Thermocline????

[kosmo]

I was out playing on the 5th.took this shot ,that I think is interesting. There appears to be a thermocline showing on 2d,with some fish at and above the thermocline,but the si/di appear to show lots of fish below the thermocline,but this can't be,right

[vic n]

I don't know about water that deep, Kosmo. I'm pretty sure a lake can get multiple thermoclines, but water this deep would be out of my league. Could it be zooplankton?

[tcounty]

I was trying to figure out where the thermocline was this weekend. Never was completely satisfied that I had found it.

[BobB]

I have a couple of thoughts about this ...... 1st, fish could be below the thermocline depending on the oxygen content .....but I don't think that is the case here since all the fish show above the thermocline on 2D.

2D ..... in my opinion .... is a much better tool than either SI or DI for determining how deep the fish are. I think I recently ticked some folks off on another forum by pointing this out.
Due to the slant range affect on DI and SI .... and the wide beam angle .....They will not accurately show you how deep the fish are ..... They might be accurate at showing the location of the highest fish .... but not the deepest fish.

Just curious if you agree with this Kosmo?

[CrappiePappy]

I don't know about that, BobB .... ALL sonar pings are going to show you the distance between the transducer and the object reflecting the signal ... don't matter if it's 2d, DI, or SI, or 360 ... it's just the nature of the beast.

I don't think the cone shape of the 2d is any more accurate at showing the depth of a fish, than a DI fan shaped beam. The only "real" accurate measurement of depth is for those fish that are directly UNDER the center of the transducer ... since that measurement of distance is the least affected by the position of the fish within the confines of the beam.

... cp

[BobB]

With a 75 degree beam angle ..... Fish on the outside of the beam will be very far from the transducer and may be painted on the image as being at that depth....

In Kosmo's image .... if you assume the thermocline is at 20', the fish at the outside of the DI beam, at the level of the thermocline, could be 77' away from the transducer with the 455 75 degree beam angle .... DI may assume those fish are 77' deep.

Run some numbers thru this calculator and the results will surprise you... Right-Angled Triangle Calculator

[BobB]

Actually, now that I think about it .... I'm not sure if the DI is a TOTAL coverage of 75 degrees, or 75 degrees from vertical .... That will make a big difference in how far the fish are on the outside of the beam....

Maybe Rickie can clarify the coverage area of DI.....

[CrappiePappy]

With a 75 degree beam angle ..... Fish on the outside of the beam will be very far from the transducer and may be painted on the image as being at that depth....

In Kosmo's image .... if you assume the thermocline is at 20', the fish at the outside of the DI beam, at the level of the thermocline, could be 77' away from the transducer with the 455 75 degree beam angle .... DI may assume those fish are 77' deep.

Run some numbers thru this calculator and the results will surprise you... Right-Angled Triangle Calculator

And on a 60deg cone angle of a wide angle 2d ... those fish at the outer edge of the cone would be shown as being at the correct depth ??? I don't think so. ALL images in sonar are painted as how far from the transducer they are ... not how deep they really are.

I already use the right triangle calculation to figure how deep my jigs are running, by how much length of line I have out, as long as the angle of the line is approx. 45deg. I already know to multiply the length of line by 0.6 to determine the depth of the jig.

But, in regards to Kosmo's screen image ... I see what you're saying --- that the fish below the thermocline (in the screen image) may not "actually" be that deep, but in fact just "that far away" from the transducer. Point taken.

It would not, however, be enough for me to go back to 2d or even go to a unit that has 2d included with DI. I'm perfectly satisfied with the picture quality images that my DI unit provides, as opposed to the narrow cone angle and finger painting images that my previous 2d 20deg cone angle units used to provide.

... cp

[genec]

NOT A TRERMONCLINE,NOTICE ON SI ON LEFT SIDE IT GOES IN AND OUT BUT WE SEE A TRAIL EITHER ZOOPLANTOM OR OLD BOAT TRAIL WHERE MOTOR STIRED UP WATER. I ON ANOTHER POST YESTERDAY TOOK DI AND 2D AND TESTED THEM IN 17 FOW AND DI COULD SEE BALLS ABOUT 1 FOOT FARTHER THAN 2D,I USE BOAT FOR MEARSUREMENT AND TURN TD AROUND IN A CIRCKLE WHILE UNIT WAS ON 2D AND DI AT SAME TIME. I KNOW THE DEEPER U GO THE MORE FARTHER APART THEY WILL BE BUT 17 FOW ALMOST THE SAME,GO READ MY POST,WILL BE DOING MORE TESTING ON DIFFERENT BOTTOMS AND HOPE TO TAKE SOME PIC

[CrappiePappy]

Actually, now that I think about it .... I'm not sure if the DI is a TOTAL coverage of 75 degrees, or 75 degrees from vertical .... That will make a big difference in how far the fish are on the outside of the beam....

Maybe Rickie can clarify the coverage area of DI.....

According to these .... it would be "total" coverage :










It's one of the main reasons I use the 800kHz setting on my DI unit ... so I know the width of coverage is roughly equal to the depth of the water I'm in. Half the depth distance to either side of the transducer. And since most of my fishing is done in depths of less than 20ft .... (or 10ft to either side of the tdx) ... then I know my 10' & 14' rods are usually within the confines of the 800kHz beam.

I don't need to "see" fish images ... I need to know the depth of the water, and the depth of the highest point of any cover that I go over. I let my baits "find" the fish

... cp