Sonar interpretation?

[Larry3215]

The next tip for spotting fish in the water column with SI also has to do with scaling. It can help a lot if you adjust your sideview range so that most of the screen is taken up by the water column and not much bottom is showing.

In other words, if the water is 20 ft deep, set your sideview range to just a hair over 20 ft. That will have the effect of making any fish returns in the water column larger than they would be if the range was say 100 ft. You wont see much bottom, but thats ok for this situation.

See what I mean in these two pics. The first one the range is just slightly more than the depth. In the second the range is much more than the depth. Any fish returns in the water column would have been invisible in the second image because they would have been too small. By the way, that second SI image is crappy because I was going way to fast for good SI - 35 mph.

[Sideburns]

So, brighter (& sensitivity?) and zoomed in screen settings for locating fish with imaging. Roger that. I'm pretty impressed that you can see any relevant imaging at 30+ mph. Either through poor transducer location, or settings, I see completely useless static on my DI screen past 10-15 mph.

[Larry3215]

I was surprised it worked that well too. I had to move the transducer to a different location and then make small adjustments to the height two different times to get it that good. All I normaly shoot for is decent view of the bottom and a good depth reading at speed. This was a nice bonus.

Getting good readings at speed is all about getting clean, smooth water flow over the transducer. My first location was too close to a strake and the turbulence it caused was a problem for the sideview even at low speeds. I have an aluminum deep V so Im lucky in a way. The bass guys often have stepped hulls and they can be a real challenge to get good readings.

[Larry3215]

That trick of increasing the Gain or brightness to see fish in the water column works great - but only for fish within a certain zone. You will only see fish within an arc under the boat thats equal to the water depth.

Any fish inside that red arc will show up on the screen inside the water column area - between the center line and where the bottom starts. The green fish will be in the black area on the screen drawn in at a distance of 45 ft.

The red fish will be drawn on top of the bottom return and you will have to try to spot it while its mixed in with all the other bottom detail on the screen.

If you turn up the Gain/Brightness enough to see the fish in the water column really well, the bottom return will wash out any fish returns outside the red arc. You will never see a fish thats further away than the water depth.

So, to see fish that are further away, you are forced to turn the Gain/Brightness back down to more normal levels. You're going to have to play with this to try to find a balance between not seeing fish returns or bottom detail and washing them all out. I dont know any way to do that but trial and error.

There are two things that help in this.

1) Larger fish tend to have fairly bright returns compared to most of the bottom, so that helps. You still need to play with Brightness and contrast settings though.
2) They sometimes throw shadows. Dense schools of fish and larger fish that are close to the bottom and close to the boat throw better shadows. Of course, bigger is much better as far as shadows. You will never see a single small bluegill's shadow.

I'll post up a couple of screen shots so we can look at details more closely.

[Sideburns]

Is it correct to assume that the visibility radius limitation is very similar with the 2d cones?

[Larry3215]

Not really.

It isnt that the fish are not seen by the transducer. The fish outside the water column arc are still being hit by the pings and those return signals are being drawn on the screen just fine.

The problem is that they are being drawn in the same place the bottom returns are being drawn. The difficulty is that the bottom returns clutter up the screen and make the fish returns from outside that arc hard to separate out.

Any returns inside the red arc in my drawing above get drawn in that nice clean black area between the center line and where the bottom starts on the screen. Thats what we are calling the water column. I know its not really a "column" at all, which is confusing.

Any fish outside that red line gets drawn on top of the bottom return - which is often very cluttered. If you turn up the Gain/Brightness a bunch to make those inside returns easier to see, the bottom return gets very bright and totally washed out. Any fish returns that are on top of that washed out bottom will be impossible to see.

Is it correct to assume that the visibility radius limitation is very similar with the 2d cones?

[Larry3215]

Maybe this will help.

In the first drawing, there are two green fish that are both less than 50 ft from the transducer. There are four red fish that are all more than 50 ft from the transducer. The 50 ft is important because thats how deep the water is.

That entire area inside the 50 ft circle is what we are calling the water column - even though its not a column at all.

This next picture is how those fish will get drawn on the screen in sideview.

The first thing to notice is there are three green fish on the screen when, in reality, there are only two in the water. That green fish right near the bottom under the boat is picked up by the left and right side cones and is drawn in mirror image on both sides of the screen. The other green fish looks like its right near the bottom, when in fact its more like 25 ft deep.

The next thing to notice is that the bottom starts getting drawn 50 ft to the side. Thats the water depth. All of the red fish are more than 50 ft from the transducer, so they get drawn more than 50 ft to the side - on top of the bottom return.

If that bottom return is washed out from having the Gain/Brightness or contrast too hi, then you wont be able to see those red fish. They will be lost in the washout of the bottom.

[Larry3215]

Lets take another look at those same two drawings, except Im going to change the screen drawing to better reflect how those fish might show up and how you might be able to tell where they are - or at least get a better idea.

Here is the first pic again - 2 green fish and 4 red fish in the water.

Now here is how it might look if some of those fish were throwing shadows.

The first thing to notice is that there are only 3 red fish drawn on the screen this time. The red fish thats near the surface on the left side probably would never be seen by sideview because of the way the cone angles work. Fish right at the surface just wont show up on sideview most of the time.

Now lets assume that all the red fish just happen to be about the same distance from the transducer. Two of them are very close to the bottom and one is well up in the water. All three look like they are the same distance to the "side" but they are not.

One way to get an idea of where they are is by the shadows - if they throw shadows. I rarely see fish shadows in real time on the water. They are usually hard to spot. But lets assume they show up. In this example, the fish close to the bottom will have shadows that are close to the fish targets. In this case, that one green fish and two of the red fish might have shadows. The fish thats not near the bottom either wont have a shadow or it will be a long ways away from the target.

Here is what it might look like if the other green and red fish did throw a shadow. Becuase of the angle from the transducer, the green fish, thats higher in the water, will have a shadow thats closer than the red fishs shadow. Imagine the transducer is a flashlight pointing at the fish and casting a shadow on the bottom.

[TrackerPro16]

Even though I knew about the shadows before this, the diagrams still help. Or irritate me, I am not sure which... Like was said, spend the money and you STILL don't really know exactly where they are. Better, but not exactly.

[Larry3215]

hehehe Yup, thats the most common reaction when people first figure out this stuff - how deceptive their sonar screen displays really are. I was really ticked off when I finally managed to over come my years of misconceptions about what I thought I was seeing on the screen. I still remember the arguments I had with Doug and Rickie and other people on-line.

The key thing for me was really getting what it means about the transducers only knowing distance and having no clue about depth or angles to targets. That and understanding how they are taking a 3 dimensional situation under the water and compressing all that data into a flat 2 dimensional screen. Its very hard to get past those things when they call it a "depth" finder and all the scales are labeled "depth". The advertising certainly doesnt help a true understanding either.

Once you start to really understand whats going on with 2D, DI and SI, you will begin to realize just how truly awesome and game changing Panoptix sonar is - especially the live view forward.

FINALLY you can actually see and know where a fish really is! With Panoptix, you can see its exact depth, its exact distance from the boat and its direction from the boat - and all in real time as its happening. Plus - no math and no close inspection of the screen and no mental gymnastics required just to make an educated GUESS about where the stupid fish MIGHT be. Or, to be more accurate, where it WAS a litte while ago, somewhere behind the boat. Who knows where it is now....

Panoptix isnt perfect and has its own limitations, but it is still revolutionary and at least an order of magnitude better than what we have had in the past.

[Larry3215]

I want to change gears a bit and look at some actual sideview pictures and see what we can tell from them. Then Im going to finally make you do some math

Here are a series of screen shots we got in one of the Chain Lakes in Coeur d'Alene.. Its shallow water that was full of crappie, bluegill and some perch. Its also supposed to contain large pike and sm bass. This section of the lake was full of timber, trees and logs which made for some interesting things to see on side and downview.

Im just going to put these out there for now, then come back with some close ups of details and talk about what they might mean.

As you can see from these pics, any fish targets, outside the black water column area, could be hard to spot even when the bottom isnt washed out and over bright. I think there are some fish showing in these shots, but they are not easy to see.

Also, note there are lots of shadows from the logs but no obvious fish shadows. I think there are some that could be from fish, but you have to look very close to spot them.

[Sideburns]

First image is using 455khz looks too blurry gain set really high?. Hard to see much detail. Odd that the 260mhz images are crisper. That surface clutter must being telling us something about the depth or bottom being different left to right?

Swarms of bait in the top and bottom images?

[Larry3215]

Good guesses!

The bright yellow line down the center just represents the boat track. It never changes.

In picture 2 and 3, the thick fuzzy line just to the left side of center is because I forgot and left my main motor lowered all the way down. If I trim it up some that line goes away.

In the top pic, the fuzzy line is on both sides and is wider. Notice the boat speed in the top pic is almost double what it was in the other two. We went from 1.42 to 2.49 mph. That thicker line on both sides is the wake turbulence from the bow mounted trolling motor set to hi power. Its a little thicker and darker on the left side because the motor is on the left of the bow and my transducer is on the right side of the keel.

You can get similar fuzzy lines when there is a lot of junk in the water near the surface. This little lake was stained but pretty clean water.

All three images are distorted to some degree because there was some wind causing the boat to track sideways to some degree. That causes the images to get skewed because the scan is not 90 degrees to the direction of travel. I'll go into that more in a while.

The top pic is worse because Im getting some interference when the sideview is set to 455. You can tell partly because of those dark ghost lines on either side of center that look like trenches. Those lines are almost invisible when Im set to 260khz. The combination of a skewed path and interference made the top pic worse.

There are subtle signs of bait in all three pics. The fuzzy stuff at the top of the top image is us heading into a weed line.

[Amx]

The bright yellow line down the center just represents the boat track. It never changes.

Is called the 'sonar burst'.

[Larry3215]

The bright yellow line down the center just represents the boat track. It never changes.

Is called the 'sonar burst'.

Is that a Lowrance thing Tom? I havent heard the term used quite that way before. On my Garmin, that yellow line is always there even if I have the transducer turned off and not transmitting.

[Amx]

The bright yellow line down the center just represents the boat track. It never changes.

Is called the 'sonar burst'.

Is that a Lowrance thing Tom? I havent heard the term used quite that way before. On my Garmin, that yellow line is always there even if I have the transducer turned off and not transmitting.

That's what I was told when I bought my first HDS10 from the guy that holds classes on depth finders, and was a Lowrance rep, so it might be just a Lowrance term. And I've never turned off the transducer, I don't know if that is possible while leaving the head unit on.

[Larry3215]

Yeah, every brand does things a little differently and they like to coin their own names for things - like Lowrance and Structure scan for sideview and Garmin now calling downview Clearvu and Humminbird's new Mega.

Doesnt make it any easier for us users.

[Larry3215]

Here are a couple of drawings showing what I was talking about with the skewed drawings and the distortion in my sideview images.

One major thing to remember when using sideview and to some degree downview, is that the boat needs to be moving over the ground in exactly the same direction it is pointed. In nautical terms, you need your Heading to match your COG or Course Over Ground.

When a car is driving straight down the road, its (almost) always pointed in the same direction its moving. Thats a case where Heading matches COG perfectly. The exception to that would be if you are sliding sideways on ice or a slick road. In that case, Heading (the direction the front of the cars is pointed) does NOT match the cars course down the road or over the ground. With boats, its very easy to have that sideways sliding thing going on and you may not even notice it.

In those pictures above, the boat was being pulled by the trolling motor in one direction, but the wind was blowing from the side which caused the back of the boat to be pushed to one side. The meant boats Heading did not match its Course Over Ground. This can often be really bad at low speeds and you may not even notice it. It doesnt matter if you are using a trolling motor or the main, if you are heading one direction and there is wind or current from the side, you will be traveling skewed to your course.

This first drawing shows what happens with your sidescan cone when the boat is angled or skewed to your course over ground. Imagine those two blue squares are some underwater structure positioned on either side of your path and you drive the boat from west to east between them while some wind or current is coming from the south or bottom of the image.

This next drawing shows how those two squares would be drawn on the screen. Note that the sidescan cone will "see" the one on the left before it sees the one on the right. That causes them to be in different places on the screen. The cone also runs into the corner of the squares first instead of the flat sides. That causes them to look twisted and can cause elongation in the direction of travel and they will be shortened from side to side. There distance from the boat is also wrong. They will show up on the screen as further away than they really are. The degree of distortion will depend on how much difference there is between the direction the boat is pointed and its actual path.

If the boat is drifting perfectly sideways, the images will be nothing but meaningless noise. Downview also suffers from this problem but it is less noticeable.

[Larry3215]

The only effect that distortion will have on fish returns is that they will be offset on the screen just like the squares were. That will make them much harder to find. Structural details on the bottom can be distorted beyond recognition.

[Larry3215]

Ok, now its time for some of that math Ive been promising

Ive been telling you all kinds of bad things about sideview, and its about to get a little worse, but its still a very valuable tool. Thats especially true if you use it with 2D and DI to help put together a better picture of whats under the water.

One of the things sideview IS good for is spotting structure. Even if you cant see fish, you can see structure where they might be hanging out. Seeing that structure isnt going to do you much good if you cant find it though.

If you see a pile of rocks or brush or something off to the side, how far away is it really? It turns out, there is some math for that!

You can use the Pythagorean Theorem to figure out the true distance of any bottom target to the side of the boat. You need to know the water depth under the boat and what the distance scale is saying on the screen. Lets call the depth D and the True Distance T and the distance that is shown on the sideview screen S.

I know all of you keep scientific calculators on the boat with you, right???