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Number at the bottom of Garmin A-scope?

Old 02-22-2015, 06:16 AM
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Default Number at the bottom of Garmin A-scope?

Hey everyone,

Can anyone tell me what the number at the bottom of my A-Scope represents? I have the Gamin 840xs w/GCV 10. My first thought was that it may the be diameter of the cone at the bottom, But it doesn't vary if I change the frequency on my b175H. If I have split frequency running 135khz and 200 khz the numbers at the bottom of both A-scopes are the same. Also, if I choose one frequency, 135 at a constant depth, and then change it to 200 at the same depth, the number doesn't not noticeably change. I had this one screenshot available and circled the number I'm asking about. It is in the bottom right corner.

Thanks for any input! I'll be on the boat later today so maybe I can get a few more pictures.

P.s. Yes I know there is a lot of clutter at the top in this picture. I was following a big boat back into the harbor.
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Old 02-22-2015, 09:23 AM
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That number is the diameter of the sonar cone at the present depth.
Old 02-22-2015, 10:30 AM
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Originally Posted by InternationalMarineMike View Post
That number is the diameter of the sonar cone at the present depth.
That is really useful information to have displayed. I wish other sounders provide the same and/or shall in the future.
Old 02-22-2015, 11:02 AM
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Most manufactures have had this A scope feature for about 10 years but they do not promote them properly to their end users.

The Kodden, Sitex, Lowrance , Furuno and many other units have it for many years.... but few ever learn to use it because the manufactures spend far too little time in the manuals explaining its use.

A Scope or Amplitude is very useful to see instant real time of what is in the cone of the ducer....and also to know the diameter of the cone at different levels.
It is also helpful in determining bottom density.




A Surface clutter, created by turbulence around transducer.
B Individual fish.
C School of fish hovering above the reef.
D Reef area, showing rocks above and harder bottom below.
E Digital readout of depth, water temp and boat speed.
F Speed of picture advance (also called "scrolling" rate).
G Sounder depth scale.
H A-Scope Picture, showing immediate, real-time returns of echoes including:
I School of fish below the boat, just above the bottom.
J Softer bottom below boat.















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Old 02-22-2015, 11:44 AM
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Cool, never knew that. Thanks.
Old 02-22-2015, 03:51 PM
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Originally Posted by greylion View Post
Cool, never knew that. Thanks.

You are very welcome.
Old 02-23-2015, 04:06 AM
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Thanks for the answers guys!
Old 02-23-2015, 04:54 AM
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Not to throw a wrench into things, but here is a wrench. The diameter you have listed is not even close to what the diameter of the B175H should be at that depth. I figured that Garmin used either the average cone angle 12 degrees or the maximum cone angle 15 degrees as a constant, instead of figuring out what the cone angle should be at frequency, to make life easier for their computer programmers. But the diameter you have listed corresponds to a 25.5 degree cone angle. The maximum cone angle of the B175H is 15 degrees, and that should have a diameter of 4.3' at 16.4' depth. That is more like the cone angle of the B175HW. Maybe a bug in Garmin software?

For reference: How to calculate transducer cone diameter at depth

Last edited by stiletto; 02-23-2015 at 05:05 AM.
Old 02-23-2015, 06:04 AM
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Even more strange is that the b175C-H has a cone angle of 6° at 210khz and 10° at 130 khz.

At 135 khz you can compute the cone diameter by using (Z)tan(X)=D. Where Z=current depth. X=cone angle. And D=diameter of the cone at the current depth.


In my current picture, (16.4ft)tan(10°)=2.89 ft diameter. Which as Stileto pointed out is way off.

I attached another picture that I have on my computer. The unit is in chirp mode from 160° to 200°? I think? Either way, the widest cone that this transducer can produce is 10° at 135khz so I'll use that since it is upper extreme for cone width. (44.9ft)tan(10°)=7.9 ft. This is WAYY off of what the unit is saying the diameter is. The unit thinks that the diameter is about 24° which would be more equivalent to the B175C-HW as Stiletto put.

This is all part of why I originally asked my question. Regardless of how I change the frequency the "Diameter of the cone value" doesn't change. This makes me think that maybe garmin is using a generic cone angle to calculate the cone diameter.

I'll look for some other shots that I have in deeper water and see if I could come up with some examples.

Gil's only gamin example he put up would need a cone angle of ~6.9°. Does this match any of the Large Low CHIRP transducers that would be used with the GSD 26?


Edit, I just saw Stiletto's link to the airmar beam width calculation. They use 2(Z)tan(X/2)=D This ends up being the same as my formula for what we are using it for.
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