Sonar interpretation?

[Larry3215]

Another very interesting thing about those shots is the difference between the bottom thickness on the 77 and 200 sides.

The 200 khz bottom line is much thinner than the 77 khz line. As mentioned before, you have probably read that the thickness of the bottom line has to do with bottom hardness, but thats not the only thing going on that determines that bottom thickness. Its actually not the main thing in this case. Both screen shots are showing the exact same bottom at the exact same time. There cant be any difference in hardness under those conditions.

Whats going on here goes back to my earlier comment that every return within the cone gets drawn on the screen at the same time and on the same vertical line at the right side of the screen. This is the same thing that causes the dead zone. In fact, the bottom thickness is a good indication of how large your dead zone is at any given time.

Assume for a minute that that shot was taken on a perfectly flat bottom. The point on the bottom exactly under the transducer will be closest "return". Lets say that point is 20 ft down. So the first part of the bottom that gets drawn on the screen will be from the center of the cone and will show up as 20 ft down.

Now look at all the points on the outer rim of the cone where it hits the bottom. They will all be further away from the transducer than the center. Again, flat bottom. That means all those returns from the outer edge of the cone will be drawn at a deeper "depth" on the screen. Lets say that the distance from the transducer to the outer edge of the cone is 22 ft. That means the bottom line will be drawn on the screen between 20 ft and 22 ft - no matter how "hard" the bottom is.

When you have a wider cone angle, you have a wider circle. That means a bigger difference between the closest bottom return and the most distant bottom return. That causes the thicker bottom line on the 77 khz screen shot - even though there is no difference in hardness.

So, because the 200 khz cone angle has a 5 ft difference in the distance measured down to the center of the circle and down to the edge of the circle, it has a 5 ft thick bottom line on flat ground. The 77 in this example will have a 10 ft thick bottom line on the screen - even though there is absolutely no difference in bottom hardness.

[Larry3215]

Now lets look at how the different cone angles effect the dead zone on sloped bottoms.

Basically, its the same thing that happens when you get a thicker bottom line with a wider cone. The wider cone also creates a thicker dead zone on sloped bottoms.

Here are some more drawings to show whats going on.

Remember what we covered about dead zones earlier - because every return within the cone gets drawn on the screen at the same time, things like fish, that fall in between the shallowest and the deepest parts of the bottom, are hidden by the bottom return when they get drawn on the screen.

This is a re-work of my earlier example with two different cone angles instead of one. Imagine you are trolling along parallel to a drop off and there are a total of 6 fish under the boat at various places.

Lets look at the 77 deg side first.

The wider 77 deg cone hits the bottom at 20 ft on the shallow side and 40 ft on the deep side. Notice that bottom line is drawn starting at the 20 ft depth extending down to the 40 ft mark. That difference in depth between the hi and low points within the cone creates a 20 ft wide "dead zone" where fish wont show up. The fish returns are there, but they are hidden within the bottom return because all the returns are drawn at the same time on the same vertical line at same time on the right side of the screen.

There are 6 fish under the boat within the 77 cone, but only the green one shows on the screen. It happens to be above the shallowest part of the bottom thats within the cone, so its arch can be seen on the screen. The other 5 fish fall within the dead zone and cant be seen on the screen.

You can see on the left side, that the green fish is actually about 7 ft vertically above the bottom, yet it gets drawn on the screen so that it looks like its almost sitting on the bottom.

Also, the digital depth number in the top left is 27 ft in my made up example, but the bottom line is being drawn on the screen at 20 ft. Thats because the 20 ft mark is the shallowest point within the cone. The digital depth number comes from some sort of averaging/calculation that the MFD does when it decides what the "depth" is based on all those different returns. Unless the bottom is dead flat those numbers will almost never match.

Now the 200 side.

The narrower cone on the 200 side hits the bottom at 25 ft on the shallow side and 30 ft on the deep side making for a much narrower dead zone. There are two fish within the 200 cone, but once again, only the red one shows on the screen. Because of the smaller dead zone, the 200's narrower cone is able to "see" fish that are closer to the bottom than the wider cone on the 77 side. It also comes closer to showing how far up from the bottom the fish really is. That narrower cone also draws the bottom depth closer to what is really is under the boat and will also give a better representation of the shape of the bottom contour as you drive over it.

The main downside to the narrower cone is you miss more fish that are off to the side of the boat. You pretty much have to drive right over the top of them to see them where as the wide cone is more like a shot gun blast and hits anything even if its off to the side.

[Larry3215]

Here is the other possible situation where the wide cone hides stuff that the narrow cone can show. Remember the hidden pothole we talked about before? This is how it looks. With the wide cone, you dont even know the hole is there and never see the fish. The narrow cone can show both - if you happen to drive right over the top of it.

[Larry3215]

Thats enough for tonight - more in the morning after some coffee,,,,,,,

[Sideburns]

Alot of the same principles at work here as your si/di explanations.

just got my new 20*/60* 2d transducer and y cable in the mail, to bypass the 20*/16* 2d elements built in to the DI transducer. It will be interesting to see the better fish arches, but I can see how I might be more mislead as to where they are!

Its eye opening to see that widely spread out fish at a precise depth show up the same as tightly grouped but varied depth fish do!

Changing the direction of travel helps solve some of the slope problems in di/si but sure doesnt do any favors to clear up the 2d slope problem..... hmmm... thats why I could never find that ravine in 2d mode!

Thanks for taking all that time drawing that stuff up!

[Larry3215]

Alot of the same principles at work here as your si/di explanations.

just got my new 20*/60* 2d transducer and y cable in the mail, to bypass the 20*/16* 2d elements built in to the DI transducer. It will be interesting to see the better fish arches, but I can see how I might be more mislead as to where they are!

Its eye opening to see that widely spread out fish at a precise depth show up the same as tightly grouped but varied depth fish do!

Changing the direction of travel helps solve some of the slope problems in di/si but sure doesnt do any favors to clear up the 2d slope problem..... hmmm... thats why I could never find that ravine in 2d mode!

Thanks for taking all that time drawing that stuff up!

Someone has actually been paying attention! I was starting to think I had put everyone to sleep

You are exactly right. The same few basics apply across the board with (almost) ALL the sonar modes we are used to seeing - 2D/DI/SI. Thats because they all work pretty much the same way with the only significant differences being how they are displayed on the screen.

Those basics again are that:

1) Sonar only knows distance and signal strength - not direction or angle or true depth.
2) The screen never stops moving and only one thin line at the edge of the screen is up-dated on every ping.
3) Every return within the cone is drawn on every ping on that same line.
4) It assumes the bottom is flat.

The exception to that is multi-beam sonar, which I will get to shortly, It adds the ability to know the exact angle and direction the return is coming from AND it up-dates the entire screen in real time on every ping - and that changes everything.

I want to go back to slopes for one more detail that relates to your comment that I highlighted above. There is actually a slight improvement you can see on 2D by going up or down hill rather than sideways on a slope.

[Larry3215]

I think I mentioned earlier that sometimes you can see part of an arch sticking out of the side of a hump or a rising or falling bottom line on the screen. That can only happen if you are going UP or DOWN the slope. Those partial arches will be hidden when you are traveling parallel to the slope.

Here is what it can look like on the screen when you are heading up or down a slope instead of parallel to it - crossing contour lines instead of going parallel to them. This is where you can get partly buried arches. As the cone first hits a fish thats near the bottom on a slope, the arch starts to get drawn as usual. But then the cone continues to climb up the hill until it hides the rest of the arch. These will not show up when you are going parallel to the slope.

Thats whats going on on the left side of this screen shot. We were drifting from deep water up toward the top of Toliva Shoal, which has a pretty steep slope, while fishing for ling cod. You can see the arches sticking part way out from the bottom line.

As we got near the top of the shoal, the contours changed so we were drifting more along the side of the slope instead of climbing it, so the bottom line is much thicker there - as is the dead zone. Those arches along the top of the hump were probably fish much further up from the bottom than the ones on the left of the screen as we climbed up the hump.

[TrackerPro16]

During the first (and only) time out with the new gear we were fighting wind and were going crooked every now and then. I actually looked for, saw and identified the 'smear' on the sonar because of your writings here. We are paying attention! Really!

[Amx]

Yup, I was playing with the khtz yesterday on my front HDS9 gen 3.

It has 200, 83, and 50. The 200 and 50 looked similar to the right image, but the 83 looked like your left image. I'll need to read the owner's manual and see what that has to say about it. It was set at 200 from the factory, yes the factory/main office/replacement office in Tenn? Kentucky? sent it to me to replace the HDS7 gen 2 that went bad. I upgraded to the 9.

[Larry3215]

New toys are always fun!

[TrackerPro16]

I am curious about your take on the Garmin TVG setting on their sonar. One video (not Garmin) I watched stated it made the size of a fish on the screen the correct size arch whether it was deep or shallow so the arches would NOT get smaller (less of the radar beam reflecting) as you went deeper. The manual does not explain a number of things very clearly...

I have re-read this entire thread a number of times to try and cement what all was gone over. If I approach it as 'sonar only knows distance' and reference everything I see from that viewpoint it makes more sense!

[Larry3215]

Interesting question and something that is another of my pet peeves - at least with how TVG is often used or marketed.

Ive seen those videos you talk about and had many discussions where people say thats how/why to use TVG - to make the returns of deeper fish look the same as shallower fish.

BUT

Thats not really its main function. TVG is primarily for reducing noise on the screen. In the Garmin manual, its listed under the "Noise Rejection" heading.

Yes, it will tend to make lower returns look like shallower returns, but there is a huge giant elephant in the room that they dont mention - where is that fish within the cone?

Remember my drawing from earlier about the size of arches based on where they are in the cone? Equal sized fish on the outer edges of the cone have shorter, weaker arches. Fish in the center have longer, stronger arches. The signal strength on the outer edges of the cone could be as low as 1/10 of the signal strength in the center and still give an arch on the screen.

The problem is, your sonar screen has no way to tell you if a fish is in the center or on the edge or somewhere else in the cone. So, even with TVG on, you still have no idea if thats a little fish in the middle of the cone or a huge fish 10 times as big on the outer edges of the cone. The arches could easily look exactly the same.

So, when people say TVG will make a 10 lb fish look the same at any depth, thats really not even close to true.

What it is good for is eliminating noise in the upper water column when you are searching for fish deeper in the water.

Oh - it can also hide shallow returns if its turned up too hi.

I havent played with TVG on my Garmin yet. The Auto settings for pretty much everything seem to be doing an outstanding job so far.

[TrackerPro16]

You confirmed what I suspected! I printed out the manual so I can look at it while playing in 'simulator' mode. It does not have a good description on the various filters so my guess is I just have to go out and play with them one at a time to see what they actually do. 'Automatic' mode is my friend! It would be nice if the weather would cooperate!

[Larry3215]

I think Ive come up with a better way than the napkin drawing to show whats going on with downview

Lets try this.

First, here is an image showing how the down "cone" looks fairly wide side to side and very narrow front to back.

Next, imagine you are driving over some structure on the bottom. On the left side is a view from the rear. In the center is a view from the top down. On the right side is how it looks on the downview screen.

Call the three tall arrows tree stumps that are sticking more or less straight up - one of which has branches. The green and dark red shapes are rocks on the bottom. Then, there is a brown log that sits across the path and is almost as long as the beam is wide.

Notice how they are spread out front to back and side to side.

Then look at the right side of the drawing to see how those shapes show up in the downview screen.

The first thing to notice is that they all look like they are lined up perfectly under the boat in a nice straight line - but that is not the case at all. Only the green stump and the center of the log are directly under the boat. Everything else is off to one side or the other.

Notice how all the items that are off to the sides in the cone appear sunken down into the bottom return to one degree or another. How far down they are is based on how far to the side they are in the cone. Even two of the tree stumps are partly sunk into the bottom return.

Finally, see how the log got bent into an inverted V shape and is mostly down in the bottom return line.

Depending on how strong the return signal is from those items, and what the bottom is like, the parts down in the bottom return may or may not be visible. So you may or may not be able to tell if something is off to the side or not.

In the next post I'll show you some actual downview screen shots that show this happening to one degree or another.

[Larry3215]

Take a look at the 2D, down and sideview images below. You can see how things that are off to the sides of the boat can get burried in the bottom return on downview and wont show up on 2D at all. Also, things off to the sides in sideview all get squeezed together so they look like they are in the center on downview.

[Larry3215]

Here are some closer looks at the side and down sections.

[TrackerPro16]

I understand the clumping and imbedding in the bottom, but the picture on the right looks reversed? Maybe I need to re-read the whole thread again...

[Larry3215]

Do you mean the one on the right side in my crude drawing?

If so, thats what the screen would look like as the boat passed over all those items on the bottom in sequence. It would have passed over the log first, so the log will be furthest to the left on the screen. Thats the oldest data. The green hexagon is the one most recently passed over, so its furthest to the right on the screen.

Or did you mean one of the others?

[Larry3215]

Admittedly, the placement of the objects in my drawing isnt perfect, but its in the ball park. Paint is a pain to deal with when you are not very artistic

[Larry3215]

ok, now imagine you drive over another set of objects on the bottom, but in this case, they are more or lees in a straight line from left to right across the width of the beam.

In this situation, they will all get picked up at the same time as the cone passes over them.

More importantly, they all get drawn on the screen on top of each other.

That can make for some very cluttered, chaotic screen images that can be virtually impossible to understand. You can have fish on top of logs on top of rocks on top of trees on top of brush all mixed in with each other and the bottom return.

It gets even worse when there is a slope and worse again if you are not traveling in a dead straight line and dont have your heading perfectly in line with COG.

Im actually kind of amazed anyone ever gets good downimages. If the bottom is at all busy, or sloped or you are crabbing, its very hard to get good images. So dont feel too bad if your are not the greatest