Sonar interpretation?

[Larry3215]

Is this subject of interest to anyone? I find it fascinating.

I started writing up a series of articles for Mike on sonar, but its not going like I hoped, so Ive decided to maybe try something else.

I think it might be better and more fun if I just started a thread on sonar interpretation, post some of my screen shots with some thoughts on what I think they mean, and let people ask questions as we go along and post their own screen shots to talk about and throw in their own ideas and interpretations.

In other words, a typical forum thread where anyone can ask questions or throw in their 2 cents worth if they want.

I would of course, win all arguments

Im planning to start with traditional sonar first, then down/sideview, then how to use contour charts to help with sonar interpretation and then new advances in sonar like Panoptix. I mainly want to cover a few things in each sonar type that are often misunderstood or are not easy to understand - like sideview and how far a target really is or isnt from the side of boat, etc.

We had a "post your interesting screen shots" thread a while back, but I was thinking of getting a bit more technical this time around, so fair warning - there will be some math

What do you guys think? Would that be worth doing? Interesting? Boring?

[TrackerPro16]

Sounds good! I do a 'little' math in my boat as I made a line angle/length/depth chart so I could tell how deep my presentation actually is in the water. That and stretched out my down rigger line with a 100' tape to check the reading on the down rigger. If I have 100' of line out with little weight on the presentation, and set the downrigger line at a 45 degree angle (1.8 to 2.0 mph on GPS with a 7 lb weight), the depth reads correctly! I check the line angles with my phone that has a level app built in with the compass app.
I think it sounds like the fish finder QA would be good.

[zen leecher aka Bill W]

Larry, you ever fish Moses Lake for walleye? We could have an "on the water" discussion.

[kodacachers]

Larry, I think that's a great idea. I often have no idea what I'm looking at so would love advice based on screen shots!

[DavidA]

I too think it is a great idea, particularly if you are willing to accept constructive criticism. Let's use this as a test of that willingness: To improve on your idea, how about I win 51% of all arguments!!?? Deal!?



In all seriousness, I look forward to it, especially since I recently went from a basic grayscale fishfinder, to a color one with GPS with lake charts, a plotter, CHIRP and downvu. I have much to learn! Bring it on!

[Larry3215]

Im glad to hear there is some interest in this! I think it will be fun.

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/self ... LE_ID=2967" onclick="window.open(this.href);return false;

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).

More later....

[Larry3215]

Larry, you ever fish Moses Lake for walleye? We could have an "on the water" discussion.

Hi Bill, we try to get over to that area a couple of times a year, but we have not actually fished on Moses yet. We always seem to end up at Potholes for some reason. After reading the preliminary notes that Mike Schmuck, the WDFW Warmwater Fisheries Biologist, sent me on the Fall Walleye Netting results, we decided to give Moses a try for sure this year. I have no idea when yet, but we could certainly try to work out a get together. Maybe we could chat about sonar and where the giant, monster sized walleye hide on Moses. We havent found them on Potholes yet

[Amx]

The Wallies see you comin, so they move to Moses. When you fish Moses they'll move to Potholes. Let me know when you go to Moses and I'll fish Potholes.

[salmonkiller]

I hope im not hi-jacking this thread, but from what i read this might be on the topic...

I have a Lowrance HDS5 -GEN2

I was wondering if someone could chime in on this….

Bottom
I was running downriggers on port and starboard side. My transducer is near the starboard side and its running parallel to the bottom of the boat just below the bottom of the boat. (The way it’s supposed to be installed)
-There was nothing stuck to my down rigger ball. Why was there so much interference?
On my friend’s boat (HDS7 exact same set-up) the down rigger ball is a small line….

middle pic
I angled the transducer down about 20 degrees down and now there is no downrigger ball (which I’m okay with) and shows fish just fine.

top pic
next day... down rigggers set at 24 feet, and transducer still angled at 20 degrees...


Is the sonar now in front or directly under my boat? Should I leave it there?

Am i getting the correct dept at this angle? Should i just take the fish icon off? and adjust my sensitivity?

Confused……

[Larry3215]

No problem with jumping in. Thats what this thread is for and those are excellent questions.

Let me give you the short answers first. Turn OFF the fish ID. They just clutter up the screen and make it impossible to tell whats really going on. Your plotter has no idea if those are really fish or not and the fish symbol hides the actual return. They could be anything from a thermocline to bubbles to noise to - maybe - fish.

On the downrigger pic, the fish ID are making that more difficult to tell whats going on but I suspect thats your flasher and lure/bait wobbling around just above the ball. Could also be a downrigger clip.

How those things look will depend on several things but mostly it depends on where in the sonar "cone" the ball and lures etc are sitting. Things like depth of the ball and boat speed and size of the ball all make a difference too. You are only moving at .9mph there and not very deep, so you probably dont have much blow back. That should put the ball pretty much in the cone making for a very solid return signal.

If your buddy has the exact same setup you do, maybe his ball is just more on the outer edge of the cone, so it shows up as a smaller, thinner line. You are getting a heavy, dark, thick line on your screen, so that ball is probably pretty close to the center of the sonar cone and maybe your Gain or Sensitivity is up.

Angling the cone more forward by tilting the transducer will do several things. After you turn off FishID, you should start seeing fish arches instead as long as the boat is moving. By tipping the transducer like that, your arches will be distorted and wont be as even and symmetrical. Thats not totaly bad, but I think they are easier to read and compare if they are nicely shaped.

It will also make figuring the fish depth readings more likely to be incorrect. If a fish shows up looking like its 20 ft down, that depth number could be off by a much larger margin than normal. It could be off by a LOT. I know this sounds contradictory, but the bottom depth readings will still be correct.

This happens to be one of the main things I wanted to go over when talking about traditional 2D sonar and sideview/downview, so IM glad you brought it up.

I need to make up some drawings to demonstrate what I mean, but your sonar does NOT tell you the depth of the fish. Its giving you the straight line distance from the transducer to the fish - which is only going to be the correct depth IF the fish is directly under the transducer. Most of the time, the depth numbers will be wrong to one degree or another. They will be more wrong if the transducer is angled. However, angling the transducer wont change the depth of water reading because the bottom is always directly under the boat. The straight line distance to the bottom just happens to also be the correct depth.

A fish that is passing through the cone on the outer edge on the other hand, will be further from the transducer than what its depth actually is. When you tilt the transducer forward, fish will seem to be deeper than they actually are.

[Larry3215]

I just saw the top pic with the transducer angled. The line from the ball is thinner because the ball is no longer in the center of the cone. Your cone is angled forward now and the ball is on the outer edge of the beam so the signal is weaker. The depth reading of the ball is probably still about as good as it was before assuming you have the same amount of blow back, The further back the ball is, the more the depth reading will be wrong, but that wont change because of the angled transducer.

Im working on some drawing that should help show what I mean.

[Larry3215]

This is a test to see if I a GIF will work...



Ok, look closely at this GIF from that Lowrance tutorial I linked to above. Whats going on here is one of the most basic keys to how things look on your screen.

On the left side is the boat and the right side is what it looks like on your plotter screen as a fish moves into and out of the sonar cone. That "into and out of is critical to the formation of arches. If the fish doesnt do the in/out thing you will not get arches.

The next thing to notice is that the arch starts out thin and at a lower depth when the fish first enters the beam/cone for two basic reasons. On the outer edge of the cone, the signal is weaker than it is in the center, so the return or arch is thinner and lighter in color. Its also slightly farther from the fish to the transducer on the outer edge of the beam because of the angle of the cone.

Take a tape measure and sit it on a table and measure the distance to the floor. Now, keep the tape measure in the same place, but angle if off to one side and measure the distance to the floor and it will be a larger number. The same thing is going on with your transducer and sonar. Any target will be closest to the transducer when its in the center of the cone and further away at the outer edges - UNLESS - your transducer is sitting at an angle. More on that later.

So as the fish moves into the center of the cone, the return signal gets closer to the transducer and the signal gets stronger at the same time. That means the arch is thicker and darker and the top edge is drawn at a shallower depth - causing the arch to form. As the fish leaves the cone, the signal gets weaker and the distance increases until the fish is out of the arch.

[zen leecher aka Bill W]

Larry, you ever fish Moses Lake for walleye? We could have an "on the water" discussion.

Hi Bill, we try to get over to that area a couple of times a year, but we have not actually fished on Moses yet. We always seem to end up at Potholes for some reason. After reading the preliminary notes that Mike Schmuck, the WDFW Warmwater Fisheries Biologist, sent me on the Fall Walleye Netting results, we decided to give Moses a try for sure this year. I have no idea when yet, but we could certainly try to work out a get together. Maybe we could chat about sonar and where the giant, monster sized walleye hide on Moses. We havent found them on Potholes yet

I've fished Moses every year since moving over here. Last year was the worst I've ever encountered. Mike's survey and also the walleye tournament results don't quite match up with our fishing results. The fish were there, the bait was there but for some reason the walleyes weren't feeding like they'd done in previous years. When we did find them they were shallower than what we've ever fished before. Hopefully 2017 won't have the same varying weather systems and will have something more consistent.

Don't know what big walleyes are but we've caught them up to 28inches out of Moses, Potholes and Banks. The key for that seems to be lure/bait selection that the bigger fish prefer.

What I'd like is a little on the water help with using side scan to id fish.

[Larry3215]

Forgive my bad drawing skills, but this is whats going on when you tilt the transducer. The green arrows are a fish entering and leaving the cone when its tilted. Pretend the fish is always at the same depth. Lets call it 20 ft deep. My numbers are just for illustration to make the point. I have not done the math to see how much error you would have with a 20 deg tilt, but it might be more or less than these numbers. It will vary a lot depending on the cone angle of your transducer and the Gain/sensitivity settings.

When the fish first enters the cone its much further from the transducer than it would be if the cone was pointing straight down. That makes the beginning of the arch start out much deeper than the fish actually is in the water. The same thing happens with the cone pointing straight down, but the error gets larger the more the transducer is tilted.

If the fish is actually at 20 ft depth, and the cone is tilted, the sonar might draw the beginning of the arch at say 28 ft instead of say 21 ft if the cone was straight.

As the fish gets to the center of the cone, you have maximum signal, so the arch gets thicker and darker, but the fish is still further from the transducer than its actual depth. So the thickest part of the arch might be draw at say 26 ft.

Then as the fish leaves the arch its at its closest point to the transducer, but its still not as deep as it is being drawn because its still further from the transducer than its water depth, so the final bit of the arch might get drawn at say 24 ft as it gets thinner and disappears.

So, your fish that is really at 20 ft might look like it 24ft to 28 ft deep. Like I said, its impossible to know for sure because there are too many variables - tilt angle, cone angle and Gain settings will all change the numbers.

BUT - the distance to the bottom from the transducer does NOT change, so your bottom depth reading will still be pretty close to as accurate as it was before. The exception to that would be if you have a very narrow cone angle and were pretty deep.

Also, the distance to the downrigger ball wont change, so it will also still show up at the same depth it did before as long as the blow back doesnt change. It will make a thinner line on the screen though because its going to be more out of the center of the cone than it was when the transducer was pointing straight down.

[Larry3215]

Larry, you ever fish Moses Lake for walleye? We could have an "on the water" discussion.

Hi Bill, we try to get over to that area a couple of times a year, but we have not actually fished on Moses yet. We always seem to end up at Potholes for some reason. After reading the preliminary notes that Mike Schmuck, the WDFW Warmwater Fisheries Biologist, sent me on the Fall Walleye Netting results, we decided to give Moses a try for sure this year. I have no idea when yet, but we could certainly try to work out a get together. Maybe we could chat about sonar and where the giant, monster sized walleye hide on Moses. We havent found them on Potholes yet

I've fished Moses every year since moving over here. Last year was the worst I've ever encountered. Mike's survey and also the walleye tournament results don't quite match up with our fishing results. The fish were there, the bait was there but for some reason the walleyes weren't feeding like they'd done in previous years. When we did find them they were shallower than what we've ever fished before. Hopefully 2017 won't have the same varying weather systems and will have something more consistent.

Don't know what big walleyes are but we've caught them up to 28inches out of Moses, Potholes and Banks. The key for that seems to be lure/bait selection that the bigger fish prefer.

What I'd like is a little on the water help with using side scan to id fish.

I wondered about that too because the reports havent been all that great. I assumed you guys were keeping things close to the vest ;) He did say there would be more details and conclusions in the final report, so Im guessing the fish/net numbers dont tell the entire story.

As far as on the water sessions, Ive been turning people down on that for a long time. It kills most of a day to do even a short session like that and a lot more if I have to cross the pass. I just havent felt like giving up the time. Now that Im retired, Ive been getting more requests and more people have actually been offering to pay me more than beer and lunch for my time ;) Thats putting a different spin on it, but Im still not decided if I want to do that very often or not. I let myself get talked into doing one paid lesson, but the guy is towing his boat up from Oregon to the Tacoma Narrows to meet me. There is some cool structure to look at under the Narrows bridge and that makes learning sideview much easier than a dead flat bottom. iI'll let you know if I decide to do more after I see how this goes. It might be a fun part time income thing....

In the mean time, I plan to go into quite a bit of detail on using sideview in this thread. Hopefully that will help.

[zen leecher aka Bill W]

My version of an "on the water" discussion would be mostly fishing with about 5 minutes of pointing at side view when it's a fish. The Humminbird tutorials are rather obvious when a fish is on side view. Real world isn't quite that easy.

I wouldn't say we're close mouthed about ML walleye fishing it's just that last year wasn't much for bragging about. The year before was a different story.

[Larry3215]

My version of an "on the water" discussion would be mostly fishing with about 5 minutes of pointing at side view when it's a fish. The Humminbird tutorials are rather obvious when a fish is on side view. Real world isn't quite that easy.

I wouldn't say we're close mouthed about ML walleye fishing it's just that last year wasn't much for bragging about. The year before was a different story.

We could easily do something like that next time we are out that way. I'll let you know when we have some plans.

Sideview is probably the worst way to look for fish. You wont see much more that a dot unless the fish is large and very close. Bait balls and large schools show up well even at a good distance, but not single fish. It gets worse in shallow water areas like Moses. The shallower the water, the less effective range sideview has.

The main problem though is that sideview draws the fish more or less to scale on the screen. When you compress 50 ft of water and bottom detail into a 4" section of screen, a 24" fish isnt very big at all. It will be about .16" long. On top of that, targets get compressed the further away they are from the boat. So a 24" fish might show up as 1/8" long on the screen when its 10 feet away from the transducer, but it will be just a dot when its 50 ft away and you will never see it at 75ft.

I'll go into that more later.

[zen leecher aka Bill W]

When I'm using side scan I typically set the range to 40-45 feet as I want a bit more sensitivity. It looks like the best use of side scan will be to effect stand off of rocks or other things and then position the boat accordingly to drag a side planer over the spot.

[DavidA]

So far I am following everything with 2 exceptions: Why won't a tilted transducer inaccurately show the 1. distance to bottom or 2. distance to down rigger ball, for the same reason it inaccurately shows the distance to fish?

[Larry3215]

When I'm using side scan I typically set the range to 40-45 feet as I want a bit more sensitivity. It looks like the best use of side scan will be to effect stand off of rocks or other things and then position the boat accordingly to drag a side planer over the spot.

Yeah, sideview is by far better at spotting structure. Each type of sonar has its strong and weak areas. You really need to be using them all to get a total picture of whats really going on under the water.