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  1. Member
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    #21
    I will get into this a lot more later too, but once again, how accurate your waypoints are goes back to the bottom slope - how much and which way it slopes in relation to your direction of travel.

    All Sidescan images are drawn assuming the bottom is flat from side to side. The transducer has no way to tell angles, or depth off to the side of the boat, so thats the only thing the software can assume. As long as the bottom is flat, the math works and your waypoints should be pretty close. The more slope there is side to side, the more error there will be in the location.

    If you are traveling parallel to a slope - like along the shore where its deeper on one side than the other, your waypoints will be off. The ones on the downhill side will be drawn on the sceen further away than they are and the up-hill ones will be drawn closer than they really are.

    Keep in mind you cannot go by the distance numbers on the sideview screen. Those numbers do not show the true distance to the side of the boat. They are the distance from the transducer on a straight line or slant line as Humminbird calls it.


    Quote Originally Posted by puhlw View Post
    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?
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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  2. Member DanR's Avatar
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    #22
    Subscribing to this thread, awesome info!

    Here's the GIF you were trying to post, it should animate.

    "Early to bed, early to rise, fish like hell and make up lies!"

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    #23
    Quote Originally Posted by Astrochris View Post
    Here is a great example of
    what you're talking about Larry..
    I love your screen shots! Lots of good stuff going on and it shows up really well. There is even more information you can get from those shots.

    Notice how the arches are wider and taller in the 77 khz side than on the 200 side. The arches are longer because the fish were in the cone longer. They are taller because they entered the cone at a greater distance from the transducer than they did on the 200 side. There are also more of them because of the wider cone and more coverage.

    There are a couple of more subtle things going on that you can really see in a screen shot. here is another one of my crude drawings to illustrate whats happening.

    Imagine 3 fish all exactly equal size and equally spaced under the boat and all at the exact same depth. The red fish is directly under the transducer and the two green fish are the same distance off to each side.

    Notice that you only see two fish in the 77 screen shot, but it gets even worse on the 200 screen shot because you only see one fish. On top of that, none of them look to be the same size or at the same depth.

    The arches on the two green fish are shorter on the 77 side because they dont stay in the cone as long as the red fish. They get drawn on top of each other - and at the wrong depth - and blend into a single arch because they are at the same distance from the transducer. So two fish blend into one arch that is shorter and thinner than the red fish arch.

    On the 200 side, you only see the red fish. That fish spends the shortest time in the cone so it has the shortest arch. That arch also isnt as tall as the arches on the 77 side bacuse the difference in distance from the edge of the cone to the center is less with a narrower cone. The top of the red arch will be at the same depth on both sides, but the start and end of the arch will be deeper on the 77 side.

    The main point here is you cant really tell how big the fish are or where the fish sit in relation to each other, or the boat, based on where the arches are drawn on the screen. The wider the cone angle, the greater the differences can be. The narrower the cone angle the less differences there will be.

    To sum up - narrow cone angles give more precise information but show fewer fish and cover less bottom and less water. Wider cone angles will show more fish, but you will have less of an idea where they are side to side and less idea of their exact depth.




    spread2.jpg
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #24
    I forgot one more point. Look at the arches at the top of the 77 side vrs the bottom of the 77 side. The fish at the top of the screen look smaller, but they could be the same size as the ones at the bottom. The top arches are shorter because they enter the cone later and leave the cone sooner than the deeper fish.

    Here is another drawing to show what can happen with fish stacked up vertically.

    Again, these three fish are all exactly the same size and stacked up vertically, but it doesnt look like that on the screen.



    Attachment 271743stack.jpg
    Last edited by Larry3215; 02-22-2017 at 02:29 AM. Reason: had the wrong drawing posted
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #25
    Thanks Dan!

    That gif illustrates how the arch is formed and how distance from the transducer translates into depth on the screen.

    How did you do that?
    Quote Originally Posted by DanR View Post
    Subscribing to this thread, awesome info!

    Here's the GIF you were trying to post, it should animate.

    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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  6. Member DanR's Avatar
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    #26
    Quote Originally Posted by Larry3215 View Post
    Thanks Dan!

    That gif illustrates how the arch is formed and how distance from the transducer translates into depth on the screen.

    How did you do that?
    I just copied the file link to the actual gif file on the lowrance site and pasted it in my post surrounded by the IMG forum tags (IMG) (/IMG) but replace ( and ) with [ and ] :)

    Dan
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    #27
    Thanks Dan!
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #28
    Lwinchester has mentioned the "dead zone" a few times in the main Garmin forum recently. Lets look at that for a minute as it ties into the "sloped bottom" thing as far as whats going on on the screen. This also ties into what I was talking about earlier about how the images are drawn on the screen - every return within the cone gets drawn on the screen at the same time.

    This is a little hard to explain, but its important for understanding what you are seeing on the screen, so I hope it makes sense. I will come back to this next image later - there is more going on than you might think.

    When you see something like this next shot on your sonar, most folks think that’s what the bottom, that the boat has just passed over, actually looks like. It sure looks like someone took a snapshot of the bottom as you passed over it.

    Unless the bottom is dead flat, that’s almost certainly NOT true for traditional sonar. Its closer to true for down and sideview, but they are just as deceptive in different ways.





    03MAY16_1225_001.jpg

    There are two reasons for that and they go back a ways. Some of you may remember when sonars were called “graphs”. Some people still call them that. That’s because the sonar screen is literally a graph showing distance to a target on the vertical or “depth” axis and time on the horizontal axis.

    There are two key things to remember here.

    1) The screen never stops moving or scrolling from right to left (for traditional and downview) or down from the top for sidescan. It can go slower or faster, but it never stops unless you pause it for some reason. Even when the boat is not moving, the screen continues to scroll.

    2) Every return signal on every ping, gets drawn in on the screen based on its distance from the transducer - all at the same time. In other words, every single target within the cone gets drawn in a single vertical line for every single ping, in the order of the distance from the transducer, all at the same time. The screen then moves over one line and the next ping gets drawn on the screen on the right edge.

    Lets start with the first one. The fact that the screen never stops scrolling is why things like downrigger balls, or your jig thats just sitting there in one place, leave a straight line on the screen when they are not going up or down. The downrigger ball is sitting there at a fixed distance and it gets pinged and a dot is drawn at that depth on the far right side of the screen. The screen scrolls over one space to the left , but the ball has not moved, so the next ping causes another dot to be drawn at the same "depth". Repeat enough times, and you end up with a nice straight horizontal line on the screen as it keeps scrolling along. As you bring the downrigger ball up, it leaves a nice angled line moving up and to the right on the screen as the screen scrolls to the left.

    Anything that just sits in the cone without moving, will leave a straight line on the screen because the screen is always scrolling. The data is always being drawn new on the far right side or across the top as the screen scrolls.


    The second thing is just as important but harder to get a grip on I think. Remember that the traditional 2D sonar beam is more or less (actually less, but lets ignore that for now) like a flashlight beam, and depending on the cone angle and depth, can actually cover a fairly large circular area on the bottom as you move along.

    I'll post a couple more crude drawings that hopefully show how that works next.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #29
    Here is whats going on with the "dead zone".

    Because every return within the cone gets drawn on the screen at the same time, things like fish, that fall in between the shallowest and the deepest parts of the bottom, are hidden by the bottom return when they get drawn on the screen.

    In this example, imagine you are trolling along parallel to a drop off and there are a total of 6 fish under the boat at various places. (Im just making up these numbers to make it easy to explain)



    hidden1.jpg

    Lets look at the 77 deg side first.

    The wider 77 deg cone hits the bottom at 20 ft on the shallow side and 40 ft on the deep side. Notice that bottom line is drawn starting at the 20 ft depth extending down to the 40 ft mark. That difference in depth between the hi and low points within the cone creates a 20 ft wide "dead zone" where fish wont show up. The fish returns are there, but they are hidden within the bottom return because all the returns are drawn at the same time on the same vertical line at same time on the right side of the screen.

    There are 6 fish under the boat within the 77 cone, but only the green one shows on the screen. It happens to be above the shallowest part of the bottom thats within the cone, so its arch can be seen on the screen. The other 5 fish fall within the dead zone and cant be seen on the screen.

    You can see on the left side, that the green fish is actually about 7 ft vertically above the bottom, yet it gets drawn on the screen so that it looks like its almost sitting on the bottom.

    Also, the digital depth number in the top left is 27 ft in my made up example, but the bottom line is being drawn on the screen at 20 ft. Thats because the 20 ft mark is the shallowest point within the cone. The digital depth number comes from some sort of averaging/calculation that the MFD does when it decides what the "depth" is based on all those different returns. Unless the bottom is dead flat those numbers will almost never match.

    Now the 200 side.

    The narrower cone on the 200 side hits the bottom at 25 ft on the shallow side and 30 ft on the deep side making for a much narrower dead zone. There are two fish within the 200 cone, but once again, only the red one shows on the screen. Because of the smaller dead zone, the 200's narrower cone is able to "see" fish that are closer to the bottom than the wider cone on the 77 side. It also comes closer to showing how far up from the bottom the fish really is. That narrower cone also draws the bottom depth closer to what is really is under the boat and will also give a better representation of the shape of the bottom contour as you drive over it.

    The main downside to the narrower cone is you miss more fish that are off to the side of the boat. You pretty much have to drive right over the top of them to see them where as the wide cone is more like a shot gun blast and hits anything even if its off to the side.
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #30
    Here is another possible situation where the wide cone hides stuff that the narrow cone can show. With the wide cone, you dont even know the hole is there and never see the fish. The narrow cone can show both - if you happen to drive right over the top of it.




    hidden2.jpg
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #31
    Subscribed. Great thread. Thank you!
    '82 Champion 201 Pro, 'Rude 235 Looper, "The Lead Sled", One Power Head in 34 years, VERY thirsty!

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    #32
    Thanks! Its difficult to know how good a job Im doing at getting these ideas across, so the feed back is very much appreciated :)
    Smokercraft Phantom 202 Yamaha F115/Merc 9.9
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    #33
    Here is what it can look like on the screen when you are heading into a slope instead of parallel to it. This is where you get partly buried arches. As the cone first hits a fish thats near the bottom on a slope, the arch starts to get drawn as usual. But then the cone continues to climb up the hill until it hides the rest of the arch.




    hidden3.jpg


    Thats whats going on on the left side of this screen shot. We were drifting from deep water up toward the top of a shoal with a pretty steep slope while fishing for ling cod. You can see the arches sticking part way out from the bottom line.

    As we got near the top of the shoal, the contours changed so we were drifting more along the side of the slope instead of climbing it, so the bottom line is much thicker there. Those arches along the top of the hump were probably fish much further up from the bottom than the ones on the left of the hump.




    03MAY16_1225_001.jpg
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    #34
    Subscribed.

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    #35
    This is really good info. Thanks for sharing it. I'm subscribed too!

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    #36
    Quote Originally Posted by Larry3215 View Post
    Here is whats going on with the "dead zone".

    Because every return within the cone gets drawn on the screen at the same time, things like fish, that fall in between the shallowest and the deepest parts of the bottom, are hidden by the bottom return when they get drawn on the screen.

    In this example, imagine you are trolling along parallel to a drop off and there are a total of 6 fish under the boat at various places. (Im just making up these numbers to make it easy to explain)





    Lets look at the 77 deg side first.

    The wider 77 deg cone hits the bottom at 20 ft on the shallow side and 40 ft on the deep side. Notice that bottom line is drawn starting at the 20 ft depth extending down to the 40 ft mark. That difference in depth between the hi and low points within the cone creates a 20 ft wide "dead zone" where fish wont show up. The fish returns are there, but they are hidden within the bottom return because all the returns are drawn at the same time on the same vertical line at same time on the right side of the screen.

    There are 6 fish under the boat within the 77 cone, but only the green one shows on the screen. It happens to be above the shallowest part of the bottom thats within the cone, so its arch can be seen on the screen. The other 5 fish fall within the dead zone and cant be seen on the screen.

    You can see on the left side, that the green fish is actually about 7 ft vertically above the bottom, yet it gets drawn on the screen so that it looks like its almost sitting on the bottom.

    Also, the digital depth number in the top left is 27 ft in my made up example, but the bottom line is being drawn on the screen at 20 ft. Thats because the 20 ft mark is the shallowest point within the cone. The digital depth number comes from some sort of averaging/calculation that the MFD does when it decides what the "depth" is based on all those different returns. Unless the bottom is dead flat those numbers will almost never match.

    Now the 200 side.

    The narrower cone on the 200 side hits the bottom at 25 ft on the shallow side and 30 ft on the deep side making for a much narrower dead zone. There are two fish within the 200 cone, but once again, only the red one shows on the screen. Because of the smaller dead zone, the 200's narrower cone is able to "see" fish that are closer to the bottom than the wider cone on the 77 side. It also comes closer to showing how far up from the bottom the fish really is. That narrower cone also draws the bottom depth closer to what is really is under the boat and will also give a better representation of the shape of the bottom contour as you drive over it.

    The main downside to the narrower cone is you miss more fish that are off to the side of the boat. You pretty much have to drive right over the top of them to see them where as the wide cone is more like a shot gun blast and hits anything even if its off to the side.
    so would you say that while searching for fish its better to use 83hrz and 200hrz to pin point fish once you have an idea where there at?

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    #37
    Wow. I've been interpreting data incorrectly. Keep up the info Larry. This is good stuff!

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    #38
    Quote Originally Posted by gohoos2011 View Post
    so would you say that while searching for fish its better to use 83hrz and 200hrz to pin point fish once you have an idea where there at?
    If possible, run both at the same time like Astrochris does in the screen shots he posted. The problem is, each one has its plus and minus points and neither one is really good in all situations.

    Lower freq's like 83, 77 etc generally have the widest cone angles, so they will let you look at the maximum/widest column of water at one time. The down sides are that you have less of an idea where the fish are - they could be under the boat or off to the right or left and you have no way to tell. You also have a less accurate idea of the true depth of the target. The wide cone also has the largest dead zone if the bottom isnt flat - and it probably isnt flat most of the time.

    The higher freq's have narrower cone angles so they have smaller dead zones, and targets you see are probably closer to the indicated depth, but you are covering a much smaller water column and could easily miss lots of fish that are well within casting/jigging range and just a little ways to the side of the boat.

    If you run both at once, you increase your odds of seeing if there are any fish there.

    BUT...

    In that example I gave above, there were 6 fish within close range of the boat but only two show on the screen even when running both freq's on screen at the same time. One shows up on 77 and one shows up on 200. How well it works is going to be up to where the fish happen to be within the two cones and how steep the slope is and how lucky you are. You might see more of those fish if they were placed a little differently or you might see zero fish if they were hugging the bottom.

    Side and down view are not going to be much more help on a slope either because of the way the images get compressed and ranging issues on a slope. Plus you will have less usable "water column" thats clear of bottom return to easily see fish. I will go into this more later when we get to side and downview - there is math involved :)

    If you are lucky enough to own a Panoptix capable mfd, you can get around all those limitations. You could easily see all 6 fish - and know exactly where they were as far as distance, direction and depth - if you had a PS21, 30 or 31. No other sonar could do that.
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    #39
    This is probably a good time to talk about using your charts along with your sonar. If you have good coverage of the lake you fish or have made some charts with Quickdraw, you can get a good idea of which way the bottom slopes and how steep it is.

    Where the contour lines are close together, the slope is steeper. Further apart means a flatter bottom or less steep slope.

    Where ever you have a choice, try to troll across the contour lines instead of parallel to them. That will help reduce your dead zones by a small amount - you will at least have a chance to see 1/2 buried arches instead of no arches at all.

    This is even more important when using side and downview. Steep slopes really really mess them both up when you are going parallel to the slope. The MFD's assume the bottom is flat ,from side to side, when drawing the screens and doing the math for side and downview. Parts of the bottom can get drawn on top of each other, backwards, compressed, stretched, at the wrong distances, etc. Again, steeper is worse than flatter.

    Here are a couple of screen shots from a couple of my local lakes. Red lines are bad directions to travel and green lines are good directions to scan in. If the contour lines are far apart, it doesnt mater much.





    contourlines.jpg

    direction.jpg

    Of course, there are lots of times you really have no choice.
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    #40
    Awesome information!! Thanks! Ill be checking back for more!

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