Thread: Question about Structure Scan

I was always told that a sonar unit can't read past a solid object, so it shows it on the screen as such (as in the case of the limbs/branches). But, a fish is mostly water ... and their air bladder is the only thing that the signal doesn't penetrate, so it's what the unit shows.

It might also have something to do with distance ... as in how far away the fish is, in relation to the transducer. Remember, you're looking at a fairly large section of the underwater world, and it's being displayed on a tiny screen (in comparison). That little "speck" has to be compared with the size of something that it's right next to, in order to really know its relative size. And since ALL the units are simply measuring distance (signal time to/from the reflecting object), in ALL the confines of whatever shape/size of their cone angle, and compressing that info into a "picture" on your screen ... it's going to be pretty hard to determine exactly whether or not that fish is "right next to" or "just in the general area of" something that you can compare it to, in order to determine "size".

... cp

So how big a picture are you expecting on the screen? Say the fish is in 25 fow, and the fish is 4 inches tall. That would be 4/(25x12) would be the fish height to water depth ratio, about 1.92%. It all depends on the depth range on the screen.

I've seen some snapshots with amazing detail of structure. I understand that smaller fish will show up as specs on the screen. Maybe I didn't ask my question correctly. Does a 80lb spoonbill or a 40lb flathead show up as a little larger spec or does the shape of the larger fish show up?

I had a picture of what I think was a big drum on mine, but couldn't remember how to take a screenshot fast enough. Several around that were blobs, but one you could see the shape, including the tail. Remember, fish aren't as uniform inside as branches. They have air bladders and open areas that are full of water.

Originally Posted by sah

I've seen some snapshots with amazing detail of structure. I understand that smaller fish will show up as specs on the screen. Maybe I didn't ask my question correctly. Does a 80lb spoonbill or a 40lb flathead show up as a little larger spec or does the shape of the larger fish show up?

heres some fish. i dont really no what size they are but they showed up fairly well

heres a spoon bill.