Number of pixels dedicated to target

[Anonymous]

I have a 480 vertical pixel 1834c/NT, with a DFF1 and TM260. If I am fishing in 450 feet of water and mark a fish with a 1 foot girth, how many pixels of the screen will be dedicated to that fish. If the fish is a whale with a ten foot girth, how many pixels will be dedicated to that return. If I mark a bait ball that is 20 feet in diameter at 200', how many pixels will be dedicated to that bait ball. Thanks for your response.
Forrest

 

[Snips]

Forrest,
Interesting question. To be honest, there are too many variables to factor in i.e. vessel speed, screen advance, gain, transducers beam width etc. Even with all that data I would still be guessing at a pixel count for you. Sorry.

Snips

 

[Anonymous]

Well let's set some parameters. TM 260- 19 degree cone at 50 hertz and 6 degrees at 200, speed- 3 knots, screen advance 1/1, gain on automatic since ocean conditions can vary tremendously and depends on marine organisms. Choose a gain if you wish. What I'm after is the number of pixels that Furuno programs into there units for a good target. Since we can see the top and bottom of a bait school, it implies that there is a way to see the lower limits of an object. If that is the case it would seem to me that any fish that is less than one foot thick will be represented by a single dot(pixel) while the larger target (whale) will be represented more like a fish arch in shallower conditions when you have more pixels per foot of water. Example: In 48 feet of water you will have 10 pixels per foot of water, more since you include the bottom,which give you much more detail and a beautiful arch that you wouldn't see if the same fish was in 480 feet of water. I'm trying to determine scale of the picture related to depth and whether or not Furuno programs extra pixels into the picture to see targets better. Sort of like when some manufacturers program fish pictures in.

 

[Snips]

Forrest,
If it were only that easy. Beam widths of transducers are calculated in a very controlled environment. Below are a couple of beam width plots with db reference, what is missing however is at what depths the 3, 6 or 10db points are measured at. For years I have been searching for frequency vs depth vs db information. If you question is centered around do we add or subtract pixels to make the targets look better, the answer would be yes but most sounders do. It comes in the way of manually deleting colors, adjusting clutter and TVG curves to better define targets. I wish I could give you a more black or white answer like a one pound fish at 100ft would equal ten pixels and a two pounder at the same depth would equal twenty, but I can't.

Snips

 

[Anonymous]

I know what each looks like on my equipment. I'm just looking for other opinions. The fish I deal with primarily in those conditions are salmon at 2-3 hundred feet that are always moving (migrating and foraging) and most people would agree that salmon are tough to see in 100 feet of ocean water. They don't inflate their bladders in cold ocean water and are always on the move. I wrote an article for our newsletter a couple of years ago. I'll send it to you if you're interested.
PS: Salmon show up really well in estuaries and fresh water when they inflate their bladders to maintain neutral buoyancy. They also show up well when the ocean warms up. Again they have to inflate their bladders in warmer water as they are the same density as cold ocean water.