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  1. #1
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    Sonar interpretation - why things look the way they do on your screen

    Lets see if there is any interest over here....

    Anyone interested in this subject enough to start a thread? Ive had some PM's suggesting this and have talked it over with some other members who think it could be a fun and interesting topic.

    I am just finishing up a similar thread on another board, so I have a lot of drawings and notes ready to go. It would lean a bit to the technical side going into the why behind what your screen is showing you and what its NOT showing you and how its not telling you the whole truth and all the truth all the time. There might be some math involved (not too much) and it might make some of you angry with your sonar..... hint - they have been lying to you from day one....

    I have some screen shots to post and dissect and I would hope others would post some of theirs as well.

    As usual, all questions, comments, suggestions, criticisms, arguments (as long as I win) and off topic asides, would be welcome. This is the internet after all
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
    Garmin 7610xsv/GT51M-TM/Panoptix PS21
    Why am I hanging out here when I could be fishing.....

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    #2
    Ready to learn!

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    #3

    Interference

    Here is one showing interference on my 7610 coming from Lowrance HDS. I think it is a good example of what interference from another unit looks like.
    Attached Images Attached Images
    My wife asks if I'm going to fish every day. I can't fish every day. Some days I might be sick.

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    #4
    I could use the help
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    #5
    Im glad there is some interest!

    First, Im going to assign some homework. I think Lowrance has the best sonar tutorial that I have seen. Some of the information is a little dated, but it covers the basics very well with some good graphics. It will help if everyone starts out on the same page and will save me a ton of typing if I dont have to repeat it all here :)

    http://support.lowrance.com/system/s...RTICLE_ID=2967

    In particular, take note of the discussion on cone angles, frequencies, fish arches, chart speed and sensitivity (or Gain or Power depending on your brand of MFD).

    This graphic is especially interesting. It shows how arches form but it also show how the screen gets drawn. That last bit is a lot more important than you might think. I hope it works here....




    fish arch.jpg


    The animation doesnt work. Fixed it - thanks DanR!!

    More later....
    Last edited by Larry3215; 02-22-2017 at 02:13 AM.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
    Garmin 7610xsv/GT51M-TM/Panoptix PS21
    Why am I hanging out here when I could be fishing.....

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    #6
    Quote Originally Posted by LWINCHESTER2 View Post
    Here is one showing interference on my 7610 coming from Lowrance HDS. I think it is a good example of what interference from another unit looks like.
    Thats a great example. Your bars are nice and tight. On my old Humminbird, I often saw them as wide fuzzy bands of static when another boat went by. I get almost none of that now with my 7610.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #7

    Air bubbles

    Sometimes we get questions of stuff to identify when it is rising columns of air. A key identify is that air never goes down. You can still get an arch though if the boat is moving faster than the air is rising. This is especially notable in deep water due to both the boat speed and the cone is so much wider in deep water. So the air close to the bottom, even though it is rising gets farther from the transducer as the boat moves past.
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    My wife asks if I'm going to fish every day. I can't fish every day. Some days I might be sick.

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    #8
    I wish that GIF had worked. It would make things easier.

    There are several seemingly simple (but very important) things you really need to understand if you want to know what your sonar screen is showing you.

    1) Your transducer can ONLY tell you the DISTANCE to a target. It has no idea about angles and does NOT tell you the depth. Your sounder is lying to you when it labels the scale as a "depth" scale. Its a distance scale. More specifically, its a straight line distance from the transducer to the target. Your transducer has no idea if that distance is to the side or down or some where in between.

    2) The screen never stops moving. You may think that a 'duh so what' sort of thing, but its actually very important to understand whats going on.

    3) Every return within the "cone" gets drawn on the screen - all at the same time - and all on the same row on the screen. This will come into play again once we start talking about sloped bottoms, drop offs, and things like that, but its critical to know.

    I'll go into these more in a bit.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #9
    Quote Originally Posted by LWINCHESTER2 View Post
    Sometimes we get questions of stuff to identify when it is rising columns of air. A key identify is that air never goes down. You can still get an arch though if the boat is moving faster than the air is rising. This is especially notable in deep water due to both the boat speed and the cone is so much wider in deep water. So the air close to the bottom, even though it is rising gets farther from the transducer as the boat moves past.
    Great example and cool screen shot.

    If you hadn't said those were bubbles, my first guess would have been that your transducer was tilted down at the back end. You get that same effect when your transducer isnt sitting level. If the lines or arches angle up to the right like yours do, then the rear of the transducer is too low. If they angle down the other way, the rear of the transducer is too hi.

    Here is a crude drawing showing what happens. This is a perfect example of how your transducer is showing you distance and not depth and what happens on the screen.





    If your transducer is tilted, when a target first enters the cone ahead of the boat it will be further away from the transducer. That means the beginning of the fish arch is drawn at a lower depth on the screen. As it moves through the cone, it gets closer to the transducer and gets drawn at a shallower depth - even though the fish never changes depth.

    angled.jpg
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
    Garmin 7610xsv/GT51M-TM/Panoptix PS21
    Why am I hanging out here when I could be fishing.....

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    #10
    This is electrical interference I got after running unshielded wires from the main battery to power the Echomap.
    The wires were running alongside the transducer wire, after rerouting the transducer wire, the interference cleared up.

    Attached Images Attached Images

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    #11
    One other interesting thing to note about Lwinchesters bubble screen shot is how consistent most of those streaks are. The majority of them are at the same exact angle.

    If they were bubbles, (and there is no way to tell for sure unless you were there and saw them), the matching angles tells me they were all rising at pretty close to the same speed. Any thing rising slower or faster would have been drawn at a different angle.

    If you look closely at the image, you can see some of them were a different angles to the majority. Those were probably active fish that were NOT rising while the boat passed over them.




    Air Bubbles.jpg
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #12
    Quote Originally Posted by Astrochris View Post
    This is electrical interference I got after running unshielded wires from the main battery to power the Echomap.
    The wires were running alongside the transducer wire, after rerouting the transducer wire, the interference cleared up.

    Good example. Sharpe vertical lines are almost always electrical noise of some sort.

    The other thing very interesting about those images is the fact you have a lot more arches and returns showing up on the 77khz side than on the 200 khz side. Thats almost certainly due to the different cone angles. The 77 will have a much wider cone angle than the 200. Those arches showing on the 77 screen are probably fish off to the side of the boat that are mostly or all the way outside the cone of the 200 khz frequency.




    77 200.jpg
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    #13
    oops I goofed
    Air Bubbles.jpg[/QUOTE]
    Last edited by Larry3215; 02-19-2017 at 09:55 PM. Reason: oops - wrong quoted post!
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #14
    Quote Originally Posted by Astrochris View Post
    This is electrical interference I got after running unshielded wires from the main battery to power the Echomap.
    The wires were running alongside the transducer wire, after rerouting the transducer wire, the interference cleared up.

    Another very interesting thing about those shots is the difference between the bottom thickness on the 77 and 200 sides. The 200 khz bottom line is much thinner than the 77 khz line. You have probably read that the thickness of the bottom line has to do with bottom hardness, but thats not the only thing going on that determines that bottom thickness. Its actually not the main thing in this case. Both screen shots are showing the exact same bottom at the exact same time. There cant be any difference in hardness under those conditions.

    Whats going on here goes back to my earlier comment that every return within the cone gets drawn on the screen at the same time and on the same vertical line at the right side of the screen.

    Assume for a minute that that shot was taken on a perfectly flat bottom. The point on the bottom exactly under the transducer will be closest "return". Lets say that point is 20 ft down. So the first part of the bottom that gets drawn on the screen will be from the center of the cone and will show up as 20 ft down.

    Now look at all the points on the outer rim of the cone where it hits the bottom. They will all be further away from the transducer than the center. Again, flat bottom. That means all those returns from the outer edge of the cone will be drawn at a deeper "depth" on the screen. Lets say that the distance from the transducer to the outer edge of the cone is 22 ft. That means the bottom line will be drawn on the screen between 20 ft and 22 ft - no matter how "hard" the bottom is.

    When you have a wider cone angle, you have a wider circle. That means a bigger difference between the closest bottom return and the most distant bottom return. That causes the thicker bottom line on the 77 khz screen shot - even though there is no difference in hardness.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #15
    One more thing on the bottom return. Notice how it fades from red to yellow at the lowest part. That yellow color in this case indicates a weaker return signal. Thats the signal coming back from the outer edges of the cone where the signal is weakest. The bottom side of the bottom line petty much always fades out like that, but it will be more noticeable and more gradual in deeper water and with wider cone angles.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #16
    Here is a slightly exaggerated example with some numbers. The red lines represent a 200 khz cone and the yellow lines a 77 khz cone. Water depth is 50 ft, but the distance to the outer edge of the 200 khz come is 55 ft and the distance to the outer edge of the 77 khz cone is 60 ft.

    That means the bottom line is drawn as 5 ft thick - from 50 ft to 55 ft - for the 200 khz screen side. Its drawn at 10 ft thick for the 77 khz side - 50 ft to 60 ft in this example.






    77 200thickness.jpg

    77 200thickness2.jpg
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    #17
    I really like this post, keep it up.

    In your example of 200khz vs 77khz, What would a hard bottom look like for both frequencies?
    Greg

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    #18
    So there have been other disscussions on side imaging and how it does really display were fish are correctly since the sonar only knows how far away the fish is, not what angle it is away.

    I assume this applies to structure as well? So if I see a brush pile on side imaging and scroll over to mark a waypoint on it will this be an accurate location?

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    #19
    Hold that question for a bit. I will get back to that in a lot more detail shortly.

    For now, the short answer is - you cant really tell a hard bottom from a soft bottom unless you know a lot more than just the color and thickness of the bottom return. The only thing the bottom line can tell you is what that section of bottom looks like relative to some other section you passed over with the exact same settings and the exact same bottom slope.

    All these things affect how the bottom line looks and is drawn on the screen:

    Gain setting - higher gain = darker, thicker bottom line - and also a wider cone angle.
    Depth - more depth = weaker signal = thinner, lighter bottom line.
    Cone width on the bottom - wider cone = thicker bottom line but not necessarily a darker one.
    Bottom slope within the cone - more slope = thicker bottom line.
    How smooth the bottom is - a smooth sandy bottom can actually give a stronger return that a rough, irregular, rocky bottom because the rough bottom scatters the signal. Its like sanded surface vrs a smooth polished surface. The smooth surface reflects more signal back to the transducer.

    Quote Originally Posted by Bassgreg View Post
    I really like this post, keep it up.

    In your example of 200khz vs 77khz, What would a hard bottom look like for both frequencies?
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    #20
    Quote Originally Posted by Larry3215 View Post
    Good example. Sharpe vertical lines are almost always electrical noise of some sort.

    The other thing very interesting about those images is the fact you have a lot more arches and returns showing up on the 77khz side than on the 200 khz side. Thats almost certainly due to the different cone angles. The 77 will have a much wider cone angle than the 200. Those arches showing on the 77 screen are probably fish off to the side of the boat that are mostly or all the way outside the cone of the 200 khz frequency.





    Here is a great example of
    what you're talking about Larry..
    Attached Images Attached Images

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