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| Read a post on another site that seems off base, how does a LCD sonar work with the depth of objects displayed on the screen? |
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Posts: 172
| 'tis true. PM me.
Edited by MNmatt 1/17/2007 8:23 PM
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| I can't, not registered here or there.  |
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Posts: 172
| it takes 10 seconds to register... |
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Posts: 32884
Location: Rhinelander, Wisconsin | I'll take a shot at this, I think I get what MN is shooting at.
Most muskie anglers use a fairly high khz rated transducer. Many Lowrance units come with a 192, which covers a 'cone' of 1/3 the total depth at the bottom. Higher KHZ are more 'focused', if you will. Lowrance also has units designed for bluewater at a lower KHZ rating that have a much wider cone and cover a larger area.
The 'error' he's talking about is the result of the transducer cone area. An object in the cone at the outside of the target area is, by virtue of the shape of the target area, further away from the transducer than an object in the center of the cone. Think 3-D here though, that also is important.
This is what creates a classic 'hook' on the screen. As the boat moves over a fish, when it's on the outside of the cone the target is 'smaller' and a bit further away, so the 'tail' of the hook is drawn thiner and 'deeper' on the screen. As the target reaches the center of the cone, the hook deepens and is thicker, appears shallower on the screen and represents better the actual depth of the target. As the boat moves away from the target, the other 'tail' is drawn. Classic 'hook'. Doesn't happen allot for those of us fishing 20' or less most of the time because the cone is only a 3.3' circle in 10' of water and about 7' in 20' of water, not allot of bottom covered.
If the target is on the outside of the cone as you pass by, and is barely 'nicked' by the signal, it will not show either the exact depth or the exact size of the fish, obviously. Is that a huge issue for us as Muskie anglers? Not in most cases, IMHO worrying about that is way over thinking the unit's capacity for our general applications.
Most important thing to remember is the sonar 'draws' only what the transducer is receiving at the instant the processor tells it to, and EVERYTHING after that is history, so what you see in the first 1/8" is the most important. With a flasher, the signal strength will give you hints, but depth of objects on the edge of the cone, especially on a low KHZ unit can be somewhat misleading when in 50' of water or more.
The sonar display is nothing but a graphic representation of the transmit signal bouncing off objects and returning to the transducer with the elapsed time measured to determine depth. 'Ghost' echoes outside the cone intended by the sonar manufacturer to be on the display can exist from repetition of the signal off the water surface and/or other objects and other factors, but most good units filter out unwanted noise.
Bottom line is don't worry about the difference in depth from an object center cone and an object on the outside edge, because it's easy to determine by the first 'tail' echo drawing on the screen if the target is on the outside of the cone, and the difference in the shallow water column most muskie anglers are fishing (under 40') is negligible, anyway. Interesting, and something to consider, but negligible, IMHO. |
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Posts: 172
| Steve somewhat touched the bases on this subject. Not sure what the confusion was about from the reader...What is important to understand about how sonar works is that the cone angle is only a designation of the half-power-point. This is how they classify transducers. A transducer will pick up a much greater area than the cone rating for objects suspended in the water column. The bottom reading you see on the sonar is ALWAYS the true bottom because bottom is ALWAYS under the transducer. When an object is perfectly under the transducer it will be read at true depth. Objects that do not pass perfectly under the transducer will ALWAY have a degree of error. The further out you get the bigger the error. How many objects do you think pass perfectly underneath the transducer? Steve's example of 1/3 the depth is equal to the coverage error is sloppy math. That is only the half-power coverage area and any transducer will read all the singals in its range. There is no perfect cut-off cone and the bottom readings can be changing as well. So if a 20-degree transducer can read a range of echo's out to an approximate 60-degree cone, the difference in depth is no longer a trival matter if you are trying to hit a certain depth, such as when trolling or trying to determine object depths.
If you want to ignore it that's fine, but always remember that you're not understanding the true picture then and not using your the tools you already have to their full potential. If you want to use your understanding of this error on the water so be it. Always nice to understand the facts regardless of if you use them or not.
-M |
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Posts: 32884
Location: Rhinelander, Wisconsin | Some builders DON'T classify the transducer as Matt says, I have a unit on one boat tha uses the 1/10 power rating. It's a color saltwater unit, an expensive one. The 'fuzzy math' Matt mentions is that used by the manufacturer in rating the transducer, not mine.
Here's what Matt is talking about, and some information from other builders. I didn't say this was not the case, I said, and continue to say, if you know by looking at the screen what signals are on the outside of the cone ( and I do) and which are close to center ( and I do) the issue isn't as dramatic as one might think. Also, as I previously said, the depths at which we fish as Muskie anglers are usually shallow enough that the effect IS minimal when placing a trolling depth run. Another point; if I am seeing baitfish schools that are definitely centered in the cone, even drawing small hooks for the ciscoes, and others that are not, basically dots and slashes, and the depth is different between the two, I am aware that the outside of the cone measures a longer distance. Let's see the average fellow apply the difference in a usable formula, read the signal and do the math on the spot to adjust depth. If baitfish in the BASIC center of the cone are 14 to 17', hooks 20', my bait's target depth is at 13 to 15 allowing for the fact the fish will move 5' with no worries, and that fish are NOT likely to be at ANY one exact depth, anyway. It's a target zone where the fish MIGHT be by what I am seeing on the sonar screen, not a bulls eye I have to hit. Put yours where you like, it's personal preference.
Lowrance:http://www.lowrance.com/Tutorials/Sonar/sonar_tutorial_06.asp
'In order to measure the transducer's cone angle, the power is first measured at the center or axis of the cone and then compared to the power as you move away from the center. When the power drops to half (or -3db[decibels] in electronic terms), the angle from that center axis is measured. The total angle from the -3db point on one side of the axis to the -3db point on the other side of the axis is called the cone angle.
This half power point (-3db) is a standard for the electronics industry and most manufacturers measure cone angle in this way, but a few use the -10db point where the power is 1/10 of the center axis power. This gives a greater angle, as you are measuring a point further away from the center axis. Nothing is different in transducer performance; only the system of measurement has changed. For example, a transducer that has an 8 degree cone angle at -3db would have a 16 degree cone angle at -10db.'
Humminbird:
'A fish arch forms as the depth sounder’s transducer moves over the fish. Due to the transducer beam angle the distance to the fish decreases as it moves into the beam, and then increases as it moves out. When the depth sounder graphs this distance change, an arch appears. The true depth of the fish is the top of the arch when the transducer is directly over the fish. Boat speed and movement greatly affect the shape and formation of an arch. When moving slowly or with a high rate of display update speed, a fish will create an elongated arch. With the boat moving at a fast speed or with a slow display update speed, the same fish will produce a much shorter arch. If the fish does not pass through the entire transducer cone angle, it will be shown as a partial arch. In any of these instances, the user may not recognize that the fish was shown as an arch.'
Want a headache if trying to apply what we are discussing? Look the the transducer possibilities here: http://www.humminbird.com/productLines_wide.asp?id=788
Another description:
http://www.vexilar.com/help/tips/tip011.html
And Matt, I didn't say I was 'ignoring' what you are pointing out, I said it generally, in my case, where and how I fish ( similarly to many muskie anglers), has minimal impact.
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| Thanks, guys, that's what I thought. |
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Posts: 172
| Steve,
How nice that you used some of the same material I previously mentioned on the other site and edited my post here. I never said it was the be-all-end-all and everyone must pay attention to it. We agree there is an error but disagree on the level of importance, which is fine with me. The question was asked and I put the facts out there, whether or not it's usefull to everyone is pointless to argue about. I appreciate your input on this though.
-M |
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Posts: 32884
Location: Rhinelander, Wisconsin | I didn't edit anything. Not sure what I didn't get to see, too bad I missed it!
I used material I got from a Google search other than the Lowrance link, that one I had bookmarked for a long time. Didn't pay much attention to whatever is bothering you about your posts elsewhere, I simply looked for resource material on the web and was not aware that the information I posted was proprietary. I could use the material Doc sends us or call him up, but thought I'd use the manufacturer's source info because I can't get into his website for some reason and it was midnight here.
As far as I knew, we were not 'arguing' we were discussing the impact to the average muskie angler of the transducer cone angle and reading the depth of objects on the edge of same, and how that might apply to everyday use for most of us.
My interest in this is no different than yours, I want folks to be able to get maximum benefit from their sonar unit.
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Posts: 172
| No prob Steve. Some of Bruce's stuff is already posted "there" as well for reference.
Edited by MNmatt 1/19/2007 9:14 AM
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Posts: 1451
Location: Kronenwetter, WI | Great responses...thanks...
Hey---For YEARS I have been thinking that you guys that do seminars would PACK THE HOUSE if you did one on using and interpreting sonar.
I think the two areas most experiences musky hunters can grow are:
1. use of electronics
2. boat control
Both would make great seminar topics.
Just a thought. |
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Posts: 5874
| Steve judt did a seminar at the Titletown Chapter in December. He covers boat control pretty thoroughly. I've seen his seminars many times, and I still come away with stuff I didn't grasp previously.
And Doc Sampson does a great job in his class on sonars. He had one in APpleton last year. Check out his website, and see where he is this year.
Edited by Shep 1/19/2007 5:43 PM
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