the bottom. That means that what you are seeing on your screen is a cross section of the cone. If I understand it correctly you are seeing everything in the red triangle in the image below:



So my question is this, how do you know if something you are seeing on the screen is directly under the boat, or out by the edges of the cross section?

I don't think you can. What shows up on your screen is what is in the cone. it may be ten feet from you or right under you.

sonar is just a sound wave that pings from your tranducer and then recieves a return signal that puts a pixel on your screen basically. so, anything in that cone that bounces back that sound wave be it bottom, bait or fish will come back as pixels to create what you are seeing on your screen. once you get good at reading it you will be able to tell what direction sometimes fish are facing and what part of balls of bait are being hit. you just have to look at it thinking about how fast the boat is moving as that will change the shapes of things like an arc will be a horizontal line when still from reurning the same sound wave over and over again. not sure about your eagle but real time for mine shows on the right side, everything to the left is history. also keep in mind your real time is based upon tranducer location.

So I guess the only way to determine where something is after you pass over it is to throw out a marker and make a perpendicular pass over the object?

Senko, when you said the thing about the solid horizontal line it made me think about something. I have never been able to drop a slab under the transducer and see it on the screen. I have always read that you can drop a slab down below where your transducer is located when you are sitting still and it should paint a solid line at the depth you have your slab at. I have never been able to do that on my transom or trolling motor graphs. I am looking at the transducer and it is pointing straight down, I drop the slab under the transducer and lower it but I am never able to see it.

BTW, you are correct about the latest being on the right, I was thinking about it backwards.

yeah, my transducer is on the transom so the second i see what i'm lookin for i throw the buoy over the back of the boat. i run an eagle 480 up front and can see a slab on screen. if there is any wind or waves you have to consider your line bowing in relation to where your slab really is. you should see a faint horizontal line when sitting stil or a series of slashes like //// faintly when hopping it. i run sensitivity around 80's on manual setting. i'll try and get a pic next time out.

I recommend for you should contact Baitfish. He has a post in the whites, hybrid, striper section about sonar training trips. I went with him this past Sunday and he not only answers any question you have but can also tell you why. This guy really knows his stuff.

BigPoppa, actually your unit will mark fish anywhere inside the cone in your drawing, not just directly under the boat. Also look for very faint arches on the screen. This usually means the fish are on the very edge of the cone. Some fish, like hybrids in shallow water, are very boat-shy. If the arches are big, though faint, I'll drop bait on them.

The sonar converts the 3 dimensional cone to a 2 dimensional view, this is the reason a fish that passes through the cone shows as an arch. Straight under the boat may be 30ft to the bottom but the outer edges of the sonar "ping" are traveling greater than 30ft to reach the bottom.(Geometry 101) Because of this, as the fish moves through the cone the sonar converts the depths to a specific pixel location on the screen, as the fish is closer to the edge the sonar thinks its slightly deepr, once in the center the arch peaks, then tapers as the fish migrates out of the cone. As long as the fish remains at a constant depth and moves through the cone, or the cone passes the fish, in a timely manner you get a nice clean fish arch. if the fish changes depth you get a variation of an arch and diagonal line, if the fish stays in the cone too long you get horizontal lines.

You are seeing everything in the cone in your diagram, not just the cross section.

BigPoppa, how much you're seeing is directly tied to the angle of your transducer cone. Before I went with Lowrance/Eagle I had a Humminbird and never saw a fish arch on it. If you lower a jig below the transducer you should be seeing it on the screen - turn up the sensitivity might help. Check out this Lowrance sonar Tutorial

BigPoppa; here's a pic I've shown previously from a Northern Wilderness trip in Canada. What you see is walleye. When I dropped a heavy jig in the 75' of water, I could actually see my line and lure at the end (wish I had taken a pic of that). So I knew when I was in the sweet spot - unfortunately, they didn't want it. I've also slow trolled for them in 20' or so of water with a worm harness trailing behind and could see the harness dragging/bouncing on the bottom - it's pretty neat.

BigPoppa:

I sent you an email with a sonar tutorial attached. It is a pdf file and is about 3MB in size. It is the best I have ever seen on the subject.

jjen,
is there anyway you could forward me the tutorial as well. I love reading stuff on sonars, as I dont think you can ever know to much. I have read some good ones on here in the past, and dont know if I have seen that one.

bryonnolan@yahoo.com

BigPoppa,
What are you fishing for, whites and hybrids? If so, I think you are making reading the sonar too complicated. Lately the fish are just a smudge on the screen when they first appear and the smudge is hugging the bottom.. Drop a slab down on them and catch one and they become excited and then you can see arches. I do not worry about the cone angle. The active fish are usually scattered over an area larger than the cone angle. Just look on the edges of humps and drop-offs to find the active, feeding fish. The slab will show on the screen, but not always.
If you are after blacks or some other species, I do not have a clue.
Don

jjen, can I get a copy of the tutorial? terryt512@yahoo.com

thanks

justgettingby and TerryT:

Tutorial has been sent to your email.