Sonar interpretation?

[Larry3215]

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?

The confusion comes from the way sonar is marketed. Its very deceptive the way its presented and most mfg's dont really want to get into whats really going on. They want you to think is showing you a lot more than it really is and that its telling you the absolute true facts when it really isnt most of the time.

The only two things a typical transducer knows is time and signal strength. Other than multi-beam transducers like Panoptix, no transducer can tell what angle the target is at. Even sideview has no idea what the angle is to the target. The most you know is somewhere on the left or right.

It sends out a ping, then waits for the signal to bounce back. It then measures how long that signal took to "return" and calculates the distance based on the speed of sound in water. It has no clue if that return signal is coming from the front or side or back of the boat. It cannot know the angle or direction. It then draws a point on the screen at that distance - and calls it the depth - and colors it in based on the strength.

Calling that distance number "depth" is what causes the confusion. A transducer is NOT measuring depth. Its measuring the distance of the target from the transducer. That target could be off to the side or front or rear or straight down.

The only time the distance is actually giving you the correct depth is when it just happens to be "returning" from a target/bottom/etc that is straight down under the boat. In every other case, the "depth" reading will not equal the true depth.

When you tilt the transducer, you are not changing the straight line distance to the bottom or to the ball, therefore the "depth" reading will not change. A ping "returning" from the bottom, or your ball, takes the same exact time to return no matter what the tilt angle is. When you change the tilt angle the distance does not change therefore the "depth" reading does not change.

The fish are different because they are entering and leaving the cone at different distances from the transducer when you tilt the transducer. When its tilted forward, the cone tilts forward so that the fish is first seen when its further forward - which is a greater distance from the transducer. That greater distance is drawn as more depth.

Here is a new pic like the one I drew above, but lets exaggerate the numbers to make it more clear and show the bottom and the ball are still in the cone.

Say your transducer is tilted far enough forward, so that now it first detects the return signal when the fish is 50 feet from the transducer. It will start drawing the arch at 50 ft "depth" on the screen even though the fish is only 20 ft down in reality.

When the fish is in the center of the beam its now 40 ft distance from the transducer, so the thickest part of the arch is drawn at around the 40 ft "depth".

The fish leaves the beam when its only 20 ft from the transducer - which just happens to be its true depth - so the top of the arch in this example is drawn at the true depth.

Note that the distance from the transducer to the ball and the bottom do not change. So the "depth" shown on the screen for both of them will not change either.

Hope that helps.

There is more to this distance/depth thing when it comes to interpreting other details on a 2D sonar screen, but I need to do some more drawings and make notes on some screen captures....

Forgot the picture...

[Larry3215]

Keep the questions coming. Its easier to answer a question than to start from scratch on some detail.

[TrackerPro16]

I have decided on getting the Panoptix 31 forward scan and have put some toys up for sale to offset the cost.
My question is:
If I tip the adjustable trolling motor mount straight down, what will I be seeing? Kind of goes with the other 'tipping' questions...

[Larry3215]

Panoptix is nothing like any sonar we have had in the past. It is multi-beam technology. From the little that I have been able to find out about the tech, its sort of (but not really) like a phased array antenna in that it can detect the direction and angle a signal is coming from plus a lot more.

Our normal transducers have no clue about a returning signals actual angle in relation to the transducer or the surface or anything else. The best you can get when using sideview is that the target is somewhere to the right or left without having any idea of how deep it is or what the angle is or what the true distance to the side is. I'll go into that more later.

Multi-beam sonar can tell you the exact distance, direction and depth - in real time.

The other big difference is that Panoptix forward view relies on reflected signals from the surface. That means the transducer should not be pointed straight down, but kept at the recomended 45 deg angle. Dont worry, you will be able to see directly under the transducer and even a tad behind it.

I should point out one thing I learned recently that might affect your decision. The PS21 and PS31 have exactly the same effective range. I had thought the larger PS31 had more range but it does not. The range limit is a function of the technology and not power or size.

The PS31 has several interesting extra functions that the PS21 does not. They each have pluses and minuses.

For me, I like the smaller size of the PS21 which makes mounting easier. There is lass drag on a stick mount. The builtin compass is handy but not really necessary. The compass wont work if you mount the PS21 to a trolling motor, so keep that in mind. Its is of course 1/3 cheaper which helps.

The PS31 has some cool extra functions like adjustable beam widths, steerable beam angles and a 3D bottom contour setting. It does not have a compass though.

I think you can download the user guides for both on Garmins site. If not, look up the user manual for the 7610 and it will have sections on adjusting settings for both models.

[Larry3215]

I have decided on getting the Panoptix 31 forward scan and have put some toys up for sale to offset the cost.
My question is:
If I tip the adjustable trolling motor mount straight down, what will I be seeing? Kind of goes with the other 'tipping' questions...

I sent you a PM but I cant tell if it actually got sent or not. If it didnt go through, let me know and I will try the email option.

[Amx]

To see if a PM did get sent, click on 'new messages' up next to 'user control panel', then look at 'out going' . If it's there it was sent, but he hasn't read it yet. If not there, look under 'sent'. If not there then it didn't get sent.

[Larry3215]

Thanks! I had no clue it worked that way

To see if a PM did get sent, click on 'new messages' up next to 'user control panel', then look at 'out going' . If it's there it was sent, but he hasn't read it yet. If not there, look under 'sent'. If not there then it didn't get sent.

[Larry3215]

It occurs to me that most of what we have covered so far is 'negative information' as far as sonar interpretation goes. In other words, we've been covering things that show up on the screen that are not really what they look like. Unfortunately, there is more of that coming. You cant really interpret whats on the screen if you thinks its showing you one thing when its actually something else.

Almost all of this comes from the distance/depth thing I covered earlier, so don’t forget that. Its going to keep coming back – especially when we get to sideview.

Ok, here are two more crappy drawings of mine that show situations that look like one thing on the screen but could easily be the result of something very different going on in the water.

The first two images shows what can happen if you have 3 fish, all the same exact size, and at the same exact depth spaced out equally, left to right, under the transducer as you pass over them.

In the first pic, imagine this is looking at the rear of the boat as it moves away from you. The first thing to notice is there are only two fish showing up on the screen when there are actually three fish. The second is that they are “graphed” or drawn at different depths even though all three are at the same exact depth. The third thing is they look to be different size fish even though they are all the exact same size.

All of these "illusions" are because the transducer only knows the straight line distance to the target and has no clue about where the fish are side to side.

The top fish looks bigger because its passing through the center of the beam and its closer to the transducer. Both of those things cause it to have a stronger return signal. The closer distance also means its drawn at a shallower depth. The other two fish just happen to be the exact same distance from the transducer in my example, so they both get drawn at the same slightly greater depth. They are both also further away from the transducer and on the outer edge of the beam, so they have weaker returns.

The second picture is a top down view of the same situation. I have the boat in the wrong place, but picture the boat driving over the same three fish. The beam spreads out in a circular fashion much like a flashlight beam. Its narrower close to the boat and wider as it gets deeper. (Thats actually not really true, but lets ignore that for now. It doesnt really change this part.)

This pic hows why the middle fish has a longer arch that starts sooner and ends later. It enters the beam before the two outer fish get detected, so its arch starts to get drawn first. It also stays in the beam longer, so its arch ends after the other two have left the beam. All of that, plus it being closer to the transducer, means it ends up with a longer, stronger return.

The final thing to notice is that the bottom arch is flatter than the top arch. That’s because the fish is traveling through the outer edges and there is less difference between the minimum and maximum return signal as well as less difference in distance from the beginning to the end of its travel.

[Larry3215]

Just to pound this a bit further, here is another situation with two fish that would show up on the screen looking exactly like the three fish example. To make it even worse, you have no clue if fish number two is on the left or right side of the boat unless it shows up on sidescan. Which it might or might not.

Keep in mind, these pics could also be showing you exactly what it looks like - one larger shallow fish and a smaller deeper fish. You have no way to tell by looking at the 2D sonar screen.

In fact, there are an almost unlimited number of ways fish of different sizes could be positioned under the boat to get that exact same exact screen.

[Larry3215]

Here is another example of what can happen on the screen that makes things look very different from how they actually are under the water.

In this example, with three stacked fish, all the same exact size, the lower fish will enter the beam first, so its arch starts out first and is the longest. The shallowest fish is last to be seen so its arch is last to be drawn and is shortest.

I didnt draw it very well, but all three fish look like they are different sizes for the same reasons as above. The top arch would be the thickest and darkest but the shortest.

Again, you have no way to tell for sure what the relative sizes are or where they are in the beam.

[TrackerPro16]

This is very interesting. Explained this way it is understandable how you could be mistaken where the fish are. I would wonder, depending on how far apart the fish are, if in 'catching' of them it would make that much difference? If the fish are sensing the vibration of a lure or flasher, or seeing a flasher in clearer water, I wonder if 'close counts'?

[Larry3215]

These things are all to illustrate the main point Im trying to make - no single type of sonar can tell you whats really going on under the water all by itself. Thats why you have downview and sideview and now Panoptix. Each one has strong and weak points and situations where they are better and worse than the others. None of them are perfect all by themselves.

It really takes all four types to get a really accurate picture and know where the fish are.

[Larry3215]

This next part is a little hard to explain, but its important for understanding what you are seeing on the screen, so I hope it makes sense.

When you see something like this next image 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.

That’s almost certainly NOT true for traditional sonar. Its much closer to true for down and sideview, but they are deceptive in different ways.

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 each 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.

Lets start with the first one. The fact that the screen never stops scrolling is why things like downrigger balls 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 like a flashlight beam, and depending on the cone angle and depth, actually covers a fairly large circular area on the bottom as you move along.

Imagine you are on your ceiling and shine a flashlight with a wide beam down on your living room. You have lamps, chairs, tables, a dog, a cat playing with some toys on the floor, maybe a big screen tv on a tall shelf.

How would all of that stuff get drawn on the screen when its all in the cone at the same time and getting hit by the same ping?

Everything in that room that returned a signal would have that signal drawn in the same vertical line – all at the same time - down the right side of the screen.

The tallest items would be show as shallowest and the short items would be deepest – but they would each just be one dot on a single vertical line on the right side of the screen. Actually, the number and color of the dots woule be based on the signal strength, so some items would be several dots and some only one.

If you looked at the return from a single ping it would just be a straight vertical line with the top at the shallowest depth and extending down to the deepest point on the screen. If nothing changed or moved, the screen would scroll over one line and a new line would get drawn exactly like the one before it.

In other words, if the boat was not moving, that very complex “bottom” would look dead flat as the screen kept scrolling to the left.

[Larry3215]

Here are a couple of drawings to illustrate this.

In the first one, imagine you are trolling along parallel to a shore line with a steep drop off to your left. Lets say your beams cone hits the bottom on the right side at a depth of 15 ft. On the left side, it hits at 30 ft and there is a fish sitting at 22 ft just off the bottom.

On every single ping, the sonar will draw every return from the bottom starting at 15 ft all the way down to 30 ft - all at the same time and all on the same vertical line at the right edge. On the next ping, it will do the same thing all over again, and again and again until something changes.

The fish return will also be drawn at the same time, but you will never see it because the bottom returns make a solid line with no breaks. In other words, your fish will never show up unless it happened to be shallower than 15 ft. In that case, it would be drawn above the solid bottom line and it would show up as an arch.


Something similar is going on in a situation like this next one. Imagine you drive over a dip in the bottom that is smaller than your beam width. You will never see that dip on the screen or any fish that might be hiding in it on traditional sonar. You would probably see the dip on downview or sideview. You have a fair chance of seeing the fish on downview but it would be long odds on sideview unless it was a big hole and a big fish and close to the boat.

[DavidA]

1. Massive amount of information! Thank you!
2. It is making my head hurt! But that is because ...
3. I am actually getting it! But ...
4. Whow! I THOUGHT I had an idea what was going on down there when I looked at my fish finder!

[18ftTrophy]

Now I know why I catch fish when there are none showing on my screen!!!!!

[Larry3215]

Whew! Im relieved that this is making some sense! These are tricky concepts to try to describe this way. Ive been wishing I knew how to make some cool GIF's or videos to make it easier to understand. I hate instructional videos though, so IM not going there

[TrackerPro16]

Now I know why I catch fish when there are none showing on my screen!!!!!

This. Makes perfect sense now.
I always presumed it was just because my line was so far back they swam into the area after I went past. Now I know it could be that or they were sitting there the whole time and my old crappy when brand new fishfinder just can't tell if there is a fish there or not!

[zen leecher aka Bill W]

Here are a couple of drawings to illustrate this.

In the first one, imagine you are trolling along parallel to a shore line with a steep drop off to your left. Lets say your beams cone hits the bottom on the right side at a depth of 15 ft. On the left side, it hits at 30 ft and there is a fish sitting at 22 ft just off the bottom.

hidden1.jpg

On every single ping, the sonar will draw every return from the bottom starting at 15 ft all the way down to 30 ft - all at the same time and all on the same vertical line at the right edge. On the next ping, it will do the same thing all over again, and again and again until something changes.

The fish return will also be drawn at the same time, but you will never see it because the bottom returns make a solid line with no breaks. In other words, your fish will never show up unless it happened to be shallower than 15 ft. In that case, it would be drawn above the solid bottom line and it would show up as an arch.


Something similar is going on in a situation like this next one. Imagine you drive over a dip in the bottom that is smaller than your beam width. You will never see that dip on the screen or any fish that might be hiding in it on traditional sonar. You would probably see the dip on downview or sideview. You have a fair chance of seeing the fish on downview but it would be long odds on sideview unless it was a big hole and a big fish and close to the boat.

hidden2.jpg

What would 2D sonar show this as?

[Larry3215]

What would 2D sonar show this as?

In my crude drawings above, the right side is supposed to be the screen on your plotter. In both cases, the bottom would just be a straight line across the screen assuming the boat was either not moving, or was following a constant depth.

In this next screen shot, there is no obvious way to tell if I have passed over a pot hole like in the second drawing. It just wont show up unless its pretty large in relation to the beam width on the bottom.

There is however, a strong hint that I am driving over a sloped bottom or an area where the bottom is not flat within the cone. Look at the digital depth reading in the top left corner. It says the depth is 202.2 ft, but if you look at the black arrow on the right side, the bottom is being drawn on the screen at about 185 ft. Thats a good indication that the bottom is not flat. You will see that discrepancy between the digital depth reading and where the bottom is drawn in many of the screen shots Im going to post later.

Im pretty sure the reason for that is because the sonar decides what to call the "depth" by averaging out the different returns that it decides are from the "bottom" and not suspended targets.