Sonar interpretation?

[Larry3215]

Ok, the first thing is to remember that if you are in 50 ft of water and you see a rock on the very edge of where the bottom return is being drawn, that rock will show on the screen as 50 ft to the side. In reality, its directly under the boat, so its true distance to the side is zero.

If you do the math in the formula above, the answer is zero.

Now lets take an example where you are still in 50 ft of water but the rock shows on the screen as 51 ft to the side - just 1 ft inside the bottom return area.

The math in that case works out to 10 ft. So that rock is really 10 ft over from the side of the boat when it looks like its 51 ft on the screen.

Now, lets take a second example. Same depth but now that rock shows on the screen as 55 ft to the side. Its actually 23 ft to the side.

[Larry3215]

Now lets look at two more examples at increasing Screen distances.

In the next one the rock now looks like its 60 ft to the side on the screen. It is really 33 ft to the side. Notice how the distance shown on the screen, and the actual distance from the side of the boat, are getting closer together.

Now look at what happens when that rock is showing as 250 ft to the side on the screen. Its actually 245 ft to the side. Close enough for fishing I think.

250 ft is 5 times the depth. At that distance ratio, the screen scale is close enough to the actual distance to not make a difference.

By the time you get to where the screen distance is around three times the depth, the error drops to about 8%. So, if you are in 50 ft of water and looking at targets 150 ft away, they will be around 141 ft. Even if the distance on the screen is only double the depth, the error is probably close enough - roughly 12%

The same ratios apply to any depth. Lets say you are in 20 ft of water. This is how the screen distances would work out.

20 ft to the side on the screen = zero true distance to the side
21 ft screen = 6.4 ft true
25 ft screen = 15 ft true
30 ft screen = 22.3 ft true
35 ft screen = 28.7 ft true
40 ft screen = 35 ft true
60 ft screen = 56.5 ft true

So, to sum up, the closer an object is to the center line, the more "off" its distance reading on the screen will be. The further away it is, the closer it will be. By the time you get to 2 to 3 times the water depth, its probably close enough.

[Larry3215]

If you are still awake after that, dont worry too much about needing a calculator on the boat. Most modern plotter/sounders with sideview will let you place a waypoint on a target on sideview so you can get to it without doing any math at all.

Those waypoints will often be wrong to one degree or another because they assume the bottom is flat between the boat and the target. I wouldnt worry about that too much though. Unless the bottom slope is fairly steep, it wont be off all that much and a little searching with 2D and DI should allow you to get close enough to the target.

You need to read the manual for your specific unit closely because the way you set a waypoint on sideview is usually different from placing a normal waypoint. If you do it wrong, your waypoint will end up where the boat is and not where the target is.

[Larry3215]

Ok, just to finish this off with some real world examples from those earlier pics.

Ive circled some targets in this pic. The water depth is 13.8 ft, so the bottom return starts being drawn 13.8 ft to the side of the center line.

The target in the yellow circle is near the edge. Lets call it 14 ft to the side. Its actually 2 ft over from the boat.

The targets in the black circles are 16', 20' and 45' to the side on the screen. They are actually 8ft, 14.5ft and 43 ft away from the boat.

Now lets look at a different example where the water depth is around 56 ft.

There is a red circle around what might be a rock on the left side showing as about 60 ft away. Its actually 21.5 ft to the left side of the boat.

On the right is what might be a fish in that yellow circle showing at about 100 ft away. If its right on the bottom, its actually 83 ft away. If its not on the bottom, you have no idea how far away it is.

There is another large chunk of bridge debris on the left that looks like it extends from about 80 ft to 135 ft to the left side of the screen. Its actually 57ft to 122ft to the side of the boat.

On the screen it looks like its 135ft - 80ft = 55 ft long. In reality, its 122ft - 57 ft = 65 ft long.

The closer something is to the center of the sideview screen, the more compressed it will look. The further away it is, the closer it will be drawn to its true scale.

[Larry3215]

Remember I said that all these measurements and the accuracy of any waypoints you mark off to the side all assume the bottom is flat.

Unfortunately, most of the time, the bottom is not flat. Sideview cant tell you what the depths off to the side are - it only knows the distance to the bottom directly under the boat.

It also cant tell you how steep the bottom slope is, but it can tell you if the bottom is rising or falling away. The bottom return will be brighter where the bottom is rising and darker where it is falling away. Shadows or black areas are places where there is no return - a hole or ledge for example.

In this image, the bottom is rising on the right and then flattens out before rising again. The really dark areas to the far right are the shoreline. You cansee some of the rip-rap along the top right. Its distorted because the boat was swinging and skewed while I took this shot.

On the left, its mostly level or falling a little on the left with what looks like a channel just to the left of center.

[Sideburns]

Earlier, when you mentioned that it can only judge distance from the transducer, the geometry of interpreting the distance to the side made sense to me. The scaling though, is shocking.

From above:
"The same ratios apply to any depth. Lets say you are in 20 ft of water. This is how the screen distances would work out.

20 ft to the side on the screen = zero true distance to the side
21 ft screen = 6.4 ft true
25 ft screen = 15 ft true
30 ft screen = 22.3 ft true
35 ft screen = 28.7 ft true
40 ft screen = 35 ft true
60 ft screen = 56.5 ft true"

That makes 1 indicated foot of image (side to side) equal to a 6.4ft actual size! (only if the object is on the bottom and directly under the boat) Further away, the visual scaling becomes more accurate...

So what happens to this scaling if the bottom is flat, but actually sloped steeply?

[Larry3215]

Earlier, when you mentioned that it can only judge distance from the transducer, the geometry of interpreting the distance to the side made sense to me. The scaling though, is shocking.

From above:
"The same ratios apply to any depth. Lets say you are in 20 ft of water. This is how the screen distances would work out.

20 ft to the side on the screen = zero true distance to the side
21 ft screen = 6.4 ft true
25 ft screen = 15 ft true
30 ft screen = 22.3 ft true
35 ft screen = 28.7 ft true
40 ft screen = 35 ft true
60 ft screen = 56.5 ft true"

That makes 1 indicated foot of image (side to side) equal to a 6.4ft actual size! (only if the object is on the bottom and directly under the boat) Further away, the visual scaling becomes more accurate...

So what happens to this scaling if the bottom is flat, but actually sloped steeply?

I need to do a new drawing to show that, but basically, if the bottom is sloping UP, then the distances shown on the screen are shortened. If the bottom is sloping down, they get longer. Its because the triangle formed by depth/truedistance/screendistance is no longer a right triangle with a 90 degree corner. That throws the math out the window completely.

[Larry3215]

Earlier, when you mentioned that it can only judge distance from the transducer, the geometry of interpreting the distance to the side made sense to me. The scaling though, is shocking.
........

So what happens to this scaling if the bottom is flat, but actually sloped steeply?

As Im working on a more detailed response for you, I realized that this is really a much more complex question than it might seem at first. Its actually a pretty interesting thing, but its going to take me a while to figure out how to present it so it makes sense. Im not sure how to describe it properly or how to draw a complex 3D situation in 2D. My art skills suck......

As Spock would say...Fascinating!

This is completely unfair, but Im going to show you a screen shot that shows just how weird this can get. See if you can figure out whats really going on here. I'll give you a hopelessly inadequate hint - Its nothing like it looks.

[Amx]

I do I do!

Ok, it's not a scientific fancy one, but I think it can do that formula. But it's in the old boat. Guess I'd better put it in the new boat. But I don't have a computer in the boat so I can't look up that formula anyway.

[TrackerPro16]

I enjoy some math but just had a flash of me in a bouncing boat in pouring rain squinting at a screen and trying to figure out where the fish actually are... I am pretty sure I will do some math but will stop when I get 'used' to knowing 'about' where stuff is.

[Larry3215]

I do I do!

Ok, it's not a scientific fancy one, but I think it can do that formula. But it's in the old boat. Guess I'd better put it in the new boat. But I don't have a computer in the boat so I can't look up that formula anyway.

LOL! I thought all us "old school" fishman types still used slide rules and pocket protectors!

[Larry3215]

I enjoy some math but just had a flash of me in a bouncing boat in pouring rain squinting at a screen and trying to figure out where the fish actually are... I am pretty sure I will do some math but will stop when I get 'used' to knowing 'about' where stuff is.

Yeah, thats why I made the point about depth to distance ratios. As long as you know things that are close to the center are closer than they look and things that are more than double the depth are close enough for horse shoes or hand grenades, you are good to go

Setting waypoints helps a bunch too - when its accurate.

[Amx]

I do I do!

Ok, it's not a scientific fancy one, but I think it can do that formula. But it's in the old boat. Guess I'd better put it in the new boat. But I don't have a computer in the boat so I can't look up that formula anyway.

LOL! I thought all us "old school" fishman types still used slide rules and pocket protectors!

Pocket protector I had. Slide rule I never could get the hang of it, technology was too new I guess.

[Amx]

Setting waypoints helps a bunch too - when its accurate.

One way to do that is find a tree that you KNOW where it is. Even drop a marker buoy over the tree. Then move the boat away a KNOWN distance and check the side view. And set a way point. Then see if you can find the tree by moving over that way point.

[Larry3215]

Setting waypoints helps a bunch too - when its accurate.

One way to do that is find a tree that you KNOW where it is. Even drop a marker buoy over the tree. Then move the boat away a KNOWN distance and check the side view. And set a way point. Then see if you can find the tree by moving over that way point.

Exactly. I had very mixed results doing that but a lot of it was user error until I actually decided to man up and RTFM!

[Bodofish]

Setting waypoints helps a bunch too - when its accurate.

One way to do that is find a tree that you KNOW where it is. Even drop a marker buoy over the tree. Then move the boat away a KNOWN distance and check the side view. And set a way point. Then see if you can find the tree by moving over that way point.

Exactly. I had very mixed results doing that but a lot of it was user error until I actually decided to man up and RTFM!

Always a hard one to do but with complex equipment, mandatory. Even though I've build a bunch of them, I'm not very good at reading them. I even ponied up and went to the Lorance University when I bought my current but old FF. I've enjoyed this thread immensely! Thanks!

[TrackerPro16]

It has been interesting! Makes my poor tiny brain hurt though...

[Sideburns]

No idea whats going on with that image.

Been snowing all day, just starting to stick. About an inch so far. Might continue like this for a few days? -Between Black Diamond and Enumclaw.

[Amx]

A few minutes ago I read the weather article on the web site for Ch 4. They said snow for Monday, then by Thursday it'll be raining, even hard rain. Tue and Wed might rain, might not snow.


edit: On the tv news broadcast looks like just rain on Tues. and Wed. Maybe snow some early Tuesday morning.

[Sideburns]

Amx: noaa is predicting snow all night and most of tomorrow, for my address... I think we are pretty close in location.