Topic: I must be confused

[Yrral3215]

Ive seen it said many times on this forum and in other places that the SI sonar cant tell direction - only distance.

If thats the case, how the heck does it know to paint a picture of a bridge column or sunken boat or rock pile on one side of the boat and not the other? It seems to me there would be no way the sonar could work unless it absolutely knows direction down to a very fine degree. Other wise it couldnt put the tree branches (or what ever) in the correct places or have the sunken boat pointed in the correct direction.

Can anyone explain how it can draw detailed pictures without knowing the direction?

Thanks,

Larry

[rnvinc]

There are 2 separate SI crystals in the HDSI xducer ...:
1 pointing left ....at a downward angle...
1 pointing right...at a downward angle...

Echo reflections in the individual beams are plotted onto the SI display in their respective "left or right" side of the image...(note that the 2d beams are also in this depiction to show total bottom coverage)...

Here is the HB description of the left and right SI beams in the HB manual...




Here are depictions of the 2 frequencies of the SI beams...with added fish to show how those fish create their own respective shadows against the displayed lake bottom...





Note how fish that are the "same linear distance" from the mounted xducer are displayed at the "same linear distance" from the SI image center line (sonar only knows distance to targets)...but also the display image clearly shows those respective fish's shadows ...which indicates how far off the lake bottom those fish are suspended over the lake bottom...the shadow is in direct line with the fish (echo target) and the xducer (sonar point of origin)...in a 90° angle from the SI image centerline...

Rickie

[234rick]

Could you tell me where  I can view those images ? I would like to study them a little closer.

[rnvinc]

Could you tell me where  I can view those images ? I would like to study them a little closer.

You should be able to right click on the image and "Save picture as" which will allow to save the picture to your pc...

Rickie

[Yrral3215]

Thanks Rickie, but Ive seen that illustration before and its part of the problem as far as I am concerned

If the sonar unit can figure out where to put the fish shadow, why cant it put the fish in the correct place? Or to say it another way, if it doesnt know where the fish are how the heck does it know where the shadows are?

It seems to me it must know the relative angles in order to place the shadows correctly. If there is a big log laying on or near the bottom, how does it know to put one end closer than the other and how does it know the angle that a limb sticks up at if it doesnt know the angles? Do you see what Im getting at?



Larry  (Still confused )

[rnvinc]

First its important to understand that the xducer is not "seeing" the entire lake bottom as in a wide angle lens camera...

It is gathering reflection echo data from a sound wave "ping" sent thru the water in a certain shape (dictated by the shape and size of the crystal element) and in a certain direction (which way the crystal element is pointing in the xducer housing)...

Lets take a minute and show how SI works....and how the sonar data is then displayed into a SI image...

The SI beam is very wide left to right (up to 180°) ...but very thin from from to back (about 1°)...This is why we refer to the SI beam as a "slice"...

The SI sonar sound pulse "ping" is sent thru the water in the direction that the xducer's crystal element is physically "pointing" in the xducer housing...

*The unit xducer is "pinging" (sending) the acoustic sound pulse thru the water at several times per second....
*The processed reflection data (fish and other reflectable objects) from each single "ping" is returned to the xducer where the processor can then display that reflection data along the topmost horiizontall edge of the SI screen (about 1 or 2 pixels thick horizontally)...
*Then the next ping's processed data is plotted onto the topmost horizontal edge of the SI screen ...pushing the previous displayed ping data down....
*This "stacking" (of each ping's data at the topmost horizontal edge of the screen and pushing the previous ping's data down).....creates the "scrolling" effect seen on the SI display...
*Because the single "ping" is only 1 or 2 pixels thick horizontally on the display ...the unit only has to plot the reflection data (in that thin slice) at the correct distance from the SI image center line (which just happens to be where the xducer is in relation to the SI image centerline)...

In the scenario of a fish echo...the fish's shadow is also in direct line from the xducer in that same slice that is plotted onto the display...

Because the sonar sound wave cannot "see" anything behind the fish in the direction that the sound wave "ping" is travelling thru the water...it leaves a "void" with no data that we interpret as the shadow...(imagine a flashlight shining at an angle toward a fish...there would be a shadow behind the fish where the light was blocked by the fish)....

So it's this thin slice shape sound pulse that allows the unit to know where to plot the reflection data ....

The unit only has to measure the time it took for the sound pulse to send-reflect-return to know how far out from the center line to plot that reflection data (in that thin slice that the unit plots on the display)...

Rickie

[newkid4si]

Very nice explanation Rickie.

[Yrral3215]

Thanks for taking the time to type that all out Rickie! I had a pretty good idea how that all worked but yours was a very nice, easy to follow explanation - but - it still leaves me with the same question.

If the sonar can use that slice of the return to accurately locate and display the fish shadows - I still dont see why it cant accurately locate and display the fish.

The same sonar ping is painting the bottom and the fish at the same time. Its the same data coming back and its all getting processed at the same time.

So why can't the sonar show the fish where they are in the same way it shows the shadows. There is some data coming back on the exact same pings that allows the sonar to know where the shadow goes and to know where the tree limbs go - so why not the fish too?

Thats where Im still fuzzy
 

[rnvinc]

Maybe I'm misunderstanding your question...

Are you seeing the fish's  shadow ....but not seeing the fish echo itself...??

Rickie

[TroyBoy30]

it does show where the fish is.  your question makes no sense

[rnvinc]

Here are a couple of my screenshots that clearly show the fish shadow and the fish echo itself...





There are 2 things that make the fish show well in the above images...
*They are very large fish (Asian carp)...
*They are relatively close to the bottom....

There are several factors which can make the fish echo itself not show very well on the SI display...
*Small fish just do not have a large reflective perspective which makes the fish echo "blend" with the lake bottom ....(this is why it's sometimes easier to spot the fish shadow first and then follow an imaginary 90° line toward the centerline until you make out the fish echo itself..)
*Chosen pallette color ...some Pallete color choices will show a better contrast of the "white fish echo" better than others ...(notice how well the "white fish echo" stands out against the Inverse pallette color in the above pic...)

There are a couple of other tricks you can use to make fish echoes stand out better against the colored lake bottom presentation...
*SI Enhance Sharpness higher will make the "White fish echo" stand out better (but the other parts of the image will look grainier ....)
*Increase Chart Speed...this, in effect will take that fish echo data (that is in the "thin slice" being plotted at the top horizontal edge of the SI display)...and "stretch" that fish data down across the SI image...but this also "stretches" all the other data being displayed in the image...

So...finding the fish echo itself is sometimes challenging...

*Look for the shadow first...then try to find the corresponding fish echo..

*Experiment with the "tricks" mentioned above (but realize other aspects of the SI image may suffer in "tricking" the image into making the fish echo itself stand out....)

Rickie

[promapper]

I am definitely not a sonar expert but perhaps I can add a couple of things. .

The first point is a question.  Does the HB SI sonar "read" multiple returns from one "ping", or pulse if you will?  or are there multiple pings for one slice?  The reason I ask is if it sends out one pulse, and because that pulse will spread with distance to some unknown size  by the time it hits the bottom it may hit multiple targets such as tops of trees, tree branches, bushes boulders, fish and finally the bottom.  So for each pulse can it read many returns calculating the time and therefor the distance of each individual return.  I am thinking it is reading multiple returns for each pulse.

Since the SI requires gps positioning and the gps unit is not updating anywhere near as fast as the sonar is pinging I assume the unit interpolates positions for each return based on the time difference between the gps position in back and in front of the pings.

Regardless, there is an awful lot of calculations taking place in real time.  Simply Amazing!

"Direction" as Yrral3215 is thinking is in relation to the direction of travel and is only computed in two dimensions x and z what I will call cross sections.  A cross section being that slice we all talk about.  with the third (Y) being made by the direction of travel and the speed. 

the 3rd point is that the sonar is not "reading" the shadow(s) but rather a lack of information or data that is being blocked by the fish or other feature.  to put it another way it is reading "nothing" where there are shadows.

Like TroyBoy30, I am confused by the question.

Please clarify or correct me if I am wrong

[Humminbird_Greg]

I'm going to through in my $0.02 here and possibly confuse promapper a little here:
The unit sends out multiple “pings” for each transmit/receive cycle and it receives multiple readings from multiple “sonar targets”.  All sonar receives readings from multiple targets or it would not be able to update so quickly.

You are correct in that our Si sonar can only display in two dimensions: distance away from the transducer and time (history of past Si sonar readings).  The sonar returns from a fish or the lack of a sonar return is plotted at the appropriate distance from the transducer as the system does this by the amount of time it took the sonar pulse to return to the unit.  Once it has done this it pushes that data downwards and displays the next transmit/receive cycle data and so on.  Because of the shading of the displayed image and due to our knowledge of how the Si sonar works; some may believe that the units are capable of showing us sonar data in a third dimension.  This is not what is happening but is instead what but our brains are telling us through the interpretation of what we are seeing.  As in many things: experienced users can “see” (interpret) far more data than what the beginner can due to their experience in using Si sonar.  I'm wondering if this is some of what Larry is questioning?

[Yrral3215]

Im off on a weeks fishing trip in 5 minutes so I cant go into details now.  I must have not expressed my question well enough but it will have to wait till I get back for more discussion.

Thanks to all!!

[Humminbird_Greg]

We will await your return.  Have a good (and safe) fishing trip!

[Sam K]

I'm confused about where the confusion is coming from.

[Yrral3215]

Thanks to everyone for trying to help me out with this!

Ok, let me see if I can explain my question better. First of all, its been said the sonar only knows distance, not the direction of the target. My problem with that statement is that it seems to me the sonar MUST know the direction of the target inoder to paint the pretty pictures properly.

For example, take the second set of diagrams rnvinc posted in his first reply to me. Note the shadows of the fish drawn on the bottom. On each side we have 2 shadows in different places. Shadow one is at (Im making up these angles just for example) say 20 degrees down from the surface relative to the transducer. Shadow 2 is say 45 degrees down on the left. Shadow 3 and 4 are also shown at different angles down from the surface relative to the transducer.

That tells me that the sonar unit does indeed know direction - very precise direction - or else how could it properly place those shadows? Add in a tree on the bottom with multiple limbs sticking up. Each part of the tree is at a different angle as seen by the transducer. The only way to properly draw all the tree parts correctly is if the unit knows the direction/angle of each part relative to the transducer and the surface of the water to a hi degree of accuracy.

Other wise the tree would show up as just a mish mash of dots.

So - back to my main question - why can the sonar accurately show something as complex as a tree with branches but not be able to tell you what direction/angle the fish is at relative to the transducer or surface or what ever?

I hope that made sense!

Thanks again,

Larry

[Rickard]

Larry,
 
The sonar does not show a true image of a tree with branches pointing in every direction, it just looks as if it does. The effect of the distance-only-capacity of a sonar is sometimes rather weird. If a straight pole that is leaning is passed, the pole will look bent. A flat deck of a wreck that is facing a camera that is mounted at the position of the transducer, will produce an overview of the deck in the camera, but a line in the sonar image (because all points on the deck surface are at about the same distance from the transducer) etc etc. A fish can, in fact, be precisely located in three dimensions if, and these are important ifs, the sea floor can be assumed to be flat, the echo from the fish is visible, the echo from the fish can be coupled to the correct shadow on the sea floor. In this case it's possible to calculate how deep and how far from the transducer the fish is.
 
Rickard

[Humminbird_Greg]

Larry,

I think what is happening is that your brain is perceiving a third dimension to these images that is just not there.  We assume that the shadows would appear on the bottom of the lake due to our experiences of seeing our own shadows showing on the ground under our feet.  In reality there could be anything in the sonar shadow area because the sonar does not ‘see’ anything there.  In the screen snapshots of the fish with their corresponding shadows that Rickie posted we make the educated guess that those fish that have a shadow close to the sonar return from the fish itself, are in fact very close to the bottom of the lake; again due to our experiences with light and shadows above water here on dry land.  The unit shows them close to or far away from the sonar return of the fish due to the distance (time) that it took the sonar signal to make it to the fish and back to the transducer versus the amount of time that it took the sonar signal to reach the bottom and back to the transducer.  Remember that the sonar unit can only display the Si data from an overhead view and not from the side like the drawing shows.

Also, the drawings that Rickie posted all have a flat bottom.  If all the lakes and rivers in the world had flat bottoms we could make a simple mathematical calculation and display the water depth of fish or other objects.  The reality is that the lake bottoms are not flat.  If the lake bottom shown above in Rickie’s drawings had a hump on the right side, the shadow from fish #4 could be shown the same distance away from the fish that it is with fish #3, yet be at a totally different angle to the transducer.  If we were in 20 feet of water we may incorrectly assume that the fish was very near the bottom of the lake in about 20 feet of water as well.  The one thing that may clue us into this not being true is that we would also see a change in what the right side bottom reading would look like versus the left side bottom reading (bottoms that are more perpendicular to the transducer reflect a stronger sonar return).

[newkid4si]

   "First of all, its been said the sonar only knows distance, not the direction of the target."


   We all can understand how distance is determined.
   I use the fact that the transmitted beam is only 4 inches wide(front to back) rather than a 360 degree cone
   to visualize how direction is determined.

          Mike

[RGecy]

The bottom line is there is just no way for the unit to determine what part or angle of the beam a fish or object is located!

I think the problem lies in how Side Imaging is being illustrated to help people understand it.  A lot of the images we use to depict SI (like the ones above) show the water column in the verticle plane like the traditional downlooking sonar view.  This is only done to help people understand the beam coverage and is not the correct way to really show how SI coverage works. Conversely, we typically show traditional downlooking sonar from above to show the cone coverage under the boat.  Side Scan Sonar should really be looked at from overhead as a thin beam emitted side to side and shown only 2 dimensions.  As the boat moves along, each thin beam is stitched together to paint the picture we see. There is no 3rd dimension such as angle to object.

As for showing the correct angle to or depth of a fish, even though the unit does show the sonar return of the fish and the shadow of the fish, it cannot calculate the exact position of the fish in the water column.  What you see on the screen is actually a lot of single pings stitched together to form the image you see on the screen.  The unit emits a pulse of sound that is very thin horizontally and tall vertically that is angled out to each side.   The unit then listens for the sound waves return echo and records what it hears for a specific amount of time (very very short duration, we are talking milliseconds).  The duration of time it listens is related to the range you set and the depth of the water, salt or fresh also affects this because of density).  As it listens, for each fraction of a millisecond it records the strength of the return at that given time.  Each ping (or pulse of sound) makes up one little sliver of our picture that we see.  The unit will emit thousands and thousands of these pings per minute to finally fill up the screen and create the beautiful Side Scan images you see.  But just like the images earlier in this post show, a fish that is high in the water column will show in the same position as a fish that is equal distance from the transducer but lower in the water column because the sonar return takes the same amount of time to return to the transducer.  There is no way for the unit to know the angle to the fish because of this.

There is software that can calculate these things using simple math, but it relies on the user to chose the objects and its corresponding shadow, and a few other factors.  But this has to be done by the user.  There is no way for the software (or an SI unit) to chose these points on its own.  This 3rd dimension you are looking for is implied and is not really recorded in the data.  It has to be calculated.

I hope that helps.

Robert