Author Topic: Cone and fish depth  (Read 620 times)

Offline 4°Fahrenheit

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Cone and fish depth
« on: Jan 22, 2021, 08:18 PM »
I'm no stranger to physics and I'm sure the topic has been covered to the point of "yawn" but searches have revealed nothing.  I'm also sure I just haven't used the correct terminology. Lol. I am new to flashers and their nuances but I'm a fly fishing maniac that studies micro current effect on emerging nymphs and how trout respond. It's a "I wanna know".  I'm am info guy.  The more info in my arsenal, the more success I achieve.

 So here goes:

A fish enters a sonar cone and swims along the specific cone diameter for that depth. (We will call it a true right circular cone for argument sake). A sonar measures distance from target to transducer with each signal sent and received.  This distance decreases as the fish gets closer to the midpoint of the diameter and then increases again. This can change its "apparent" depth across the graph. Right?.  Now I know this distance is virtually negligible but can come into play with wide transducers and greater depths.  So my question: Can you discern this apparent change in depth on your flasher(outside of the target fish's mark getting bigger on the flasher)?  Does your strategy incorporate this inherent inaccuracy of sonar?  To me this fraction of an inch sounds absurd but when I discuss micro currents and Leisenring lifts,  I get some that call me absurd; but my tactics are proven on some of the highest pressured tailwaters in the U.S.

Offline river_scum

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Re: Cone and fish depth
« Reply #1 on: Jan 23, 2021, 04:47 AM »
im sure they took depth and fish position(off to side) into consideration when developing it.  but im not that into it either.  all I wana see is the mark turn red and come up ;D
real fishermen don't ask where you caught them.

Tim- member since -2003- IN.

Offline badger132

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Re: Cone and fish depth
« Reply #2 on: Jan 23, 2021, 02:54 PM »
You are correct- The sonar can only measure distance from the transducer, by computing the time from pulse send to return, and multiplying times the speed of sound in water. A fish off to the side of the cone appears smaller and deeper than it actually is. That is why if you look at summer sonar pictures, on a display that scrolls, fish appear as a little crescent shape as you pass over them.



This effect can be more than inches- a 20 degree sonar cone (10 degree half angle) the cosine of 10 degrees is .84, so a fish 20 feet deep, and at the edge of this cone, would be 23.81 feet from the sensor, and that is how deep he appears. 


If a fish comes in from the side, it always looks like he is coming up, and the return gets stronger. You just need to remember that. If he swims away to the side, it looks like he is getting smaller as well as going down.

Also, a fish off to the side and on the bottom is difficult to separate from the bottom, since his return is marker below the level of the bottom directly under the sensor.

Since your only choices are to go up or down, there is not much you can do about it anyway.

Offline ice dawg

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Re: Cone and fish depth
« Reply #3 on: Jan 23, 2021, 03:22 PM »
To me it doesn't matter if the target is coming up from below or in from the side. What matters to me is what happens directly below the transducer where my bait is. When my target changes from green to yellow to orange to red is what matters most to me. Sometimes I set the hook when my bait and the target both change to red and become one.
It seems to go from zero to hero all some have to do is lie.

Offline Fisherman 1

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Re: Cone and fish depth
« Reply #4 on: Jan 23, 2021, 03:36 PM »
I'm no stranger to physics and I'm sure the topic has been covered to the point of "yawn" but searches have revealed nothing.  I'm also sure I just haven't used the correct terminology. Lol. I am new to flashers and their nuances but I'm a fly fishing maniac that studies micro current effect on emerging nymphs and how trout respond. It's a "I wanna know".  I'm am info guy.  The more info in my arsenal, the more success I achieve.

 So here goes:

A fish enters a sonar cone and swims along the specific cone diameter for that depth. (We will call it a true right circular cone for argument sake). A sonar measures distance from target to transducer with each signal sent and received.  This distance decreases as the fish gets closer to the midpoint of the diameter and then increases again. This can change its "apparent" depth across the graph. Right?.  Now I know this distance is virtually negligible but can come into play with wide transducers and greater depths.  So my question: Can you discern this apparent change in depth on your flasher(outside of the target fish's mark getting bigger on the flasher)?  Does your strategy incorporate this inherent inaccuracy of sonar?  To me this fraction of an inch sounds absurd but when I discuss micro currents and Leisenring lifts,  I get some that call me absurd; but my tactics are proven on some of the highest pressured tailwaters in the U.S.

You answered your own question but look at Badgers picture upside down, now you see the curve the way you interpret it.  The F/F see's it the other way, at the edge of the cone it appears further down, as the fish swims through the middle of the cone, it appears to be coming up, but it doesn't.  Think of it this way, fish don't swim like they're hooked to a yoyo, usually in a horizontal line.  Now if you have a fish chasing your lure up, what you see,  pretty much a line on the rise.

Offline badger132

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Re: Cone and fish depth
« Reply #5 on: Jan 23, 2021, 08:13 PM »
The key is to remember that the sonar is not measuring depth- it knows only the time the return echo came back, and the intensity. We interpret that as depth and size, which is close, but not exactly correct.

If we assume the sonar transducer os pointing down, straight down is zero degrees. You also have to remember that zero depth is measured at the bottom of the transducer. If you have 2 feet more cable out than your buddy, he will see 2 feet more "depth"

A fish that is not along this zero degree line is going to appear "deeper" than it is. Trigonometry says that if it is 20 feet deep and 10 degrees off from the zero degree line, it will measure 20/(cos 10 degrees)=23.8 feet from the transducer.

If it swims perfectly level from there to the zero degree line, it will appear to rise to 20 feet.

Fish signals which appear to grow are swimming closer to the center of the cone, or are changing direction to put a larger cross section perpendicular to the transducer. They also are sometimes several small fish getting further separated from each other.
If is swims perfectly level in a circle 10 degrees from the zero line, it will not move on the sonar.

Offline pugman

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Re: Cone and fish depth
« Reply #6 on: Jan 24, 2021, 08:46 AM »
I have a Vexilar FL-12 the Ice-Ducer went bad so I was checking the different sites for am replacement, went to Vexilar's website they have video's showing how a ice -ducer cone actually looks and works, I ended up going with their pro-View 9 degree, which you can adjust your gain to change the cone degree, you might want to check out website.

Offline slipperybob

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Re: Cone and fish depth
« Reply #7 on: Jan 24, 2021, 07:41 PM »
I learned from Vexilar website.
For more information read my MN nice journal

 



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