Sonar interpretation - why things look the way they do on your screen

[RoughThumbBG]

This is all awesome stuff. Really helps to understanding. I am looking forward to hearing about how Down and Side Imaging are similar and different than traditional 2D.

[Larry3215]

Yea Larry, while I was writing that I thought you might throw that back at me. So Instead of taking the long explanation, like you have I did not write depth displayed on the screen and assumed the distance from transducer because that is how I think with Sonar. Also I wrote "compensate" and not eliminate, as to not over sell the feature. One more thing, I have been somewhat talking about sonar in general as you have been more specific with Garmin features so there can be small differences between brands and what their settings do to manipulate the returns. Good catch though!!!

Sorry about that Russ. I came across a little harsher than I meant to. I really didnt have any gripes with what you said. Its mostly with how TVG has been marketed. I have similar gripes with how all brands market sonar though :)

As far as different brands being different, I dont think I have mentioned anything so far that is specific to Garmin only - other than Panoptix? Or did I forget something?

Of course, they sometimes use different terms for the same thing - clearvu/downvu, sidescan/sideimaging, etc. As you say, the details will vary, depending mostly on the beam widths/cone angles and frequencies in use, but the theory Ive been covering is the same for all brands. They all work based on distance and the screens all get drawn the same way, and returns vary the same way depending on where they are in the cone, etc.

Anyway, sorry of I sounded like I was getting on your case too hard.

[Larry3215]

This is great info that I've never really thought that deeply about before. Thanks for sharing, I don't want to detail this but do have a question (sorry about my inexperience). Now, I have the GT52 which is a chirp 160-240... Just to confirm, I don't have much option to tighten up my cone as I don't have 77 capability... Not sure if there would be much benefit in 160 over 240 (or if I can even select between the two)? Any advice on 2d settings (93sv unit)?
Looking forward to the SV and DV posts!

You should be able to choose a single frequency within that range - I think. Check your owners manual. I just did a quick check and I cant find any specs on the Garmin site for the cone angles for that transducer, so I dont know how much difference you could expect in bottom coverage, dead zone, etc by doing that.

Its listed as an HW transducer which means high/wide = hi frequency wide angle - but I have no idea what those angles are.

[Larry3215]

Larry, when you start going on side and down looking imaging are you going to be very specific with Garmin? I find there are bigger gaps and differences between brands in this area and assumptions in between brands and transducers can be way off.

Not if I can help it Russ. The theory is the same for all brands. Some of the terms are different and of course there is a large difference in quality and some of the frequency ranges - especially with the new Humminbird Mega setups. I will mostly be using the Garmin terms, but other than that it will be as generic as I can make it.

If you do spot something that is different with a different brand, in any area, by all means please bring it up.

[Larry3215]

isn't di and si a slice? not a cone?

Yeah, like Russ said, downview/clearvu cones are long skinny ovals. They are very narrow front to back but tend to be very wide side to side. Typically front to back angles will be less than 1 degree to maybe 2 or 3 degrees depending on the transducer and brand and the frequency. Typically higher frequencies will have narrower front to back angles which is why they give more fine detail.

Side to side angles can be 50 to 90 degrees more or less.

Most of the time, downview cones are wider, side to side, than 2D cones.

Sideview is the same thing more or less but with even wider cone angles if you add up the left and right sides.

Im going to go into all that in boring detail - with some math - shortly :)

[slabbacks]

Subscribed...great info I've not really thought about. Can't wait for the SV section to kick in hard. Keep it up!

[Larry3215]

This is great info that I've never really thought that deeply about before. Thanks for sharing, I don't want to detail this but do have a question (sorry about my inexperience). Now, I have the GT52 which is a chirp 160-240... Just to confirm, I don't have much option to tighten up my cone as I don't have 77 capability... Not sure if there would be much benefit in 160 over 240 (or if I can even select between the two)? Any advice on 2d settings (93sv unit)?
Looking forward to the SV and DV posts!

Someone posted a link to a Garmn Transducer specs PDF on the main board. Your CV52 is listed.

https://www8.garmin.com/transducers/d/Garmin-Transducer-Selection-Guide.pdf

CHIRP High
Wide (150-
240kHz)
455 kHz
(425-485 kHz)
800 kHz
(790-850 kHz)
DownVu/SideVu

Trad/CHIRP
24-16
Down/Side
2.0x50@455
1.0x30@800

So at 150 khz the cone angle is 24 deg and at 240 khz its 16 deg. Thats not as big a difference as we have been talking about, but it does have an effect.

At 40 ft depth for example, the 150khz will cover a circle thats just under 18 ft across on the bottom. The 240 khz will have a circle thats just under 11 ft across.

Assuming a perfectly flat bottom - if you do the math, that works out to a bottom line thickness on the screen and a "dead zone" thats about 1 ft thick on the screen at 150 khz.

The bottom line and dead zone is about 4" thick, or roughly 1/3 the size, at 240 khz. If the bottom was sloped, the difference could be much larger.


Also, going back to the downvu and sidevu cone angles - on that transducer, the front to back cone angles are 2 deg at 455 khz and 1 deg at 800 khz. The cones are 50 deg wide and 30 deg wide at the same two frequencies. So in this case the down cone widths are both wider than the 2D cone widths side to side by a significant amount. Thats how it almost always is.

[Larry3215]

After posting that last it occurred to me I hadnt really talked about the dead zone on a flat bottom. There is still a dead zone even on a perfectly flat bottom, but its much smaller. Again it really depends on the cone angles and the depth. Deeper water and wider cones make for larger dead zones.

The main difference on a flat bottom vrs a sloped bottom, is that fish in the center of the cone will still show up. Its fish on the outer edges that get hidden.

If we take a different transducer with a bigger difference in cone angles - like the GT41 - we get much larger differences in the bottom line thickness than with the CV52. The GT41 has a 50/200 freq range and cone angles of 40 deg and 10 deg.

In 50 ft of water, that works out to a bottom line thickness of about 3 ft for the 50 khz but only 3" for the 200 khz. The 200 khz will have almost no dead zone and a very thin bottom line but the 50 khz will have a 3ft thick bottom line and a correspondingly larger dead zone.

The 200 khz will be less effected by slops and the 50 khz dead zone will be greatly magnified on sloped bottoms.

Im also not sure I covered this before either - deeper water magnifies these effects and shallower water reduces them. So, when you fish shallow, you probably dont need to worry about dead zones nearly as much as in deeper water.

In this drawing you can see the green fish just fine but the blue one is hidden. Its outside the cone angle on 200 khz and its in the dead zone on 50 khz.



flat dead.jpg

[SHOTGUN RUSS]

Larry, hope your feeling better, no worries about your TVG description or correction. I was more meaning that because I took a shortcut in describing the setting that I kinda wondered if you would catch me on not giving the full rundown. Keep it up! Humminbird DI has a selectable narrow, medium or wide cone, if I remember right the narrow has just about the same width as their 2d 200khz cone (if i ever use DI on "Bird" it is always on narrow). Yes the theory is all the same in all brands, how they implement them in the software (which we will never know) and fine tune for the brand can have different effects (read how Humminbird sells clear mode vs max mode on switchfire, clear mode used to be their TVG setting) As for what I meant about the different brands and transducers with DI and SI being very different that holds true. Cone coverage angles, how the image is made, where the crystals are located in the transducer and processing software are making the generalities far less general.

[Larry3215]

I think we are on the same page Russ :)

Im not going to go into transducer details or specifics on the different brands for the most part. It would just take too much time. My plan is to focus on the general theory. Feel free to jump in with specifics and/or anything you think it relevant or helpful or fills in anything Ive skipped over or left out, corrections, etc.

[Larry3215]

Before I start on down and side, louievito sent me some images from his 7sv that he wanted me to post for him.

He had some specific questions, but I was an idiot and deleted his emails after saving the images to my hard drive. So... between that and being on some good drugs recently, I cant remember what he wanted to know

Im going to post a few in each of the next posts grouped the way I think he wanted them. Hopefully, he can chime in and let us know what he was interested in.

20170226_093242.jpg20170226_092204.jpg20170226_092409.jpg

[Larry3215]

The next 3....20170226_093450.jpg20170226_112712.jpg

[Larry3215]

next some sidevu shots

I think the main question here was why the dark lines on each side of the images. Those are ghost lines. They are caused by having both sidevu and downview on the same frequency. Try changing one of them to a different frequency and they should go away.


20170226_100857.jpg20170226_103038.jpg20170226_104719.jpg20170226_105032.jpg

[louievito]

thanks larry hope you are feeling better. yes the main question was the lines. I could understand the both frequencies, but the last two shots above are side scan only and the bow unit transducer is not in the water and still lines. also, in the first set of pics, the first shot is down scan brush pile and I was concerned about how unclear it is. second shot, the depth on the upper left corner doesn't match the depth on the graph. this is what is was talking to you about earlier. I watch my jig go down on ascope and when it hits the red it disappears but I still have a few feet of fall left to the bottom. the third shot in the first group, I don't know what the heck it is, there is no wood to get hung on, im not sure whats going on in that shot. larrys second post has a shot of a brush pile on 2d and down view. the downview isn't to clear? what says you?

I appreciate you posting he pics and look forward to everyones replies! have a great weekend

[Larry3215]

I'll take these one at a time.

thanks larry hope you are feeling better. yes the main question was the lines. I could understand the both frequencies, but the last two shots above are side scan only and the bow unit transducer is not in the water and still lines.

The Striker SV model does downvu and sidevu at the same time. Even when you are only looking at sidevu, the downvu is still sending out pings. Look at the first combo picutre - both down and side are on 800. Thats why the lines. Change one of them to 455 or what ever the other frequency is and the lines should go away.

also, in the first set of pics, the first shot is down scan brush pile and I was concerned about how unclear it is.

Its really hard to tell from a phone picture, but there are a lot of reasons why down and sidescan can be fuzzy or look distorted. I will go over those shortly. The most likely reason is you were drifting sideways to some degree. The boat needs to be traveling in the same direction the transducer is pointed. Any side wind or side current will mess up the images. Or of the transducer is on the TM and its turned to one side or turning while scanning, it will mess up the images.

second shot, the depth on the upper left corner doesn't match the depth on the graph. this is what is was talking to you about earlier. I watch my jig go down on ascope and when it hits the red it disappears but I still have a few feet of fall left to the bottom. larrys second post has a shot of a brush pile on 2d and down view. the downview isn't to clear? what says you?

I appreciate you posting he pics and look forward to everyones replies! have a great weekend

Go back and look at posts 28, 29, 39, 68. Your jig is disappearing into the "dead zone". The depth numbers dont agree with the bottom line on the screen for the same reasons. You are fishing on a sloped bottom and/or the jig is falling into the outer edges of the sonar cone.

the third shot in the first group, I don't know what the heck it is, there is no wood to get hung on, im not sure whats going on in that shot.

I dont know either but I would guess some sort of vegetation. Again, if you were drifting with any sideways motion things get distorted badly making it very hard to know whats going on in down and sidevu.

[The Force]

Thanks for speaking to the GT52 Larry and thanks again for the articulate explanations. Get better soon!

[Larry3215]

Before we start on downview, lets go back briefly to the question louievito asked earlier about why the digital depth numbers didnt match the depth where the bottom line was being drawn.

Here are a couple of screen shots showing what that looks like.





03MAY16_1615_00.jpg
28MAY16_1615_00.jpg

When ever you see something like that going on its a pretty good chance your on some degree of sloped bottom. The portion of the bottom closest to the transducer and within the cone, will be drawn on the screen at the shallowest depth. Thats where the bottom line shows up on the screen.

The software on MFD on the other hand, will have some sort of algorithm that it uses to calculate what it thinks is the "depth" and that number is whats shown int h top left in my pics. I dont know it it is doing an averaging calculation or some sort of strongest return or a mix of both. What ever it is, it seems to always be deeper than what is drawn on the screen. The main thing to know when you see that difference - along with a thicker bottom line - is that you're most likely on a slope or some sort of very irregular bottom.

[Larry3215]

Before I start on down and sideview I want to confuse things just a bit by going back to "cone angles". Everyone - including me - always talks about cone angles as if they were something well defined and exact or fixed in some way. Thats actually a gross exaggeration and over simplification.

I mentioned before that what was going on in some of my drawings wasnt quite accurate. In fact, talking about cone angles at all is really a gross over simplification of a very complex situation. I would not even bring this up but its important to know that the examples I gave above about how fish interact with the "cone" and how they display on the screen aren't always the case. Those are not "rules" you can count on all the time.

Remember in the Lowrance tutorial I linked to earlier? They hinted about this when they mentioned that the specified "cone angle" was not the same as the "fish detection" angle. A transducer that says it has a 20 deg "cone angle" could have a fish detection "cone angle" of 60 degrees or more - but - that angle will vary greatly depending on the depth and the gain settings and where the target is within that "cone".

Here are a few pictures showing what the true "cone" is more likely to look like on any transducer. The cone isnt really shaped anything at all like my drawings or the ones you see in any fish finder manual or advertisements. Those simplified, perfectly geometric, nicely triangular cones help to get the basic idea across, but they are complete and utter lies.

Its even worse because there is no industry standard on how to measure cone angles or what numbers to use in for cut offs. I believe Airmar uses -3DB (1/2 signal strength) and I think Humminbird uses -10DB (1/10th signal strength) and and I have no idea what Garmin or some of the other brands use because they dont advertise those specs.

And remember - everything about a cone varies with depth and Gain settings - so saying a transducer has a specific cone angle is really not very accurate at all.

Im still going to use the simple triangular cone examples I have been because its pretty much impossible to do anything else and have it make any sense. Plus I cant do that good a drawing :)







cone coverage 03.jpgbeam_patterns_450.gifSonar Cone.jpg

[Larry3215]

Lets recap a little and then get started on side and down views.

Traditional 2D sonar is great for seeing if there are fish somewhere under the boat as long as its in the "cone" - except for the dead zones on slopes and in holes - but - you have absolutely no idea where the fish are left to right and only a rough idea of the true depth much of the time and no idea of how the fish sit in relation to each other unless they are very close together.

Still, 2D is by far the best way to see if there are fish somewhere close to under the boat.

Downview and sideview on the other hand, suck for seeing fish unless they are close in and fairly large or in a school. Things like a bait ball or a school of crappie or a school of perch will show up quite well, even at a good distance, but a single perch or crappie or even a salmon will be very difficult to spot unless its very close in and you have settings just right. Thats mainly because of scalling. Side and downview draw the fish returns closer to their actual size, so they tend to get lost on the screen as tiny dots.

lwinchester2 has a trick he likes to use that gets around that limitation pretty well under the right conditions and with tweeks to gain settings. I will let him jump in with an explanation of that.

Downview and sideview are really designed for seeing structure on the bottom and thats what they excel at while 2d sucks for seeing details about bottom structure but shows individual fish very well. They compliment each other nicely.

I think sideview and downview are more difficult to interpret - or maybe easier to MIS-interpret - for a few reasons.

For starters, DI and SI really make it look like you have taken a picture or snap shot of the bottom. The level of detail is far far greater than with traditional 2D sonar. You can scan over a sunken ship or a bridge or a tree and, as long as you do it right, it looks like a ship or a bridge or a tree on the screen.

BUT...

Si and DI are still subject to the distance/depth thing. They both are still "graphing" sonar returns based only on the distance from the transducer and the strength of the return signal. This is especially deceptive when looking at an SI screen.

The second thing that makes it difficult to interpret whats on the screen is the way they both try to represent a three dimensional situation on a two dimensional screen. That causes things to look one way when they really can be very very different - in some ways worse than with 2D sonar.

[Larry3215]

The first thing you need to understand is how the "cones" for SI and DI are different from 2D sonar. Again - Im going to stick with the simplified geometric cone thing from here on out - mostly because trying to draw the real cone shapes is impossible and it's just too confusing. For these discussions it really doesnt make any difference anyway.

You can think of the cone for 2D sonar is like a flashlight beam that spreads out in all directions in a circular pattern as you move further from the transducer.

Di and SI are flattened versions of that cone shape. Imagine cutting a narrow slot in a piece of paper and shining your flashlight through the slot. You would end up with a long narrow area where the light hit a wall. The SI and DI cones are wide from left to right of the boat, but very narrow front to back.

Here are some drawings showing how it works.

This first one is a top down view showing a circular 2D cone and the left and right SI cones. The left and right cones actually over lap in the center. The left SI element in the transducer actually sees a little ways to the right side of the boat and the right side element sees a little ways to the left side of the boat. The down cone over laps all of the 2D cone and a large portion of both side cones.



sidetoptop2.jpg

On some transducers, that over lapping area of the two sideview cones in the center is where the down image comes from. Better, newer transducers have a third element that is dedicated to the DI image. For these discussions, it doesnt really matter that much. The theory is the same.

Its important to understand that the SI sides do overlap in the center. Thats one of the key things to understanding the side images.

Here is a rear view of the situation without the 2D cone. This is just the left side cone and the right side cone and the overlap area plus Ive added a third downview cone.



side back.jpg