Topic: DI optical illusion..?? Not hardly...

[rnvinc]

Here is a DI screenshot of a bridge piling I captured using the 800kHz beam of my 1197....(DI from SI technology)..



The interesting thing about this screenshot is that upper 10ft of the piling to the water's surface is absent from the image... And on top of that is another 55ft of bridge piling that carries the roadway over Ky lake...
(here is a pic of the bridge mentioned and it, in fact does extend from the lake floor to the roadway above... 



So, where is the upper portion of the piling on the DI image...?? Why does it look like the bridge piling is "only" about 12ft tall..??

The answer lies in the shape of the 800kHz SI sonar beam....
** point 1 - The DI image in my 1197 is derived from the information received in the SI crystals....(so I actually "side imaged" the bridge piling while viewing the DI view)..
**point-2 : the 800kHz SI beam only covers 130 degrees....(the upper  25 degrees on each side is not actually in the 800kHz SI beam coverage)....

Here is a depiction of how I scanned the 80ft tall bridge piling and why the upper portion to the water's surface is not showing on the DI image ...



Is the DI image an optical illusion...?? Not hardly.....

The DI image can only display data that is received from the sonar beam shape.... and in this case.... The upper portion of the bridge piling is just "not in that 800kHz 130degree beam shape"...

The sonar beam shape of each of the separate frequencies in any sonar unit is an important basic fundamental in understanding what that unit can show on it's display...

This is such an interesting technology...

Rickie

[SonarTRX]

Hi Rickie,

Keep in mind that the sonar display shows the data based on range, not depth.

Even with a beam that covers 180 degrees, the bridge piling would still not look like it goes all the way to the surface.
(...unless you run your boat right next to it, so the range is close to zero.)

Tore

[rnvinc]

That's interesting Tore....

I've got to get back to that piling and do even more experiments...

I never get tired of learning...

Rickie

[xSilmarilSx]

Like SonarTRX said, people often confuse depth with range in the SI/Di views..

SI and DI don't know the depth value. All they can tell you is the distance from the transducer to the object.
So, an object that is 50 ft in straight line from the transducer will show the same return if it lies on the bottom or on the top of the water.

[Humminbird_Greg]

Very correct Tore and Sil.  So many fishermen think that they are looking at an actual picture and forget (or did not know) this.


I take it that you imaged this with the Down Imaging Beam Width menu set to the Wide setting Rickie?

Nice picture and drawing to go with this.  Now go back and image the same bridge column using the 455kHz setting as well and post that as a comparison.