Thread: Getting The Most From Your Graph (Sonar, Sounder, Fishfinder Thangy, etc.)

Questions on how to properly adjust sonar units and decipher what is shown on the screen have come up many times on this board. The following is what I have learned from using several different graphs over the years and may be of some help to folks trying to get the most out of their sonar units. A lot of this is pretty basic, long and drawn out (boring) but I've tried to make it helpful for someone that has never used a graph before.

Fish Arches - What Am I Seeing?

In order to get a “fish arch” (like the manufactures lead us to believe we should be seeing on our "fish finders") a fish must pass completely through the “cone” or view of the transducer from one side to the other within a narrow range of speed depending on several variables without changing depth when either the boat passes over the fish or the fish swims under the boat.

Think of your graph screen as a composite picture of individual sonar returns. The transducer sends intermittent sound waves or “pings” down into the water and records how long it takes for each ping to return to the transducer as well as how loud that signal is when it bounces back up from the bottom or suspended objects and returns to the transducer.

The returning sound waves ("return") are "painted" on the graph screen by filling in pixels with shades of gray or colors depending on whether you have a "Grayscale" or a color graph. The sound waves are broadcast into the water in somewhat of a "cone" or circle that is narrow at the transducer and widens until it meets the bottom. Only objects that are in the cone or view will return sound waves and show up on the screen.

Think of each horizontal pixel on your screen as one ping or one still photograph of what was in the view at that instant. As long as the object being painted on the screen stays in the view of the transducer the pings will continue to paint or fill in the pixels across the screen. Think of each vertical pixel as one volume setting. The louder the ping comes back the more vertical pixels it will paint and the darker or "louder" colors (on a color graph) it will paint them depicting higher volume. Higher volume means the sound wave bounced off a larger and/or denser object.

Also, note that the display on your screen is simply a short history of what your boat passed over or what passed under your boat. The most recent information comes into view on the right side of your screen and then moves across and off your screen to the left. Depending on your boat speed and scroll speed (speed at which information passes across your screen), what you're seeing on your graph isn’t necessarily under your boat and can actually be quite a ways behind you if you are moving.

Arches are created because as a fish passes through the view or “cone” of the transducer it is farther from the transducer when it is at the edge of the cone and closer to the transducer when in the center of the cone. The width of the arch depends on several factors including, how fast the fish passes through the cone and also how fast your Scroll Speed is set.

The cone angle also affects fish arches because at a given speed (boat or fish) and scroll speed it takes longer for a fish to pass through a wide cone than a narrow cone at any given depth. Varying depths of fish also affects the arches because the cone gets wider as it gets deeper so the deeper a fish is the longer it takes it to pass through the cone and visa versa because of the difference in the distance across the cone at different depths.

The faster the fish passes through the cone and/or the slower your Scroll Speed is set the shorter the arch is going to be up to the point where it is just a blob instead of an arch. The slower the fish passes through the cone and/or the faster your Scroll Speed is set the longer the arch is going to be up to the point that it goes all the way across your screen.

If the fish stays in the view of the cone - either because the boat is not moving or the fish is moving at the same speed and direction as the boat – and doesn’t change depth then you’re going to see a solid line all the way across your screen instead of an arch.

If you’re sitting still or moving very slowly over a brushpile the only way you’re going to see fish arches is if a fish swims through the view of the transducer fast enough – probably somewhere between .5 and 3-mph depending on a variety of factors as mentioned above.

If you get over an active school of fish that are moving up and down in the water column you're going to see lines that go up and down as they scroll across your screen. Crappie however are not usually that active and if you’re sitting or moving very slowly over a brushpile you can expect them to show up as solid lines across your screen rather than arches.

If a fish enters the view of the transducer and leaves in less time that it takes for that information to move across the screen but not fast enough or centered enough to make an arch then you’ll have a line on your screen that starts and stops.

To see actual sonar returns - lines or arches - and no fish symbols turn your Fish ID off. Set your overall sensitivity so the bottom shows up fairly dark (hard return) and cover – brushpiles, bamboo condos or weed beds, etc. – show up a lot lighter (soft return). Most graphs now have several different “views” – Whiteline or Grayline, Inverse, Structure ID, Bottom Black, etc., which are a variety of ways information is displayed on the screen. I have mine set to Whiteline (Humminbird) or Grayline (Lowrance), which seem to be the best for differentiating between the bottom and cover.

How Do I Adjust My Graph For Optimum Performance?

First, make sure your transducer is pointing straight down and be sure there is no oil or wax residue on the face, which could cause small air bubbles to stick to the face and interfere with the sound waves. Also, make sure you have a good connection where your transducer cable connects to your graph - there should be no moisture or corrosion in this connection.

Depth or “Keel” Offset

The very first adjustment you should make on your graph is for Depth Offset or Keel Offset. This is an adjustment to correct for the difference between the depth of your transducer and the surface or the difference between the depth of your transducer and lowest point of your boat that is apt to run aground. For fishing purposes you’ll probably want to make adjustments so your depth reading is the actual depth from the surface to the bottom of the lake.

The easiest way to accurately make this adjustment is to position your boat over a smooth, level, hard bottom and measure the depth with a tape measure or string with several ounces of weight at the bottom and a mark for a known depth. Then adjust your Depth Offset or Keel Offset up or down until your depth reading matches the actual measurement from the surface to the bottom.

Changing the Depth Offset or Keel Offset may or may not correct the Target Depths when using Fish ID or the Grid Lines on your screen depending on the graph you have. To find out, put a 2-ounce bank sinker on a length of fishing line or string that is marked at 15 or 20-feet and matches a Grid Line on your screen. Then lower the sinker directly under your transducer to your measured mark and see if the return from the weight shows up at the Grid Line of the same depth.

You can also check your Fish ID to see if the Target Depths are corrected by turning up your Fish ID sensitivity until fish symbols start showing up on the sonar return from your weight. If the depth of the Grid Lines and Target depths are accurate then they have been corrected with your Depth Offset or Keel Offset. If not then you’ll need to mentally adjust for the depth of fish and cover returns if you want to know exactly how deep they are, which can be very important when crappie fishing.

Adjusting Sensitivity

This is probably the next most important adjustment on your graph. I think one mistake folks make is to assume that a higher sensitivity setting means you see more on your graph, which isn't necessarily true. Increased sensitivity simply makes everything darker (or more of the colors that depict more solid objects on a color graph) and decreased sensitivity makes everything relatively lighter (or less of the colors that depict solid objects on a color graph).

With sensitivity properly set, harder (rockier) bottom will show up darker because stronger signals are coming back to the transducer and mud or silt bottom will show up lighter because some of the signal is absorbed and less of the signal is returned to the transducer. Increased sensitivity also shows more surface clutter and electrical noise and visa versa.

You can distinguish between cover and the bottom because wood, bamboo, weeds, etc. have a different density than rock, silt or mud. Knowing what type of cover you are over, either because you put it there or have seen it when the water level was down, helps a lot in learning to identify what you’re seeing on your graph.

You can also distinguish between fish and cover because fish have a different density than wood, bamboo and weeds, etc. Probably the easiest identifying feature of a sonar return made by a fish that stays in your “view” for any length of time is that it has a constant thickness or “volume”, which shows as a consistent line across your graph. Bigger (thicker) and shallower fish will show up as thicker lines because the ping comes back louder than it does on a smaller (thinner) or deeper fish but each fish will show a consistent thickness or volume as it’s sonar return is displayed across the screen.

As the fish moves towards the edge of the cone its sonar return may be somewhat lighter or thinner because it is farther from the transducer than when it is in the center of the view but it won’t change much. Tree limbs on the other hand vary in thickness and therefore show up with an inconsistent thickness or volume. Bamboo is not very dense so it has a fairly light but consistent sonar return and shows up very similar to a school of baitfish so it’s very easy to see sonar returns (arches or lines) made by fish that are down in bamboo cover.When you are confident that you can distinguish between fish and cover you may want to turn your Fish ID back on and set the Fish ID sensitivity so it only shows fish symbols on some of the better fish returns. I use the Fish ID on my Humminbird 797c2 SI so I can glance down and quickly see the depth of fish on the screen by the target depth shown with each fish symbol. In bamboo and other cover that isn't too dense I can still see the actual sonar returns to verify that the computer generated fish symbol is actual a fish. Once properly adjust, I trust that when my sonar shows fish symbols it usually is a fish. In heavy cover or steep slopes the actual sonar returns of fish are harder to distinguish because they are obscured by other sonar returns so using Fish ID can be an advantage if it is properly adjusted.

Depending on what graph you have you may or may not be able to set it to show both fish symbols/depth and actual returns. If you have to choose between one or the other you definitely want to turn off the Fish ID so you can see the actual returns.

One useful method for fine tuning your overall sensitivity and your Fish ID sensitivity is to use a 1/16 to 1/8-ounce lead head jig suspended in the view of your graph. The width of the view of a 20-degree cone angle transducer is only about one-third the depth of the object your seeing so if you’re looking for your jig at a depth of 10-feet it will have to be within about a 3-foot diameter circle centered under your transducer. It’s easier to keep your jig in the cone if you put it down 15 or 20-feet but even then it will need to be within a 5 or 6-foot diameter circle directly under your transducer.

To adjust your overall sensitivity turn your Fish ID off and suspend a jig in the view of your transducer and slowly move it up and down a foot or two at a time. It should show up on your graph as a line that moves up and down on the screen just like you are moving the jig with a split second delay as the graph processes the information and displays it on the screen. If you have a flasher bar on the side of your screen it should show the jig moving up and down in real time.

If you don’t see your jig, slowly increase your sensitivity JUST until it starts to show up as a faint line. If your jig shows up real dark then reduce your sensitivity until it only shows as a faint line. Now your overall sensitivity should be about right but you can make minor adjustments from there if needed.

Most graphs now have an “Automatic Sensitivity” setting, which adjusts for different water conditions and depths. Even with the Automatic Sensitivity turned on you can manually set the desired sensitivity range. The Automatic Sensitivity then makes adjustments for different depths and conditions so your display is consistent but with a bias towards level you have chosen. If you’re going to use your Automatic Sensitivity then be sure it is turned on before you make sensitivity adjustments.

If you are going to use your Fish ID turn it back on after you have adjusted your overall sensitivity. Now move your jig up and down as before. If you’re graph shows fish symbols along with the line depicting your jig then reduce the Fish ID sensitivity until it no longer shows fish symbols. If your jig no longer shows up as a line then you may have to increase your overall sensitivity if you are going to use your Fish ID. If your jig still doesn’t show up as a line or if it only shows up with your sensitivity set so high that you can’t distinguish between cover and the bottom then you probably want to turn your Fish ID off and reduce your overall sensitivity until your jig is just a faint line again.

With even a lower end graph properly adjusted you should be able to move over a brushpile or other cover and quickly determine how deep the bottom is, how tall the cover is, whether or not it is holding fish and how deep and how close to the cover those fish are holding.

With that information you should have a good idea about how deep to start fishing and whether or not the fish are actively feeding. If you’re not marking very many fish and don’t get a bite within a few minutes knowing that you are presenting your bait at the correct depth then you can either move to another location or sit and wait for more fish to show up or the few that are there to start feeding. On the other hand if you are marking a lot of fish all up and down the water column you may have to try several different depths to determine which fish if any are actively feeding.

Taking the time to learn how to properly use your graph will save you a lot of time and help you put a lot more fish “in the box”.

Man...

Jerry I really appreciate this...
I mean really... Thanks.

Your a real stand up guy...

I don't care what these other crappiers say about you:D

I will definately put this to good use and I'm sure many more will to...

Indeed, Thank you.
That is a help. almost nails what i asked in the other thread. YDM!

To add to this, ill share what a called a "Pearl of wisdom" in my other post.

If you decide to switch up and fish for catfish or any other scaleless fish (why would you though ) you can tell a catfish apart from other fish in the water with your depth finder.

To do this you will need to manually adjust your depth range for double what you are currently fishing in.

So for example let's say you are in 30 ft of water...
You will set your depth range manually from 0 - 60 ft.
Now when you look at your screen you see the actual bottom at 30 ft, and a double echo of the the bottom at 60 ft. This one will be lighter and not as defined as the one at 30.

Now lets say you have a scaled fish at 20 ft. You will see the "real" echo at 20 ft, and the double echo at 40.
Now with a catfish say at 25 ft... You will see the "real" echo at 25 ft, but there will be no double echo at 50 ft.

FWIW
-SS

Originally Posted by Slab_Stalker

Now lets say you have a scaled fish at 20 ft. You will see the "real" echo at 20 ft, and the double echo at 40.
Now with a catfish say at 25 ft... You will see the "real" echo at 25 ft, but there will be no double echo at 50 ft.

FWIW
-SS

Why is that ?? Do you know ? I was always under the impression that the depth finder was reading the air in the fish's air bladder ... and that was the "return" signal that was processed into a arch or fish symbol. Catfish have an air bladder, as does any "scaled" fish ... so, why would there be no second echo ? Fish are like us, in that they are mostly made up of water ... and the signal goes thru the body of the fish, almost as easily as it goes thru the water the fish is in. The air in the air sac is what (I believe/been told) "interrupts" the signal ... so why would it be different for a "skin" fish, over a "scaled" fish ??
I'm really interested in any substanciated info on this "pearl of wisdom". It would be great to know "how" this works !! Got any articles or web links that you can direct me to ?? ......... cp

Originally Posted by crappiepappy

Why is that ?? Do you know ? I was always under the impression that the depth finder was reading the air in the fish's air bladder ... and that was the "return" signal that was processed into a arch or fish symbol. Catfish have an air bladder, as does any "scaled" fish ... so, why would there be no second echo ? Fish are like us, in that they are mostly made up of water ... and the signal goes thru the body of the fish, almost as easily as it goes thru the water the fish is in. The air in the air sac is what (I believe/been told) "interrupts" the signal ... so why would it be different for a "skin" fish, over a "scaled" fish ??
I'm really interested in any substanciated info on this "pearl of wisdom". It would be great to know "how" this works !! Got any articles or web links that you can direct me to ?? ......... cp

I believe what the graph picks up and displays as actual sonar returns is the sound waves bouncing off the fish’s body. Air does interrupt the sonar signal and a graph may use that information to process and display a fish symbol using the Fish ID system. But arches, as I understand it, are created by the difference in the distance from the transducer and the fish depending on where the fish is in the cone. As the fish enters the cone (or the cone begins to pass over the fish) it’s farther from the transducer, which causes it to show on the graph deeper and lighter than it does when it is in the center of the cone where it is closer to the graph, which causes it to show up shallower and thicker (louder) on the screen. As the fish passes from the center of the cone out the other side it then paints the other side of the arch for the same reason. A suspended stick or any solid object will also make an arch on your screen if you pass over it at the correct speed.

The height of the arch is relative to the width of the cone and the depth of the fish. The wider the cone and/or the deeper the fish the bigger difference there will be between the transducer and the target whether it is in the edge of the view or center. Of course the height of the arch displayed on the screen is also relative to the overall depth displayed on the screen.

I have no clue about the scaled vs. fish without scales thing. Seems like they would show up about the same relative to their size. A larger fish should show up better on a double return than a smaller fish I would think. I usually figure the fish I'm seeing right close to the bottom are walleye or catfish but I'd have to catch one to be sure.

Originally Posted by crappiepappy

Why is that ?? Do you know ? I was always under the impression that the depth finder was reading the air in the fish's air bladder ... and that was the "return" signal that was processed into a arch or fish symbol. Catfish have an air bladder, as does any "scaled" fish ... so, why would there be no second echo ? Fish are like us, in that they are mostly made up of water ... and the signal goes thru the body of the fish, almost as easily as it goes thru the water the fish is in. The air in the air sac is what (I believe/been told) "interrupts" the signal ... so why would it be different for a "skin" fish, over a "scaled" fish ??
I'm really interested in any substanciated info on this "pearl of wisdom". It would be great to know "how" this works !! Got any articles or web links that you can direct me to ?? ......... cp

If the air pocket is what reflects back to the transducer, then why does brush and other water-logged structure show up on the sonar?

I've never heard of the air bladder producing the signal. Do you have any articles or web links to support your "impression"?

Originally Posted by Matt Smith

If the air pocket is what reflects back to the transducer, then why does brush and other water-logged structure show up on the sonar?

I've never heard of the air bladder producing the signal. Do you have any articles or web links to support your "impression"?

Second paragraph under the section "Transducer Frequency", in this article:

http://www.boatus.com/boattech/SelectTransducer.htm

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This, from a Furuno Color Depth Sounder article:

The airbladder of fish is what returns the echo to the fish finder. So the larger the fish, the larger the air bladder and thus the larger the target on your screen. Bait fish show up on the screen as a ball, because the echo is being returned on their collective air bladders. The air bladder on a single baitfish is too insignificant to return an echo, but as a school, their air bladders can be picked up by the fish finder. Remember, the denser the target the darker the color will be on your display.
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This, from Pro-Troll.com article about using depth finder to selectively pick out fish species, based on their air bladder size/shape:

" Fish finders cannot read through air. Most fish have air bladders. The locator beam put out by your fish finder passes through water but cannot pass through air. Fish finders read the air bladder of a fish, and the image is bounced back to you."
*******************************************

And the Vexilar article, "How Sonar Works":
http://www.vexilar.com/help/tips/tip011.html

And, I've read other articles that basically say the same thing.
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I've used depth finders since the first "flashers" came on the market. It has always been "reported", by these fish/depth finder mfg's, that the signal is bounced off of "anything" that's denser than water ... be that rock, weed, wood, etc. They also state that the air, in a fish's body, will interrupt the signal ... and even "warn" users to not place the transducer where "air bubbles" (caused by turbulence) will pass over it, as this will create false signals or signal clutter on your screen (and, essentially block out some of the return signals .... those of the "fish/cover/bottom").

The "ping" doesn't travel thru air very well ..... that's why you can't get a "bottom" reading, on your depth finder, when the boat is on land and the depth finder is still running. It's "sonar" ... not "radar". :D

To answer your question about why we can see "waterlogged structure" -
The "other objects" (brush, trash, thermocline, rocks, etc) are "denser" than the surrounding water ... and they reflect or absorb (change) some of the signal. That's why we get their "picture" imprinted on the screen ... the differences between the strengths of the returning signals are processed by the unit, into the pixel picture which you see on the unit.

............. cp

I would think that the body parts of a fish like muscle, bone or even scales would return a signal better than air in an air bladder. If air "interrupts" the signal rather than reflecting or returning it to the transducer then it wouldn't be possible for the transducer to "see" it.

The transducer sends out a sound wave and has no way of knowing what part of that signal has been interrupted. All it can process is the small fraction of that signal that is reflected or bounced back from an object. I've dumped remains from filleted fish that no longer have any air in the air bladders over the side of the boat and they show up on the graph very much like live fish do.

When I have time to do some experimenting, I'll tie a filleted fish carcass on a line without a hook or jig and suspend it under my graph to see what it looks like.

If a graph can "see" a 1/16-ounce jig I have no doubt it will pick up bones in a fish's body.

It doesn't really matter though what part of a fish returns the signal to the transducer. What is important is having the graph adjusted so it will display fish on the screen and for the operator to understand what they are seeing on the screen.

Originally Posted by Jerry Blake

I would think that the body parts of a fish like muscle, bone or even scales would return a signal better than air in an air bladder. If air "interrupts" the signal rather than reflecting or returning it to the transducer then it wouldn't be possible for the transducer to "see" it.

It doesn't really matter though what part of a fish returns the signal to the transducer. What is important is having the graph adjusted so it will display fish on the screen and for the operator to understand what they are seeing on the screen.

But, the word "interrupts" was MY interpretation ...... I assume that the air "reflects" a signal back. I was equating both words, to mean the same effect.
And, yes, you are quite correct ...... it doesn't really matter what reflects the signal, or why, in the greater scheme of things. I was just challenging the idea that a catfish won't show a double echo, just because it has skin (and not scales). IF the return signal is, in fact, being reflected by the air in the air bladder (as stated by most sonar mfg's) ... then why would it not show up on fish with skin, in the secondary reading, since those fish do have air bladders ?? I wasn't disputing anyones "beliefs" ... I was seriously wanting to be shown some evidence. .... cp