Thread: Misunderstanding my side imaging

if I was going to mark the diver with a buoy I'd throw it out 8ft behind the boat in the water and inline with the left back corner

Thanks Kosmo, that was my logic also but wanted a confirmation!

Originally Posted by kosmo

Sonar counts the time from were the ping hit the target and bounced back to the transducer.its twelve feet deep and the unit placed the diver about half way to the bottom ao six feet.if the diver were twelve feet away the unit would have placed the diver at the bottom of the water column.

This is a very good way to explain it...

Rickie

But according to Ricky's graphics, the depth of the diver is in relation to the transducer. I agree that he is more than halfway in the water column...let's say 6'+
But that means he is 6'+ from the transducer. His shadow gives us a clue where he is in relation to the transducer. He is obviously to the left, but the proximity of the shadow to where the water column meets the bottom return means he is not very far left of the transducer. In other words, if he were only 1' under water, but 12' to the left, the return would show him at the bottom of the water column....but his shadow would be way, way to the left. We have to interpret what the machine is telling us.

Originally Posted by vic n

But according to Ricky's graphics, the depth of the diver is in relation to the transducer. I agree that he is more than halfway in the water column...let's say 6'+
But that means he is 6'+ from the transducer. His shadow gives us a clue where he is in relation to the transducer. He is obviously to the left, but the proximity of the shadow to where the water column meets the bottom return means he is not very far left of the transducer. In other words, if he were only 1' under water, but 12' to the left, the return would show him at the bottom of the water column....but his shadow would be way, way to the left. We have to interpret what the machine is telling us.

yes i agree. The shadows location would be the key in determining the divers true location

I sort of understand the concept although it does not make a whole lot of sense to me I somewhat grasp it now as far as depth. However what should your minds eye use to determine distance out to the side of the boat?

Originally Posted by 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

Notice how close sideways the shadow is to the target (diver)...



Now, in the next depiction....look at how much difference there is in shadow distance of "fish #1" and "fish #2" ...
*the diver's shadow above is more similar to "fish #2"...
*the red dot's shadow above is more similar to "fish #1"...



Try to imagine a straight line from the xducer - thru the diver - to the shadow...

The scenario of the "red dot" in the OP pic is correct...the red dot AND the diver are the same distance from the xducer ...making both painted at the same distance from the SI center line...(because the SI center line is the xducer)..

Now imagine the straight line from the xducer (SI center line) - thru the targets - to each's shadow....for your "mind's eye" to determine how high in the water column each target is...

Rickie

Good explanations and interpretations guys!