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Woodlander
Senior Member
Username: Woodlander
Post Number: 2159
Registered: 02-2003
| | Posted on Sunday, August 02, 2009 - 06:04 pm: | 
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Part III
How objects are displayed on the screen
A word of encouragement. I balked several times years ago when trying to learn to read a depth finder. It seemed like at times it was just not worth the effort and I’d go back to fishing the way I always did. I basically lacked confidence in myself to learn. This changed my mind:
I saw a guy in an old aluminum boat way off the bank. He was down close to the dam where the main river channel takes a big bend below Hunter Creek. He must have been in water 80 feet deep. I thought he must be broken down at first but he was jigging his rod up and down. I thought “good luck to him” because he wasn’t likely to catch anything fish there. Well, as luck would have it, he was pulling out at the same time I did. As it always is during the week in the summer, there were few people at the ramp so I decide to walk over to him and asked how he did. He opening up his cooler and it was full of fish - crappie and bass! You could have knocked me over with a feather. He was extremely nice old crusty fisherman - like me now! lol Anyway, he showed me everything he was doing. He was using a ½ oz jig head with a small fluke type bait and jigging the tops of the standing timber. He had a paper graph. For those not familiar with a paper graph it was the next generation depth finder after the flashers. It was like an EKG machine in that it recorded sonar signals on real paper. He said he could see his jig in the water and could see the tree tops and fish. He was just lifting his jig up and down in the fish’s face. Super nice guy but I got the impression he was no more a rocket scientist than I was. I left the ramp that day knowing if he could catch fish that way so could I!
A lot of this is not going to make good sense the first time or two through because it a different concept than the way we are use to interpreting a picture. I’m going to try to explain it the best I can but there might be a better way to word it so don’t hesitate to ask questions. Your questions might help me to reword something to make it make more sense. If you have a question it probably means other people have the same question but might be too shy to ask it.
I’m going to limit this article to the conventional vertical depth finders. I’m going to use my Lowrance 520c to provide most of the sonar pictures. Both Lowrance and Hummingbird make excellent depth finders and I’ll later go into what makes a good depth finder. Black and white depth finders are great too. In fact some people think they do a better job than color. I have a bunch of black and white sonar pictures but since I had to take they with my digital camera and then put them in the computer and crop them, they don’t do real life picture justice and might mislead someone on the quality of black and white units.
There are two important concepts to grasp in learning to interpret a depth finder. The first is that a depth finder measures the distance between an object and the transducer. When an object appears on the screen at 20 feet is doesn’t necessarily mean the object is in 20 feet of water but that it’s 20 feet away from the transducer.
The second is that the sound waves are coming from a three dimensional environment but the screen is only two dimensional. All the information get flattened to get it all on the screen. You can’t see depth. The new side imaging and side scan technologies are changing that but they have their limitation too, but that's another story.
Let me explain first how the distance between the transducer and an object it has picked up is displayed on the screen.
When the transducer picks up an object in the water the information is sent to the unit and a recording of the object is placed on the screen on the far right side. The sound waves come out and back so fast that as long as the object is in the range it will continue to be recorded on the screen.
Think in terms of a jet way up in the sky leaving a contrail. The lower arrow is pointing to the jet and the upper arrow to it’s contrail. The jet has left a recording of where it was but it’s gone from the main picture. I’ll explain screen shots in much detail later.
Let’s say you were in 30 feet of water and you had a 1/4 oz sinker tied on your line. You lower the sinker to 20 feet and hold it there and turned on your depth finder. You would begin to get a continuous line running across your screen at 20 feet. As long as the sinker is within range it will continue to be recorded. Similar to what’s happening here. I pulled the sinker up and out of range so it disappears from the screen.
If you raise your rod you can see the sinker rise up in the water because it’s getting closer to the transducer. If you drop it you’ll see it sink on the screen because it’s getting farther away.
It’s the same reason why you get that saw toothed pattern on the bottom when a boat runs by near you. The waves lift your boat and transducer up and then sets it back down multiple times.
All that makes perfectly good sense.
Now let’s say you suspend that same 1/4oz sinker on a buoy so it floats in 20 feet of water over a 30 foot bottom. If you can maintain a distance of say 4 feet away from the buoy you’ll again get a line on your screen that will run across your screen at a depth mark of a little more than 20 feet. If you start to troll towards the buoy, the line will start to rise on the screen until you run right over the buoy and it pecks out on the screen at 20 feet. As you start moving away from the buoy the line will start dropping and you’ll maintain contact with it on the screen as long as it’s being picked up. You’ve made an arch that everybody talks about!
Now lets see what happens if you actually did that. I did this experiment a couple of years ago before the snap shot feature was introduces so the pictures aren’t great. I took three balloon - I tried one but they look so much like fish and attracted fish so that I couldn’t find it for sure and had to use three! I tied the buoy cord to an anchor and doubled the line in the buoy cord and tied an overhand knots at 5, 10, and 15 feet from the anchor leaving loops in the line of about an inch. I blew up the balloons, pushed the open ends into the loops and tied them to hold the balloons in place. I dropped everything in 30 feet of water. Of course the buoy cord unraveled and the buoy float to tell me where they were.
I idled and trolled over the buoy to see what I could see. This is a picture where I trolled of the balloons at a moderate pace.
Here I went over them slower.
And here I went over them pretty fast.
Here’s one were I move fairly rapidly towards the balloon - on the far left - and the turned and trolled parallel to them for a while before turning back and trolling right over them. The blue is the return from the buoy cord as I banged into the buoy.
Now let’s look at the second important concept. The one about making a three dimensional environment fit on a two dimensional screen.
Take a look at those two fish on the screen the arrow is pointing to. Our mind tells us they are right under the boat, that one fish is bigger than the other, and the bigger fish is in 20 feet of water and the smaller one is right above him in 19 feet. Actually none of this is probably true. The “bigger” fish would be in water shallower than 20 feet while the “smaller” fish could actually be bigger and possibly on the opposite side of the boat.
Let’s assume the white arrows in the next picture are fish that are all in 20 feet of water and they are all 8 feet away from the boat.
If you were stationary and the fish were too, they would all appear on the screen at the exact same spot in - at about 21.5 feet.
Fortunately for us the companies that make the depth finders have added clues to help us make good judgments on the size of objects and where they’re located in relation to us.
I’ll go back to an earlier picture.
I painted the middle of the pictures yellow to help explain what the depth finder companies call the “cone”. Most depth finders for fresh water use a transducer with what they call a 20 degree cone. What they are telling you is that your view is limited to 20 degrees - or the yellow area - and you might not see anything else.
Actually what they are doing is filtering the sound waves to eliminate everything but the 20 degrees straight down. They feel like this will eliminate a lot of unwanted or useless clutter that would otherwise be on the screen and interfering with what you see.
You can see how this works with a flashlight. Cut a hole about the size of a nickel in a piece of cardboard. In a dark room shine the flashlight against a wall. Now hold the cardboard out in front of the flashlight and see how the hole confines the beam to a much smaller area on the wall - much like a strong spot light.
Some companies use stronger filters on their beams than others. By that I mean you can see objects that are located outside the 20 degree cone in some depth finders better than others.
I turned up my sensitivity to make this even clearer. See that big black arch in this picture starting at 18 feet and going up to 4 feet and back down to 16 feet.
That is this bucket that is floating about 3 inches in the water.
The depth finder was actually able to pick up that bucket from 18 feet away on the surface of the water and following it as I got closer and then move away. (That blank area and the corresponding line down below at 20 feet is a little hiccup I get sometimes with my unit when the sensitivity and the surface clarity filter are turned up high.)
So now you can see that my Lowrance unit does not completely filter out all the signals that are outside the 20 degree cone. What it actually does is paint the signals that come from in the 20 degree cone yellow and the ones from outside the cone blue. Humminbird uses different colors - in fact three different ones - with their color units. A black and white unit will use shades of gray.
I know those are confusing concepts but you don’t have to keep them constantly in your mind while fishing. I don’t really think about them consciously anymore. They just tell you why you’re seeing what you are. When you know the “why‘s” you’ll be able to use this information to find fish.
I’ll post enough pictures later that it will become automatic for you. The thing to remember is there’s really not that much in the water to confuse you. You’er basically, with few a other possibilities, looking at either fish, cover, bubbles or some other kinds of clutter. Screen are really very easy to read.
Let me throw in a couple of pictures for the fun of it to show how much you‘ve learned.
I think you can all now see that that’s a single fish close to the bottom that’s moving along with me. The bottom is changing depth so you know we’re not standing still. He’s obviously curious about my spoon because he’s come up a couple of times to check it.
You can count at least 6 fish here as they swim along with me. You can tell they’re moving because fish don’t just bob up and down. They’re just out cruising and certainly not interested in what I have. It’s hard to get them to bite this time of the year unless they’re relating to bait fish. If I had to fish for bass now I think a drop shot might be better than a spoon. That’ll change a lot in a month or so!
Thanks for all the great responses.
The next part will be on "how to adjust a depth finder". It's going to take me a little while because I don't have the pictures I need. I'm going to show the different menu screens and also have pictures of what effect the adjustments have on the picture you see. |
Slabchaser
Intermediate Member
Username: Slabchaser
Post Number: 39
Registered: 08-2008
| | Posted on Monday, August 03, 2009 - 11:04 pm: |
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I"m learning , great post |
Olegreymule1
Intermediate Member
Username: Olegreymule1
Post Number: 35
Registered: 04-2007
| | Posted on Tuesday, August 04, 2009 - 07:37 am: |
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great help. thanks. keep-um comming |
Catch1
Senior Member
Username: Catch1
Post Number: 1100
Registered: 10-2003
| | Posted on Tuesday, August 04, 2009 - 09:37 am: |
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Great work Woodlander. Keep it coming. I am learning more and more..... |
Rwill99
Senior Member
Username: Rwill99
Post Number: 200
Registered: 02-2007
| | Posted on Tuesday, August 04, 2009 - 10:18 am: |
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So with the bucket thing. It would be safe to say that when you see an arch to top of the arch is the depth that the fish is at, more or less? |
Woodlander
Senior Member
Username: Woodlander
Post Number: 2163
Registered: 02-2003
| | Posted on Tuesday, August 04, 2009 - 04:08 pm: |
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Thanks guys,
Rwill99,
The top of the arch is actually the closest you got to the fish - instead of it's actual depth in the water. It could be it's depth in the water if you ran right over the top of him.
Good question. Later in the part on reading a depth finder I'll show you how the cone helps you tell. |
Catch1
Senior Member
Username: Catch1
Post Number: 1115
Registered: 10-2003
| | Posted on Friday, August 07, 2009 - 09:38 pm: |
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where's the next installment?.....you done got something started........I would like to see some things on different bottom structures such as rock, sand, mud, etc..... if possible....from Wedowee....Thanks..... |
Ironbishop
Advanced Member
Username: Ironbishop
Post Number: 49
Registered: 04-2009
| | Posted on Saturday, August 08, 2009 - 01:24 am: |
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Great work!!! |
Yank
Senior Member
Username: Yank
Post Number: 148
Registered: 04-2005
| | Posted on Thursday, August 13, 2009 - 10:32 pm: |
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Great work Woodlander. There is no doubt that you know what you are talking about. Once these guys start learning how to use their units and start catching a bunch of fish, you'll probably start getting a bunch of Xmas presents every year. lol The fun of sonar fishing is the thrill of finding fish and being able to consistantly catch them. |
Woodlander
Senior Member
Username: Woodlander
Post Number: 2192
Registered: 02-2003
| | Posted on Thursday, August 27, 2009 - 01:40 pm: |
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A much better term would be “Distance Finders”. I got this interesting email yesterday:
___________________________________________
It tells me that the term “Depth Finder” is just as misleading as the term “Fish Finder”. The units are more than a fish or a depth finder. I appreciate his email and I thought it was interesting enough to a response with some pictures. This seemed like the most relevant Part to place it in.
I hunted up some pictures from January of 2008 where I had done some more tests with the bucket. This was from the first day and I tried to show the screen and bucket at the same time. You can tell the bucket is barely floating one inch down in the water. I tied a buoy on thinking the bucket might somehow sink even with a lid on - it‘s an O/C disorder! lol I also wanted to show the bucket’s handle.
I went out another day with three buckets and got these pictures. There was some wind that day and I had more trouble keeping those bucket hemmed up than I would have a bunch of 5 year old! lol They kept blowing over and away from each other every time I’d troll back around to get a good sonar shot.
This is my “depth finder” picture that was obviously not taken at the exact time I took the bucket picture. You can not only see the buckets but the handles too. I put an arrow on the picture above to show about where the last bucket would have been to get this sonar shot.
There’s actually no meaningful “depth” reading on the above picture. The buckets obviously aren’t in 5 to 10 feet of water. Even the read out of the depth of the water would be off by a good bit considering I was over a steep drop - I’ll get to that in Part VII.
Everything has to do with the distance from the transducer to the object and very little about to do about depth.
You don’t have to float a bucket to try that experiment. If you have a Lowrance, next time out on the water, troll up to one of the buoys that the marine patrol puts out. You’ll pick up the buoy and the buoy cable on your screen. I had some pictures doing this but I can’t find them. I’ll get some more later to post here.
In that Bazz Clazz video on depth finder done with a pro and a Lowrance representative they show a dog swimming up to the boat and then away. It looks like the dog is swimming up from 10 feet below the surface and then back down.
Now to answer his transducer questions. I’m using the basic puck transducer that they send you. I can’t remember the number but it’s the same one that I used with my black and white Lowrance X125. It’s mounted on my trolling motor and point as straight down as I can get it - so perpendicular and about 2 to 3 feet deep.
To answer the other commit, the only way you can get a straight line on your screen is to stay the exact same distance away from an object for a period of time - either by staying still over an object that’s not moving or moving at the same speed and direction as the object.
Here I’m stationary over the balloons while lowering my sensitivity to see the effects.
Any movement by me away from the balloons would start the formation of a arch on the screen like this one when that fish came over and I dropped my drop shot and started drifting!
I appreciated the questions. It makes me have to think and explain things better.
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Catch1
Senior Member
Username: Catch1
Post Number: 1138
Registered: 10-2003
| | Posted on Thursday, August 27, 2009 - 03:33 pm: |
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Woodlander, I am going to try the bucket test with my Humminbird and I will post the results. I have looked at lake bouys but they start at an angle and then come up even though I know the line goes straight down. |
