Sonar interpretation?

[Larry3215]

Lets recap before we start 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 they 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. I'll get into the why later.

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.

Im going to use the Humminbird terms SI and DI for sideview and downview to save me some typing from here on out.

I think sideview and downview are more difficult to 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 are not - much like with 2D sonar.

[Amx]

I've seen many more fish on my down scan than I have on the 2-d, even schools of perch and crappie. At least those close to the bottom, and the schools at any depth, on/at, and above the bottom, and even caught some of those fish. Bass and perch, mostly bass. Up from the bottom a ways I see more fish on the 2-d.

[Larry3215]

The first thing you need to understand is how the "cones" for SI and DI are different from 2D sonar. 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.

The cone for 2D sonar is like a flashlight beam and 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.

That over lapping area in the center is where the DI image comes from. Some transducers have a third element that is dedicated to the DI image and some only have two elements. It doesnt really matter that much.

The key thing you need to know is that the DI image is the area where the two SI sides over lap in the center.

Its also critical to understand that the SI sides do overlap. Thats one of the key things to understanding the SI images.

Here is a rear view of the situation without the 2D cone. This is just the left SI cone and the right side SI cone and the overlap area.

[Larry3215]

I've seen many more fish on my down scan than I have on the 2-d, even schools of perch and crappie. At least those close to the bottom, and the schools at any depth, on/at, and above the bottom, and even caught some of those fish. Bass and perch, mostly bass. Up from the bottom a ways I see more fish on the 2-d.

Ah ha! A perfect example of how many of the things I am telling you are NOT hard and fast rules. There are exceptions in many cases.

Your experience is entirely possible depending on your settings for 2D and DI and what transducer your using and the bottom contours and depths you're fishing. You can tweak the settings on DI and SI to show fish returns better, but you will - usually - be making the structure returns worse when you do that. Things like increasing the brightness, gain and contrast will all make fish show up better, but they also tend to washout structure details and increase noise. Also, Im betting you're fishing relatively shallow when you see individual fish on DI. Schools of fish will show up quite deep on the other hand.

[Amx]

Anywhere from 20 to 56 feet for the down scan. And the same for the 2-d, but the mid depths on the 2-d seem to be from 10 feet down to about 50 feet. I don't remember the sensitivity setting on the 2-d, and I decreased it the other day. I think it was auto at mid/50 percent selection, and it's now at about 40 percent to get rid of all the little spotting and clutter. The contrast for the down scan is about 40 or something, I'd have to look.

Most of what I get on the down scan are short lines just off the bottom. And when I catch one it turns out to be a smallie about 12" to 15" long , or a perch of about 8", or even a trout of about 14" to 16".

The small even spaced round dots seem to be crappie, when I catch a fish that's what I get. The longer lines in groups are perch, and LONG lines in big groups can be perch or some of that green slime.

[Amx]

And lots of the times when I see a group on one, it doesn't show up on the other. Or it'll show up different, spots or lines on the down scan, and a blob on the color 2-d. Yes I do need to play with each settings, and I have a little, but I'd rather fish. lol

[Larry3215]

My brothers agree with you Tom. They tell me "Quit messing with the sonar and fish!" They usually start telling me that when I point out there are fish down there and they are NOT catching them!

Yeah, its extremely rare to see anything like an arch on downview and you will never see one on sideview. Dots are by far the most common. If you are seeing lines above the bottom, the fish are moving. If its attached to the bottom it may be grass or weeds the fish are hiding in.

This is a little out of sequence, but here are a couple of screen shots that more or less prove your point and mine at the same time

These are good examples to look at because they were all taken at the same time in the same place and show what the exact same bottom and fish details look like on 2D, DI and SI. These were taken in one of the Chain lakes in Couer d'Alene. The depth is very shallow and I was playing around with single frequencies on 2D, so you wont see as many fish as when I had it in CHIRP mode.

In this case, more "fish" show up in the DI than in the 2D, but the 2D targets are easier to see on the screen. Its almost impossible to see any fish targets in the SI screen unless you look very close. Its interesting to note the different details that show up in each shot or dont show up. This lake has crappie (we caught a few in the 6"-8" range), small bluegill 3"-4", tons of small 2" minnows, trout, pike and some big small mouth.

That V shaped set of logs are the same ones in each shot. They are at the very top of the SI shot. Note that on the SI shot, they show up on both sides at the same time. Thats because they are directly under the boat and are captured by the left and right side scans at the same time. There was a school of the minows on the surface and a school of crappie right on the V logs and another smaller school of minows to the left of the V and some larger fish on the bottom. Probably bass but they were not biting. Im fairly sure you can see some pike or really big bass off to the left and right sides and very close to the bottom in the sideview but its really hard to tell for sure.

Im going to come back to these shots again later in more detail.

[Larry3215]

oops....will add something more intelligent later

[TrackerPro16]

You are more likely to see things like that hole on downview, but it all depends on the size of the hole vrs the beam widths on downview. Its a little more complex than it might seem. And yes, you will see a creek bed more easily if you hit it at 90 degrees, but again - size matters

Im about to get into that in more detail......

Old style fishers usually have a paper map in the boat so they have an idea what the bottom looks like (for bigger lakes)

Hey! Just because I pull out my detailed waterproof paper map about every other time I am out does NOT mean I am,,,, wait,,,, what were we talking about??? Where am I? Oh yeah! Old!!! Ok, at 62 I am pretty much getting there...

[Sideburns]

I'm not sure how other brands of down imaging work, but I thought it would be useful to note that with the newer dedicated down imaging units from Humminbird, the down image is from a single fan shaped beam, pointed straight down. (75* or 45*) So, much different than a side imaging unit with down imaging view that is produced by the overlap. I think that the newest humminbird HD SI units can actually add a separate down imaging transducer and selector switch to see the dedicated type of down imaging view.

I have a dedicated humminbird DI unit, but trolling for mostly suspended kokanee, trout and salmon, the 2d is definitely more useful for me, to locate and also differentiate fish species. Down imaging just cant see the suspended fish as often. The down imaging view saves my butt quite often though, and keeps me from targeting submersed cables and trees etc.. that are crystal clear with DI and look like a school of fish on 2d views. (Im sure SI is just as useful for structure identification) The humminbird DI transducer is goofy though, and has compromised, weaker and narrower angled 2d elements than most other 2d transducers. (that's what the recent transducer selection thread was about).

[Larry3215]

I'm not sure how other brands of down imaging work, but I thought it would be useful to note that with the newer dedicated down imaging units from Humminbird, the down image is from a single fan shaped beam, pointed straight down. (75* or 45*) So, much different than a side imaging unit with down imaging view that is produced by the overlap. I think that the newest humminbird HD SI units can actually add a separate down imaging transducer and selector switch to see the dedicated type of down imaging view.

I have a dedicated humminbird DI unit, but trolling for mostly suspended kokanee, trout and salmon, the 2d is definitely more useful for me, to locate and also differentiate fish species. Down imaging just cant see the suspended fish as often. The down imaging view saves my butt quite often though, and keeps me from targeting submersed cables and trees etc.. that are crystal clear with DI and look like a school of fish on 2d views. (Im sure SI is just as useful for structure identification) The humminbird DI transducer is goofy though, and has compromised, weaker and narrower angled 2d elements than most other 2d transducers. (that's what the recent transducer selection thread was about).

Yes, some transducers (mostly newer models) have a separate dedicated element for downview, however, the sideview elements still always overlap in the center to one degree or another.

Also, you reminded me of another reason why Tom might see more fish on DI than he does on 2D. In addition to settings, the DI cone is almost always wider than the 2D cone. My drawing above was actually wrong in showing the 2D circular cone being wider than the overlap section where DI is.

Those Humminbird cone angle you mentioned above are fairly typical for DI - 75 deg and 45 deg. Most 2D cones angles are rated between 15 and 40.

Thats definately whats going on in those images I posted above. When I set the 2D to a single frequency, I locked the beam at somewhere around 20 degs but my DI beam width was still around 60 deg. So there were a lot more targets in the DI cone than were in the 2D cone from side to side.

[Larry3215]

Ok, here is a more accurate drawing of the relative cone angles.

Again, there are exceptions to this. Some transducers have narrower DI cones and very wide 2D cones depending on the frequencies selected at the time and the relative Gain settings.

[Larry3215]

Here are some more shots that show how the overlap on sideview affects what you see on the screen in both SI and DI.

In the first drawing, this is a view from the back looking forward. Lets say there are a series of targets sitting on the bottom - rocks, fish etc. From left to ring A,B,C, D, E.

Target C is dead center under the transducer. Remember the distance/depth thing? The second pic will show whats up with that.

Targets B and D are off to the sides a short distance but still within the DI cone range on both sides. Then, A and E are far enough to the sides to only be on the right or left.

This shot shows how those different targets would get drawn on the screen.

Target C is the closest to the transducer, so it gets drawn at the shallowest depth = closest to the center line on SI and the tallest point on DI.

Targets A and E dont show up at all on DI because they are to far to the sides.

Because targets B and D are both within the left and right SI zone, but further away from the transducer, they both get drawn on the SI screen a little further from the center line AND because they are at the same distance from the transducer, they get drawn on top of each other.

In the DI image, because they are slight further from the transducer, B and D get drawn slightly below where the "bottom" line is and also on top of each other.

In the 2D shot, B and D might or might not show up in the 2D cone, so they might or might not get mixed into the bottom return with C. Ive drawn them as lower case because they would be weaker returns due to being further away.

Confusing?

[Amx]

Ok, just a note. A few years ago I did the math and with the down scan the 'cone angle' give me a width of 7' in 21 foot of water. I forget the 'cone angle'. Same with the 2-d, I forget the 'cone angle'. And I never did figure out the width of the cone for the 2-d. What I do is watch the 2-d, if I see a mark I watch the down scan. If the mark shows up on the down scan then I know the fish is directly under the boat. If the mark doesn't show up I know the fish is off to one side or the other, outside that 7' width, and I toss my lure in accordingly. Someday I have do the math for the 2-d cone.

[Larry3215]

Ok, just a note. A few years ago I did the math and with the down scan the 'cone angle' give me a width of 7' in 21 foot of water. I forget the 'cone angle'. Same with the 2-d, I forget the 'cone angle'. And I never did figure out the width of the cone for the 2-d. What I do is watch the 2-d, if I see a mark I watch the down scan. If the mark shows up on the down scan then I know the fish is directly under the boat. If the mark doesn't show up I know the fish is off to one side or the other, outside that 7' width, and I toss my lure in accordingly. Someday I have do the math for the 2-d cone.

Lets see if this works. Ive attached a spreadsheet that shows the cone width on the bottom for a wide range of angles and depths. In your case Tom, 7 ft wide at 21 ft works out to just a hair over 20 deg cone. Fpr 2D, that probably means you were using 200 khz. Without knowing what transducer you have and the specs, I dont know what the cone angle or bottom width would be for your DI, but I'd be willing to bet its wider than 20 deg. I cant think of one Ive ever seen that was that narrow and none that were narrower.

I suspect that your settings are whats causing what you are seeing, but its really hard to tell. On the other hand, it could be that you are just an exceptional person

That didnt work. Is there a way to attach a spreadsheet?

Ok, I'll try a screen shot....

[Amx]

I should go look at the info for the depth finder and see what they say the cone angles are. Too cold to go out and see what the down scan and 2-d are set at for khz.

[Amx]

And I know the down scan is narrower than the 2-d because of experimenting with it a few years ago. Run over a rock, log, or brush, or dropoff that I know is there, and it shows up first on the 2-d because of the full round cone, then on the down scan. Then run to the side of that object, about 5 or 10 feet away, and it'll show up on the 2-d, but not on the down scan, but will on the side scan.

[Amx]

According to the comparison page at Lowrance this is the results for the 2-d transducer;

Depth
21ft


at Frequency
200kHz

Angle
22°

Beamwidth
8ft


at Frequency
83kHz


Angle
52°

Beamwidth
20ft

[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 degrees 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.

[Larry3215]

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.