Sonar interpretation?

[Sideburns]

Just looked again....and snow likely through tuesday

[Amx]

Yup, that's what I got out of the forecast on tv on the 4 or 5 o'clock weather news. Most likely on into some time Tuesday morning.

[Larry3215]

NOAA still saying Gig Harbor area will have off/on snow through Wed. Up to 4" accumulation tonight in my area. Only have 1/2" so far.

[Bodofish]

I'm in Woodinville and it's just really started coming down.

[Amx]

Just looked, very little falling here. Big flakes tho. A THIN 1/2" on the porches. With thinner snow on other area of the porch.

[Larry3215]

Before I try to tackle the sloped bottom thing, lets finish up on shadows. They can tell you a lot sometimes.

First, all this is assuming a flat bottom. In fact, pretty much everything about sidescan assumes a flat bottom. Thats largely because its a nightmare of unknown possibilities if the bottom isnt flat and all the math goes out the window. The brightness of the bottom return can give you an idea of slope, but you have no clue how much of a slope it is or what the depths are anywhere but under the boat. That means you cant accurately calculate anything if the bottom is sloped.

Unfortunately, the bottom is almost always sloped to some degree. Luckily, for moderate slopes, its still a matter of close enough for horse shoes and hand grenades.

So, how do slopes effect shadows? Imagine you are on flat ground and there is a 3 ft tall pole and the sun, at that time, is causing a 3ft long shadow. Now imagine the same pole at the same time of day, but sitting on the side of a steep hill with the ground rising up behind the pole so the shadow is on the hillside above the pole. If you measure that shadow, it will be shorter. The steeper the hill, the shorter the shadow.

Just the opposite happens if the shadow is pointing down hill. The shadow will get longer with steeper hills.

Take a look at this next screen shot again. In this case, the bottom is actually pretty flat, so we are good. I have arrows pointing to various things that are casting shadows. Most of those things look to be more or less the same length, yet they are casting very different shadows. Things that stick up higher in the water, will cast longer shadows. The shadows will give you a good idea of how tall various things are. If there is no shadow, it doesnt stick up from the ground much at all. If there is a shadow, but nothing seems to be casting it, then maybe there is a fish return or something else hidden in the clutter.

That thing with the black arrow is odd. There is a long shadow but I dont see anything that looks like it matches that shadow. There is a bright circle at the end of it but I cant see how that could be casting that shadow. The other nearby log already has its own shadow. A mystery!

[Larry3215]

In this next one, look in the top left corner where I have the circle. There are some more odd things in the shadows.

One of the long shadows has a bright spot in the middle of it. That pretty much has to be a fish. Probably a good sized one. There is also a shorter shadow just under that that doesnt seem to have a source. Fish?

Above that long shadow is another shadow with a bright streak in front of it that doesnt look like a log. That may be another big fish - maybe.

The trick is trying to spot those things on the water. It can be tough. Thats one reason I like saving screen shots and sonar recordings so I can look at them after I get home. I often find all sorts of cool things I missed when we were out on the water.

[Amx]

8 1/2" of snow on the back porch. Got all 3 porches cleared, now to do the top of the dog house. :-)

A couple bigger limbs down,but they can wait until spring.

[Amx]

I'd say that bright circle at the end of that shadow is a post that happens to be pointed directly at the boat, thus you can see only the end.

[Amx]

The power went out and they sent me an e-mail telling me the power is out! How the heck can anyone read the e-mail if there is no power to power the computer. And at 3:30am anyway. It just happened that I was awake when the pellet stove quit and I knew the power went out for 3 1/2 hours. Not bad tho, I figured it'd be out for many hours with the snow.

Looks like it'll go out soon again, lights are dimming a couple times.

[Larry3215]

I'd say that bright circle at the end of that shadow is a post that happens to be pointed directly at the boat, thus you can see only the end.

Thats what I thought at first, but if the post was pointed directly at the boat/transducer, it wouldnt have a shadow. Just like the sun at noon on the equator = no shadows.

[Larry3215]

Ok, lets go back to the trick question on the image I posted earlier. That image was captured as we drifted past the bridge pilings that hold up the new and old Tacoma Narrows bridge.

There are actually several things that make this image a challenge to interpret.

First, the boat was drifting and not under power. It was sideways to some degree or another almost all the time which creates distorted images. Remember the squares earlier that show up as diamonds on the screen. These are nice square concrete pilings yet they look very distorted. On top of that, the boat actually spun in a circle at least once as we passed by.

You can tell that because it looks like there are two sets of pilings on the screen. One further away and looking smaller in the bottom right side of the image. There are only a single set of pilings though.

This is our approximate path.

The squares are the two bridge pilings. The blue line is the boats path. The red line is how the sideview beam/cone would have been pointed at various times as we went by. Note that we started further away and got pretty close at one point. Under 50 ft away.

Here is how is would have looked under the water at our closest approach point. Note that the distance to the piling is LESS than the water depth under the boat.

This is an extreme example of what can happen when the bottom is not flat.

More or this later....

[Larry3215]

Oops - here is the image again. I can only put three in a single post.

[TrackerPro16]

I'd say that bright circle at the end of that shadow is a post that happens to be pointed directly at the boat, thus you can see only the end.

Thats what I thought at first, but if the post was pointed directly at the boat/transducer, it wouldnt have a shadow. Just like the sun at noon on the equator = no shadows.

A waterlogged pole standing straight up but just under the surface and slightly off to the side?
The closer to the boat the wider the shadow would be, I would think, so it could be fairly small if it was close to the transducer?
On second thought there would be other indicators of that... Hmmmm.

[Larry3215]

I'd say that bright circle at the end of that shadow is a post that happens to be pointed directly at the boat, thus you can see only the end.

Thats what I thought at first, but if the post was pointed directly at the boat/transducer, it wouldnt have a shadow. Just like the sun at noon on the equator = no shadows.

A waterlogged pole standing straight up but just under the surface and slightly off to the side?
The closer to the boat the wider the shadow would be, I would think, so it could be fairly small if it was close to the transducer?
On second thought there would be other indicators of that... Hmmmm.

Thats a possibility. Sonar signals reflect off of changes in density. Air bladders or things like rocks, metal etc are best. Anything either more or less dense than water. A water logged tree or branch could be less reflective for sure. I would still expect to see something, but maybe its just blended in to the background too much.

[Larry3215]

Someone asked earlier what happens in a sideview image when the bottom is sloped. That can actually get really messy and its all down to geometry and the fact that things are drawn on the screen in the order of their distance from the transducer and NOT based on depth or angle.

Ive been trying to come up with a good way to show how this works and I havent been able to come up with anything that is easy to understand that I am able to draw with my poor artistic skills. Im debating trying a short video, but I hate videos....

Anyway, here is my first shot at this. Lets take a situation where the bottom is sloped at roughly a 30 deg angle sloping up on the right side of the boat. Lets take 5 places on the bottom and see how they get drawn on the screen.

A is to the far left and only falls within the left side cone. It doesnt show up on the screen at all in this example
B is to the left but is in both cones. It will be a strong image on the left side and weak on the right side.
C is directly under the boat and will be equally strong on both sides of the screen.
D is to the right side, but because of the slope of the bottom it is closest to the transducer. This is important. That means D gets drawn first on the screen as we move out from center out to the edges of the screen.
E is farthest to the right but still in the left cone. It will be strongest on the right side of the screen and weak on the left side of the screen.

Here is what you would expect to see on the screen if these same points on a flat bottom with no slope. On the left is the sloped bottom and the five points. On the right is how the screen would look if the bottom was flat. You would expect, the points go from the center out to the right and left in the proper order - C, D, E on the right and C, B, A on the left. Point C being directly under the boat.

BUT....... that’s NOT how its going to be drawn on the screen when the bottom is sloped.

Several things are happening with the screen because the bottom is sloped and because things on the screen are drawn in the order of their distance from the transducer.

1) The point that looks like its under the boat is now D instead of C. So what you think is directly under the boat is actually more to the right side.

2) Everything on the right side will be drawn closer than it actually is and will be more compressed than normal. Everything on the left side will be drawn further away than it is and will be stretched out more than it is.

3) This was the hardest thing for me to get my head around. Points C and E are far apart on the bottom, but they are the same distance from the transducer so they will get drawn on top of each other on the screen. Even though C is to the left of D it get drawn to the right of D and on top of E.

In other words, a sloped bottom can really really really mess up how things look on the screen. Completely different sections of the bottom can get drawn on top of each other, and in the wrong order, as far as their real distance to the side of the boat. Plus the perspectives and distances get thrown all out of whack.

Im working on another example to illustrate this another way.....

[Larry3215]

Here is another way of showing the same thing with arrows showing whats on the bottom and how it gets drawn on the screen.

[Larry3215]

Here is one last example with an under water ledge. The exact details of how things get drawn on the screen will depend on where the boat is in relation to point A or what ever point on the bottom is closest to the transducer. There are an infinite number of variations.

The primary distortion here is that the red and blue bottom sections get drawn on top of each other.

The over all point is that anything other than a perfectly flat bottom will create distorted images on the screen to one degree or another - over lapping sections drawn on top of one another, stretched or compressed or reversed sections of the bottom.

When you think about the number of ways to mess up a sideview image, its a wonder anyone ever gets a good one.

[Larry3215]

Have I lost everyone? To much useless information? I hope this is making at least a little sense...

Lets go back to downview for a bit. Downview can give you some extra information that isnt always easy to spot if you dont know what to look for. Sometimes its lost in the clutter but sometimes you can see useful stuff.

Because of the wide side to side cone angle, downview "sees" a lot more of the bottom than you might think by whats shown on the screen. Because every return is drawn on the screen in the order of its distance, you can sometimes pick out extra information that you might only see on a sideview screen. Hopefully, these next pictures can show what Im talking about.

[Larry3215]

Ok here are a series of images to show what Im talking about when I say that every return signal inside the cone gets drawn on the screen.

In this downview image, you can see an upside down U shape inside that circle in the lower right corner. That is actually a log that I drove across the center of.

That log was laying on the bottom at an angle to my path and the boat passed almost directly over the center of the log.

Here is what it looks like on sideview.

I have circled two other logs that are in almost the same positions - part showing up on the right side and art on the left side.

The downview image is a combination of both sides drawn on top of each other at the same time. Sometimes you get to see good details and sometimes you dont. Things can sometimes just get too confused or muddled to see anything useful, but sometimes it works out great. The details that do show up will look like they are below the ground level when, in fact, they are off to the sides of the center line.

It all has to do with how the 2D screen image gets drawn based on a 3D underwater situation.