Misunderstanding my side imaging

[crappieseeker]

I am misunderstanding the concept of SI so here is a picture and I will explain how I interpret it according to what videos I have watched and you tell me if I am on the right page. I am under the understanding that the line in the middle is directly under the transducer and the dark black to each side is the water column out to each side of the transducer. I at first was thinking the dark area was the water column right under the transducer as well but now think I was missing it all along. It is still a little misleading to me. I think for instance that the diver is to the left side of the transducer about halfway down the water column if I am thinking correctly, and since the shadow is close to the leading edge of the bottom closer to the boat it seems as if the diver would be about 15 feet out to the side if the side range was set to say 80 feet since it is closer to centerline. I was also at first thinking for example like the red dot I put on the screen being something suspended toward the outside of the dark column that it meant that it was further away in the column therefore casting it's shadow further out on the bottom like the black dot instead of it being more so the depth. Please tell me if I am starting to get the hang of how it works. My final verdict is that the divers location in the dark area relates more to his relative depth and the divers shadow over the bottom is more relative to how far out to the side he is which from the picture looks to be rather close to the side of the boat. Is this the correct interpretation of this?

[kosmo]

Neat picture.Im just guessing but i think he's under the boat and the shadow that he cast would be close to him the closer he it to the bottom and farther away the higher he rises in the water column.

[Redge]

Great question, I think he is under the boat also a little to the left. I'll be watching this thread!

[Redge]

Great question, I think he is under the boat also a little to the left. I'll be watching this thread!

[kosmo]

I've changed my mind.i think he's six feet off to the left side.

[Redge]

Great question, I think he is under the boat also a little to the left. I'll be watching this thread!

[crappieseeker]

I watched a SI video from one of the main guys saying the dark area was water column to the side, not under the boat, that is why I am thinking the diver is off to the side a few feet.

[Redge]

Kosmo how did you come up with 6 feet? Trying to learn something!

[rnvinc]

I copied this from the sticky in the Side and Down imaging forum....

The SI image "dark water column" has frequently been a topic that is difficult to interpret... and even more difficult to explain....

One of the first things to understand is how the SI unit processor takes a 3 dimensional underwater world and renders it into a 2 dimensional display...
(for info understanding the SI image rendered into 3d...see... SI interpretation...in 3d - (tutorial)

Once we have a grasp on how the unit takes the "vertical dimension" water column (that resides somewhere under our boat) and lays it down flat in the SI image.... then the question arises... What exactly is this "dark water column" and what is it comprised of...??

With a lot of help and advice from some of the guys over at
Humminbird Side Imaging Forums
I hope that the following depictions will give a little better understanding of What exactly is this SI "dark water column"...??

I will refrain from a long drawn out tutorial at first.... and just post the diagrams (one for the 455kHz SI beam and one for the 800kHz SI beam)....

Then, as questions are posted, maybe we all can get a better understanding of ...

What exactly is this SI "dark water column"...??

This is the depiction for the the 455kHz SI beam...


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This is the depiction for the 800kHz beam...



Rickie

[rnvinc]

In a nutshell...what this means is...

Sonar basically measures time...the time it takes for the sound pulse (ping) to leave the xducer, reflect off targets, and return to the xducer...

Then, because the speed of sound in water is constant ....the unit can calculate distance to reflective targets...

If we take a rope just long enough to touch the lake bottom directly under the xducer...(no matter the depth as long as the rope touches the lake bottom)...

Then we tie a ball on 1 end and tie the other end to the xducer...

Then we swing that ball in an arc all the way to the surface to the left...
Then we swing that ball in an arc all the way to the surface to the right...

Any reflective target within that ball's arc (rope included)...would lie in the "dark water column" in the SI image...

The distance from the echo target to it's own shadow is what gives the "mind's eye" the perception of how high in the water column that target actually is...

Notice in both my depictions above, I have drawn fish targets at the same distance from the xducer....because the xducer does not know where in the SI beam shape that target actually is...the unit only knows "how far" that target is from the xducer...therefore the units "paints" both targets directly in line with each other away from the centerline....even tho the targets are at completely different depths in the water column....

The individual shadow of each target is what your "mind's eye" uses to understand how high in the water column each of those target are...

Rickie