Author Topic: 2-D Sonar - depth of fish when they are on the sonar cone's edge  (Read 11470 times)

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Offline ITGEEK

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Someone told me that if a fish is just on the edge of the
2-D sonar cone, they will actually be shallower than the
depth shown on the sonar screen.

Can anyone verify this?
« Last Edit: July 31, 2012, 01:48:14 PM by ITGEEK »


Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #1 on: July 31, 2012, 02:33:45 PM »
This is absolutely true.  Ive learned over the years to NEVER fish the marks on sonar. Generally you will be too deep. Now i might be fishing different than alot of people on here but I troll lake erie for walleye and steelhead and found that if i fish the marks i generally dont do that well.  Ive read and talked to other captains about this very thing and weve all come to the conclusion that if you target 10-15 ft. above your marks you will have a way better success rate.  There is a reason for the fish being shallower than marked on the screen and it has to do with the cone angle.  Depending on the depth your fishing, the deeper the water the bigger the cone gets. Now sonar doesn't see distance.  It "times" the send and return and takes that amount of time and graphs it for the depth.  Because the cone shoots in an angle, the fish on the edge get marked deeper because the beam shoots out farther on the edges than if it would shoot straight down.  So the return takes a little bit longer time wise than if the same fish was straight down under the boat.  Its a little hard to explain without pictures.   Here is a link that explains it pretty well.  Please take some time and really read and explore this website.  There is a ton of information on it and it really helped me out considerably. http://biggeorgiaspots.com/boats-and-electronics/2d-traditional-sonar/73-understanding-sonar-cone-angle.html?showall=1
Hope this helps,
RH
« Last Edit: July 31, 2012, 02:36:42 PM by Rough House »
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Offline Humminbird_Greg

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #2 on: July 31, 2012, 02:37:03 PM »
Excellent answer Rough House and far better than the meager explanation I was going to post!
Greg Walters at Humminbird
gwalters@johnsonoutdoors.com

Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #3 on: July 31, 2012, 02:41:08 PM »
Well Thank you Greg.  Ive been down this road before and it took awhile to figure out it for myself.
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Offline lajo1976

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #4 on: July 31, 2012, 02:49:08 PM »
Excellent answer. I often drop shot fish for larger fish I see on the sonar and if i don´t go right over the fish the arc often appear twice the depth of actual depth of the fish. Those fish are often seen as black high arcs(on my lowrance sonar).

Offline ITGEEK

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #5 on: July 31, 2012, 02:58:32 PM »
Rough House:
Thanks for the reply.

I prefer trolling also.
I can see the depth being off maybe one foot, or a maximum of two feet.

But 10 to 15 feet is unacceptable in my book.
If it's off that much, I don't see the point of even using a sonar.

At some point in your fishing, you are going to go directly over fish,
then the sonar should be dead on.  In your scenario, you won't
be fishing deep enough for those fish.
« Last Edit: July 31, 2012, 03:28:10 PM by ITGEEK »

Offline ITGEEK

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #6 on: July 31, 2012, 03:27:04 PM »
Hey Greg,
Is there any way that you could provide information about
how much the depth of the fish seen on the sonar
can vary from their actual depth,  based on the depth
of the water.

I'm using the 200/83 khz beams for my 2-D.

Thanks.

Offline Humminbird_Greg

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #7 on: July 31, 2012, 03:44:52 PM »
Already on it ITG.  I ran some numbers and here is what I got (I think I got the math right):

Greg Walters at Humminbird
gwalters@johnsonoutdoors.com

Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #8 on: July 31, 2012, 04:37:24 PM »
Ive had walleye come up from 75 feet to hit a plug running 25 foot down. I could actually watch the fish come up on the sonar. The water clarity really has an impact and erie is extremely clear.  Ive been told by wreck divers that on a good day they can see over 200ft in 100-120 feet of water.  So the fish can see, feel, and sense baits above them for a long ways.   In walleye fishing especially, you always want you baits above them.  You will never catch walleye with your baits below them because they dont feed down.  only up.  What i do is find where most of the fish are marking.  so lets say im marking the majority of the fish between 45-55 feet down.  Ill put my baits anywhere between 30-40 feet down.  I know that some fish are going to be down at 55 so one thing to do is stagger baits until you find exactly what they want.  So maybe ill run my downriggers and dipsys at 45-55 but my board rods will always be in that 30-40 ft. range.  The sonar can be off quite a bit the deeper you go but without the sonar you wouldnt be able to tell if fish were even there.
  One way of Recognizing if the fish is on the edge of the cone or in the middle is the shape of your arches.  If there really nice and uniform arch where it looks like a perfect arch its usually an indication of the fish caught in the 20 degree cone (83khtz).  If one side is longer than the other It usually means that there off to the left or right of the boat and getting picked up with the 60degree cone (200khtz) (if your running dual). The 83 is more accurate depth wise than the 200 but it doesn't give as much information.  I run it in dual. I never run the fish ID but to my knowledge the fish id shows a different color fish id to which beam it hits.  Gold and Blue for 83 and 200.  so you can judge what fish are off to the side and which ones are straight down.  I dont remember which color is for which signal though. Have to look that up.  its nice to have the side imaging to see if the fish your marking are to the right or left when you pass over them. 

Heres a pic of what im talking about.  This is a shot of a school of perch with walleye under neath the school. Judging the sonar pic you can see that the archs of the perch aren't uniform which tells me to some extent that most of the fish are being picked up by the 60 degree cone. (off to the left or right of the boat).  The walleye underneath the school of perch (on the bottom) are really nice uniform marks (more horizontal than the perch) which tells me that there mostly being picked up by the 20 degree cone (right below the boat)  Hopes this helps, RH
« Last Edit: July 31, 2012, 04:51:56 PM by Rough House »
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Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #9 on: July 31, 2012, 04:47:47 PM »
Thanks for that chart greg.  Thats some really good information.
I never did the math over 100 ft.  Thats crazy that at 200ft the 60 degree can be off up to 24 feet.  WOW!!!   Keeping this chart on the boat.
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Offline lajo1976

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #10 on: July 31, 2012, 04:54:01 PM »
Usually when you got arcs like that the transducer is tilting a bit forward/backward.

Attach a picture from a Lowrance unit. Do you got those high arcs with Humminbird sonars?


Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #11 on: July 31, 2012, 05:02:36 PM »
That has not been my experience with it.  You can see that the perch marks are very different than the walleye marks.  The perch marks look like the lower case r.  Where most of the walleye marks look similar to a sideways parentheses  ( *flip to the right. lol. When i went over this pics the side imaging showed the perch slilghty to the left of the boat and the walleye were straight down.  Caught both perch and walleye when i hit this spot.  Thats why i know that these are perch and walleye. FYI.
The 60 degree cone hits the fish at an angle and thats what is shown like this on the graph. (assuming that the fish are swimming horizontally)
« Last Edit: July 31, 2012, 05:12:06 PM by Rough House »
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Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #12 on: July 31, 2012, 05:06:12 PM »
Usually when you got arcs like that the transducer is tilting a bit forward/backward.

Attach a picture from a Lowrance unit. Do you got those high arcs with Humminbird sonars?

What beam are you using when you took this pic.  Those arches really look like the fish are directly under the boat and caught by the 20 degree cone.  and im assuming that the other marks are you downriggers balls???  Thats what my downrigger balls look like.  Looks like you sent one down to the bottom and brought it back up towards the fish
« Last Edit: July 31, 2012, 05:09:15 PM by Rough House »
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Offline lajo1976

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #13 on: July 31, 2012, 05:08:26 PM »
ok, seems like bird and lowrance shows arcs a bit different. Thinking of a switch to bird for SI but I really like the 2d sonar on lowrance.

Offline Rough House

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #14 on: July 31, 2012, 05:14:42 PM »
ok, seems like bird and lowrance shows arcs a bit different. Thinking of a switch to bird for SI but I really like the 2d sonar on lowrance.

They both have great 2d sonars.  Im really partial to my Hbird.  Had a lowrance hds before getting the Hbird and i really prefer the Hbird in all aspects. Just my preference.  Both units are great.
« Last Edit: July 31, 2012, 05:34:56 PM by Rough House »
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Offline ITGEEK

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #15 on: August 01, 2012, 08:19:38 AM »
Thanks a lot for that information Greg.

So, in light of this information, this is the conclusion that I have made for myself:

If I'm trolling lures, and I am trying to target fish at a specific depth,
and I am using the dual 2-D frequency (200khz, and 83khz), and
I have the fish ID on:

Then I should trust that the depths shown by Golden fish
(200khz - 20 degree beam), are much more accurate than
depths shown by Blue fish (83khz - 60 degree beam).

Does that sound right?

Offline Humminbird_Greg

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #16 on: August 01, 2012, 11:23:05 AM »
Thanks a lot for that information Greg.

So, in light of this information, this is the conclusion that I have made for myself:

If I'm trolling lures, and I am trying to target fish at a specific depth,
and I am using the dual 2-D frequency (200khz, and 83khz), and
I have the fish ID on:

Then I should trust that the depths shown by Golden fish
(200khz - 20 degree beam), are much more accurate than
depths shown by Blue fish (83khz - 60 degree beam).

Does that sound right?

Yes it does ITG.
[The manual states that those are "orange" colored fish, not gold]

Remember though that the above calculations are all based on 20 and 60 degree sonar beams that were measured at the -10db point. Real-life conditions can add or subtract to the effective sonar beam with.

Rule of thumb: the narrower the sonar beam, the more accurate the water depth of any fish shown.  With a wide sonar beam there is an increased chance that the depth of a fish may not be accurate.
Greg Walters at Humminbird
gwalters@johnsonoutdoors.com

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #17 on: August 01, 2012, 11:27:28 AM »
ok, seems like bird and lowrance shows arcs a bit different. Thinking of a switch to bird for SI but I really like the 2d sonar on lowrance.

They arch fish differently due to the shape of the transducer sonar beams.  If we designed our transducers with stronger side lobes we would be able to have these super wider arches for fish and other sonar targets as well.  The flip side of that is that you sacrifice bottom detail.  With a narrower sonar beam you gain bottom detail and water depth accuracy but sacrifice water column coverage and the ability to show very wide fish aches.  The reverse is true of wide sonar beams: less accurate bottom detail and water depth but increased water column coverage and fish arching.

Greg Walters at Humminbird
gwalters@johnsonoutdoors.com

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Re: 2-D Sonar - depth of fish when they are on the sonar cone's edge
« Reply #18 on: August 06, 2012, 03:15:31 PM »
Rough House: The first high arch is a 14 lbs zander(walleye) I first passed with boat. I reversed the boat and then dropped my bait 2 feets above the fish. In a later moment the fish bites the lure.


What beam are you using when you took this pic.  Those arches really look like the fish are directly under the boat and caught by the 20 degree cone.  and im assuming that the other marks are you downriggers balls???  Thats what my downrigger balls look like.  Looks like you sent one down to the bottom and brought it back up towards the fish


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