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Fishing Charts depth Vs Sonar

Old 03-01-2013, 11:53 AM
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Default Fishing Charts depth Vs Sonar

Hello I am new to using a sonar/fishing charts and I noticed too big difference in the numbers of depth (of the sonar and the charts). For example I may be somewhere that shows 330m on the charts and the sonar displays 520m!!!

What is wrong? Which is the correct?

01MAR13_1348_00.bmp I tried to pass several times over this reef but in none case I found it. Could the map be wrong? I never found so big decreases in the depth or anything close to 160m.

01MAR13_1522_00.bmp What is the "M" symbol in the chart?

01MAR13_1551_00.bmp What are the weird symbols that look like a tree in the chart a bit more up from the the center of the screen?

01MAR13_1457_00.bmp My transducer is the 8pin P66 (without any 6 pin to 8 pin adapter) transducer connected to Garmin GSD 24. Its 600Watt. Why the diagnostics say 500Watt?

Please note that in all examples I was in 50KHz
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Last edited by koutroul; 03-01-2013 at 12:11 PM.
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Old 03-01-2013, 01:10 PM
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Keep in mind that a P66 at 1000' depths has a cone of about 800' across at 50 kHz.
That is a bit too wide to get accurate depth under the keel at those depths.
You need to use a ducer with a narrower cone .

Last edited by semperfifishing; 03-02-2013 at 11:55 PM.
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Old 03-01-2013, 01:44 PM
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Thanks once again Semper!

So in the case that in this "circle area" it covers on the bottom it will find depths from 100-1000 meters it will display 100? 1000? or something randon between?

So the depth in meters it displays are irrelevant numbers to what depth is exactly below and the exact depth can be found only from the fishing charts and I must believe in the fishing charts? Also in the example you can see 276m in the chart on the right I can not see anything close to 200m so where does it see the 276m?

What about the rest of the questions. Can you guess for example why in the diagnostics it says 500W instead of 600W. I have the latest software installed on the GPSMAP 5212/GSD24 and the P66 is the latest 8 pin version of the sensor.

Originally Posted by semperfifishing View Post
Keep in mind that a P66 at 1000' depths has a cone of about 800' across.

That is a bit wide to get accurate depth under the keel at those depths...especially if the bottom contour itself varies greatly.
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Old 03-01-2013, 02:17 PM
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On the chart on the left it shows that the boat is at 180m. The degrees at the 50Khz for the P66 is 45. Using the calculator I found that the diameter should be 45 meters so 22.5 meters around the boat.

In the chart around the boat there is nowhere (at a radius of 22.5m) 276m...

So what is wrong?

Here is the link of the calculator:
http://www.furunousa.com/LearningCen...alculator.aspx

Also I can't understand why they say use 50KHz for more depth and they have bigger cones in degrees for 50KHz than 200KHz. It does not make sense to have more degrees for something that will be used for deeper.... Why should I care if there is a fish on the bottom and the fish may be 500 meters from my boat?
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Old 03-01-2013, 04:54 PM
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Originally Posted by koutroul View Post
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Also I can't understand why they say use 50KHz for more depth and they have bigger cones in degrees for 50KHz than 200KHz. It does not make sense to have more degrees for something that will be used for deeper.... Why should I care if there is a fish on the bottom and the fish may be 500 meters from my boat?
My guess....the larger wavelength penetrates farther...just like sound in air!

You would think that the cone angle for 50 should be less then that of 200. That's the question.
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Old 03-01-2013, 04:55 PM
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You are talking about a transducer that s meant to be used in shallow water...thus the 45 degrees at 50 kHZ.

For deeper water use such as aT M260 then you have a 19 degree cone....a bit more accurate for reading depth.
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Old 03-02-2013, 01:00 AM
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Originally Posted by semperfifishing View Post
You are talking about a transducer that s meant to be used in shallow water...thus the 45 degrees at 50 kHZ.

For deeper water use such as aT M260 then you have a 19 degree cone....a bit more accurate for reading depth.
So the P66 could be more capable in deep water if its 50KHz used 19 degree cone. So in this case just because of this I must install the TM260 with 19 degree at 50Khz for depths over 100m.

But still I cant understand why to make the P66 shout at 50KHz at a wider angle than the 200KHz... In the deeper case (50Khz why didnt they make 10 degree to be more representative)...

Also if it heats depths in the range 160-200m within its coverage are how can it display 276m?
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Old 03-02-2013, 06:55 AM
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This has to do with physics. Think sound and speakers. High frequency sound is created by a small transducer, a tweeter. It's sound is very directional and it fades very quickly when you don't have the tweeter facing you. Low frequency sound is created by a large transducer, a subwoofer. It propagates farther and radiates wider and your losses are little when it is not facing you.

The B60 has a single element trying to produce both frequencies. When you try to achieve so much with so little you have to make compromises. Once you step up to a multi element transducer that need for compromises goes away.
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Old 03-02-2013, 08:40 AM
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I'm a commercial fisherman so have spent thousands of hours staring at sounders. From your bottom mapping display I don't think the contour is all that steep to throw you that far off. Your bottom lock picture looks weird to me. Too fuzzy. I've seen times where the sounder gets confused resulting into it thinking a double echo is the bottom instead of the proper echo. Sound wave goes down, bounces off the bottom comes back up and then back down again...that sort of thing. Or a thermocline messes it up. When this happens you might try changing scales to get it out of the phase.
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Old 03-02-2013, 11:45 PM
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Originally Posted by koutroul View Post
Thanks once again Semper!

So in the case that in this "circle area" it covers on the bottom it will find depths from 100-1000 meters it will display 100? 1000? or something randon between?
There are too many variables involved to give a hard-and-fast answer; but most likely something roughly approximating "A" or "C".


So the depth in meters it displays are irrelevant numbers to what depth is exactly below and the exact depth can be found only from the fishing charts and I must believe in the fishing charts?
Try to not read too much into the answers you're getting -- or what your chartplotter/sounder is telling you, for that matter.

The fundamental problem here is that you are using a transducer which is far from ideal for the conditions you're operating it under. Notwithstanding the claimed "maximum depth" rating (which should always be taken with a grain of salt anyway), the Airmar P66 is designed primarily for waters less than 100 Meters deep; arguably, much less. It also has a rather high "Q", and will thus produce plenty of ringing, which will further confuse the sounder (possibly leading to "double-bounce" readings, among other things).

You're also driving it from a sounder capable of severely overloading it; and unless you have taken specific steps to avoid this, you may well be trying to cram up to 2kW though a transducer which is rated for 600-watts at most. If that's the case, all sorts of "odd" behavior (up to and including including destroying the transducer) is entirely possible.


What about the rest of the questions. Can you guess for example why in the diagnostics it says 500W instead of 600W.
Given that, you apparently got lucky and the sounder DID somehow recognize the P66, and therefore automatically scaled back its output to protect it. The selection of 500W is/was probably the only appropriate choice, out of a small handful of fixed reduced-power settings the sounder has to choose from. Besides, the difference between 500W and 600W is too trivial to worry about. Just be glad it DIDN'T try to feed 2,000 watts into that P66!


Originally Posted by koutroul View Post
On the chart on the left it shows that the boat is at 180m. The degrees at the 50Khz for the P66 is 45. Using the calculator I found that the diameter should be 45 meters so 22.5 meters around the boat.

In the chart around the boat there is nowhere (at a radius of 22.5m) 276m...

So what is wrong?
Again, you're trying to read too much into too few isolated data points.

Bear in mind that all of these quoted "cone angles" are nominal, and based on the -3dB points. It's not like the beam maintains exactly 100% strength within that cone, then instantly drops to 0% immediately outside of it. I suggest you download http://www.airmartechnology.com/uplo...s_pc_rA_lr.pdf, and refer to the graph at the bottom-left of Page 1. This shows (an idealized version of) the polar plot of a typical transducer's beam pattern -- and remember, it is an idealized plot; real life is a lot messier.


Also I can't understand why they say use 50KHz for more depth and they have bigger cones in degrees for 50KHz than 200KHz. It does not make sense to have more degrees for something that will be used for deeper.... Why should I care if there is a fish on the bottom and the fish may be 500 meters from my boat?
It is a function of the transducer's design, as influenced by the laws of physics. As stiletto pointed out, acoustical energy in water acts pretty much the same as acoustical energy in air, only slower and less efficiently. Lower frequencies are inherently less directional than higher frequencies. But they also penetrate the water better, which is why they are generally preferred for deep-water soundings.


Originally Posted by koutroul View Post
So the P66 could be more capable in deep water if its 50KHz used 19 degree cone.
If the P66 had a 19-degree cone angle, it wouldn't be a P66.


So in this case just because of this I must install the TM260 with 19 degree at 50Khz for depths over 100m.
Frankly, I'm not at all sure why you even installed the P66 in the first place, given your choice of sounders. It's really NOT a good match.

If you already have that TM260 you mention, it would be a FAR better choice for what you're doing. But if you have not as yet purchased it, you can probably do still better; for example: http://www.airmartechnology.com/2009....asp?prodid=56.



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Old 03-03-2013, 11:52 AM
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Thanks guys I got it now! P66 was created for shallow waters only (0-100m). So for 400-500m its better to go for the TM260.

I dont have the money now for 2KW (the price is 2 times the price of TM260) and even when I have it I may sell the GSD 24 and go for the GSD 26 so better get something Transom that can be removed easily and dont loose a lot of money during that sale.

I can proceed now with a TM or M 260 and in the future I can go CHIRP.

Originally Posted by Itteldoo View Post
There are too many variables involved to give a hard-and-fast answer; but most likely something roughly approximating "A" or "C".




Try to not read too much into the answers you're getting -- or what your chartplotter/sounder is telling you, for that matter.

The fundamental problem here is that you are using a transducer which is far from ideal for the conditions you're operating it under. Notwithstanding the claimed "maximum depth" rating (which should always be taken with a grain of salt anyway), the Airmar P66 is designed primarily for waters less than 100 Meters deep; arguably, much less. It also has a rather high "Q", and will thus produce plenty of ringing, which will further confuse the sounder (possibly leading to "double-bounce" readings, among other things).

You're also driving it from a sounder capable of severely overloading it; and unless you have taken specific steps to avoid this, you may well be trying to cram up to 2kW though a transducer which is rated for 600-watts at most. If that's the case, all sorts of "odd" behavior (up to and including including destroying the transducer) is entirely possible.




Given that, you apparently got lucky and the sounder DID somehow recognize the P66, and therefore automatically scaled back its output to protect it. The selection of 500W is/was probably the only appropriate choice, out of a small handful of fixed reduced-power settings the sounder has to choose from. Besides, the difference between 500W and 600W is too trivial to worry about. Just be glad it DIDN'T try to feed 2,000 watts into that P66!




Again, you're trying to read too much into too few isolated data points.

Bear in mind that all of these quoted "cone angles" are nominal, and based on the -3dB points. It's not like the beam maintains exactly 100% strength within that cone, then instantly drops to 0% immediately outside of it. I suggest you download http://www.airmartechnology.com/uplo...s_pc_rA_lr.pdf, and refer to the graph at the bottom-left of Page 1. This shows (an idealized version of) the polar plot of a typical transducer's beam pattern -- and remember, it is an idealized plot; real life is a lot messier.




It is a function of the transducer's design, as influenced by the laws of physics. As stiletto pointed out, acoustical energy in water acts pretty much the same as acoustical energy in air, only slower and less efficiently. Lower frequencies are inherently less directional than higher frequencies. But they also penetrate the water better, which is why they are generally preferred for deep-water soundings.




If the P66 had a 19-degree cone angle, it wouldn't be a P66.




Frankly, I'm not at all sure why you even installed the P66 in the first place, given your choice of sounders. It's really NOT a good match.

If you already have that TM260 you mention, it would be a FAR better choice for what you're doing. But if you have not as yet purchased it, you can probably do still better; for example: http://www.airmartechnology.com/2009....asp?prodid=56.


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Old 03-03-2013, 11:56 AM
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Your GSD24 and a 260 series ducer will more than be fine for your depths.
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Old 03-03-2013, 12:10 PM
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Originally Posted by semperfifishing View Post
Your GSD24 and a 260 series ducer will more than be fine for your depths.
Thanks once again Semper!
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