Sonar interpretation?

[Larry3215]

They don't have the LSS-2 on that chart. The guy on the phone said the angle was 3.3 degrees on the down scan at both khz settings.

And on the side scan 12 degrees at 455 khz, and 6.4 at 800 khz.

So I'm going to have to see what I have the depth finders set at khz wise as I have been playing with that recently.

That is when it warms up above 50 degrees in the garage air temp wise.

Ah, ok. Those numbers are the front to back cone width. The side to side numbers will be much greater.

[Amx]

Did they list any specs on the DI and SI cone widths? It would probably be for 455 and/or 800 khz frequencies.

Lowrance is calling sidescan Structure Scan now. I forget what they are calling DI.

That info was in my next post.

And yes they call it Structure Scan, but there is still 'down' and 'side'. Structure Scan is the package, I still call it side and down as that is what is shown in the different panels on the depth finder. When I set up the panels I pick what is showing in the panels, the choice is 'side' and 'down'. So that is what I call it, and so does the manual. Maybe side view and down view, but same same. I have to check again.

[Amx]

They don't have the LSS-2 on that chart. The guy on the phone said the angle was 3.3 degrees on the down scan at both khz settings.

And on the side scan 12 degrees at 455 khz, and 6.4 at 800 khz.

So I'm going to have to see what I have the depth finders set at khz wise as I have been playing with that recently.

That is when it warms up above 50 degrees in the garage air temp wise.

Ah, ok. Those numbers are the front to back cone width. The side to side numbers will be much greater.

Ya, I thought that was pretty darn skinny for the WIDTH of the cone slice - from side to side.

[Larry3215]

Well, I cant find any specs on the side to side angles for LSS2. Either I suck at searching or they dont want to advertise those numbers.

[Larry3215]

As mentioned earlier, Humminbirds angles for the width of SI and DI vary from 45 deg to 75 deg or so. So in 20 ft of water, Humminbirds downscan would cover 17 ft to 28 ft.

Garmin is in the 50-60 deg range, so it would cover 19-23 ft wide swath at 20 ft of depth,.

They both use different dB levels to get those numbers, so its hard to compare between brands. In both cases though, the 2D cone angles in the 200 khz range are a lot narrower than the Di widths - about the same as your 20 deg width. Even the lower freqs are only maybe as wide as the narrower ranges on downscan.

[Amx]

I asked the guy if that was included angle, or just one side. He didn't know what I was talking about. lol

[Larry3215]

LOL I am not surprised!

I asked the guy if that was included angle, or just one side. He didn't know what I was talking about. lol

Ok, back to the right/left/center/overlap thing for a bit. Here are three more screen shots showing the exact same piece of bottom structure and how it looks on 2D, DI and SI.

This is under the Tacoma Narrows Bridge and is looking at some of the old collapsed bridge pieces. I just happened to pass almost directly over one large chunk. Its just slightly more to the left side.

You can see how it gets drawn on both sides of the SI image but shows up as a single piece on DI. It looks like just some blob with a beard on 2D.

[Larry3215]

One of the key things to notice in the Si image is the distance scale along the bottom. Some brands have it at the top.

Most people think that scale is showing them how far to the side of the boat some piece of structure is.

Thats is NOT true.

In that SI picture, that piece of structure that is split on both sides, is actually directly UNDER the boat.

If you look at the DI image, you can see its about 50 ft to the bottom and the top of that piece is about 40 ft down.

On the SI image, it looks like those two matching chunks are 40-50 ft to the side of the boat, but they are really 40-50 ft straight down under the boat. All of the black in between is the 50 ft of water under the boat.

[Larry3215]

Oh - those circles are the same fish seen on 2D and DI. It doesnt show up at all on SI.

[Larry3215]

Here is another example of how SI can fake you out when it makes you think something is way off to the side when it really isnt.

This is a screen shot of a large herring ball on sideview. Schools of fish show up pretty well on SI and DI even though individual fish not so much.

In this first shot, it looks like that herring ball starts about 100 ft to the side of the boat out to 175 ft or so and is touching the bottom.

Thats not even close to being true.

That herring ball is actually almost directly under the boat but mostly on the right side. Its depth goes from about 100 ft to about 175 ft down, but it does not actually touch the bottom at all.

Here is a DI picture of that same herring ball taken at the same time.

There is a clue in the first picture about where the herring ball is but its hard to see in my screen shot. If you look closely on the left side you can see a faint mirror image of the herring ball. That tells you that part of the ball was in the left side cone, but probably still on the right side of the boat.

The reason the herring ball looks like it touches the bottom in the SI image, is because that part of the herring ball is further from the transducer than the bottom thats directly under the boat.

[Larry3215]

And here is that same herring ball 2 minutes later after some seals and diving birds and salmon chased it back under the boat and up above the bottom.

[Sideburns]

Humminbird down imaging units, that are NOT capable of reading side images, are 75*/45*. The side imaging transducers, with seperate left and right elements, are claimed to be 180 degree total, or double 90* cones.

[Larry3215]

Ah, thanks for clearing that up.

[Larry3215]

Here are some more drawings showing whats going on with targets in the water column under the boat and off to the side.

Im still pounding on this issue of sideview not really showing you what it looks like its showing you.

The blank or black area on a sidescan screen, between the center line and where the bottom starts to get drawn, is showing you the water under the boat - sort of.

Again, this goes back to most transducers only being able to tell straight line distance and NOT depth or angle.

In this first picture, lets say you are in 50 ft of water and there are 5 fish on the right side of the boat. The key thing here is that all 5 fish are exactly 45 ft from the transducer but at different depths. They all are on the circular arc exactly 45 ft from the transducer.

Because they are all the sane distance from the transducer, they will all get drawn on the sidevew screen the same distance over from the center line. That means they will all look like they are 45 ft to the side of the boat when actually only fish #1 is close to that distance to the side. They will all get drawn right on top of each other if they are lined up perfectly.

All 5 fish will look like they are 45 ft to the side of the boat and right near the bottom - when that is not true at all. Only #1 is 45 ft to the side and only #5 is near the bottom. Plus you will see 6 or maybe even 7 fish on the screen when there are only 5.

Fish #5 and possibly even #4 will also show up (probably faintly) on the left side of the screen - also at 45 ft - because they will be in the center overlap area.

The point here is that if you see a fish in the water column you have no idea where it really is unless its showing up on both sides. Fish #1,2,3 and 4 could be 40 ft to the side and 12 ft down or 40 ft down and 12 ft to the side or anywhere on that arc.

Most of the time, the "depth/distance" scale on the sideview screen does not really tell you anything all that useful, all by itself. You need to look for other clues if you want to know how far to the side a target really is. Without those clues, you can not tell where a fish or a rock or a sunken boat really is based on sideview.

I'll get to more of those clues next - but you may have to do some math

[Sideburns]

Wow, I cant believe how misleading those images are... and how commonly misunderstood... Im seeing now that much of the same inacuracy is true for si di and 2d.

[Larry3215]

Yup. Just imagine how many sales they would loose for hi end sidescan sonar if they were 100% honest in advertising!

"Please buy our super duper, latest greatest $3000 sonar! Its guaranteed to make you THINK you know where the fish are!"

Unfortunately, Im not done yet......

[Larry3215]

I was just reminded of a couple of things on another board.

One of them is what I think is one of the best sideview videos ever made. Doug is one of the Humminbird guys and really knows his stuff. He skipped over some points but the over all presentation is excellent for getting a handle on sideview without any extra mumbo jumbo.



The other thing is a post by rbvinc (Ricky) on this crappie forum. He has some excellent graphics and another video (which I didnt really like much) showing how the "water column" actually works on sideview. Ricky is another really sharp sonar dude

http://www.crappie.com/crappie/side-dow ... ost3275934" onclick="window.open(this.href);return false;

[Larry3215]

Ok, here is one more screen shot that helps illustrate another thing that is misleading on the sideview screen.

The lines on the screen, where the bottom starts to be drawn on each side, are always directly UNDER the boat. In this screen shot it looks like the edge of the bottom is further away and closer in various places, but thats not true. The depth directly under the boat is changing and thats how it gets drawn on the screen. Every point on that line that I have circled is directly under the boat.

That line is zero distance to the side of the boat.

The really confusing thing is, there might be a fish showing right next to that line and the fish could be a long ways to the side of the boat or not.

[Larry3215]

Lets change gears for a bit and look at some practical things you can do to help with finding and catching fish using sideview.

First, lets sum up how you get around some of these "fake news" aspects of sideview.

Lets start with fish in the black area - the water column. Thats the space between the center line and where the bottom starts to get drawn.

1)Any fish that shows up in the water column is closer to the boat than the waters depth. If the water under the boat is 20 ft deep, then any fish in the water column is closer than 20 ft to the boat. If the water is 50 ft deep, then the fish showing in the water column are closer than 50 ft from the boat. They could still be at any depth or any distance to the side.

2) Any fish that shows up in the same place on both sides of the screen - a mirror image - is (more or less) under the boat. If its darker on one side, then its more to that side than the other, but still under the boat.

2A)If the fish is mirrored on both sides (and only then), then you can get an idea of how deep the fish are by how far to the side of the center line they are. So if you have a fish showing on both sides and its 20 ft from the center line, then that fish is 20 ft deep and under the boat. if the fish is NOT mirrored, then you have no clue about the depth or distance - unless its throwing a shadow. We will get into shadows shortly.

2B) You can also double check by looking at the DI and 2D screens to to see if the targets are showing up there. The depths on those screens will be close to correct if the fish is under the boat.

Remember the herring ball in these shots. Its mirrored on both sides, but more to the right, so its pretty much under the boat, and the depth starts out at 100ft and goes down from there - but its not touching the bottom. You can figure out most of that from the sideview pic but the DI pic confirms it.

[Larry3215]

Ive said several times that Down and especially sideview are the worst ways to spot fish unless they are schooling - like the herring ball above.

A lot of that has to do with scaling. Fish targets in DI and SI are drawn more or less to scale. That herring ball above is almost 100 ft tall and over 150 ft long. A 3 ft long salmon isnt going to show up as more than 1 pixel wide with typical settings.

If you look closely at the DI image, you can see a small dot a little to the left of the herring ball at 150 ft deep. Im pretty sure that was one of the large seals that was chasing that herring ball. That seal is much larger than any salmon and yet its still only a small dot on the screen and hard to see. Its especially hard to see on the sideview because the scaling is that much worse. More distance is crammed into a smaller screen area. You can still see it just barely on the left side of the sideview image and even fainter on the right side. So that seal was about 150 ft down under the boat but off a little to the left side and the herring ball was under the boat and a little to the right side.

I have my side and down setting optimized to show bottom details and not fish. However, you can cheat a little bit to make the fish returns in the water column stand out better on DI and SI - if you are willing to give up seeing bottom structural details.

The trick is to turn up the GAIN or Brightness or Contrast, or what ever its called on your brand of sonar, to the point where lots of noise starts to show on the screen, then back off a bit. That will make the fish returns much brighter and bolder, but it will washout the bottom area on the screen completely.

I dont happen to have any screen shots showing that because I almost never use DI or SI that way. I use them both to hunt for structure and I have Panoptix for spotting fish off to the sides. However, there is nothing wrong with doing that and it can be very effective.