Thread: HB HD DI (dedicated crystals) vs Lowrance SS DI

Lets look at this analogy...

Lets say we tie a string to the DI xducer and measure down that string to point 10 ft away from the xducer...And tie a knot at that 10ft distance..(the knot represents a fish echo)....as long as that string is hanging straight down, then that knot is going to be 10 ft deep straight under the xducer....

Now, keeping the string taut, swing the knot away from vertical 37.5 degrees (half of the 75 degree DI sonar beam)...

That knot now is still exactly 10ft away from the xducer... BUT...that knot is now approximately 4ft away from its original position and only approximately 8ft under the water....

The DI technology cannot measure the water depth at this knot's new location...(8ft water depth).. It can only measure the distance from the xducer to the knot (10ft distance)..so it still displays this knot at 10ft depth... Even though it is now only 8ft water depth..

And the deeper the water, the more this difference in water depth to the fish is compounded...

Rickie

I think what Rick is trying to point out is .... the shape of the cone is crucial to understanding how the picture on the screen is formed. When using 2d ... your cone still won't show you "exactly" where the fish is located within it, but it's a small circle that you're looking at, so the distance from the center of the cone is less than what it would/could be with DI ... since the DI cone is an elongated oval shaped area.

The fish may very well be right where the screen picture shows them ... but, they may not be !! Doh

Because of the shape of the transducer cone ... a DI unit is showing a oval shaped "swath" of water column territory. Anything coming into the elongated oval is going to show on the screen, relative to the distance away from the transducer. As such, it won't (& cannot) show you what side of the oval the fish are on, nor where they're located in relation to the rest of the picture ... just how far away from the transducer they are. All the info coming back into the unit is made into a composite picture, showing what's returning the pings that make up that oval shaped slice of water column.

Don't be fooled by the pretty DI pictures ... they're not an underwater camera view of what is directly under your transducer. They are a composite picture of everything that's within that oval shaped slice of water/bottom.

2D = a cone shaped section of the water column, that draws a circle on the bottom
DI = a oval shaped section of the water column, that draws a elongated oval on the bottom

Both pick up signals from the front of their "shape", then process the info into a "picture" that runs across the screen from right to left. The newest/most accurate info is what's coming on the right side of the screen ... everything else is "history" (in relation to time/speed of the forward movement of the transducer, & the direction you're headed vs your position in the boat/transducer location/& head unit location).

That's how I interpret it, anyway. Correct me if I'm wrong ... because I'm one of those "trying to learn" how to use & accurately determine what my DSI unit is telling me.

... cp

Deleted by rnvinc..

Wrong subforum

Originally Posted by rnvinc

Lets look at this analogy...

Lets say we tie a string to the DI xducer and measure down that string to point 10 ft away from the xducer...And tie a knot at that 10ft distance..(the knot represents a fish echo)....as long as that string is hanging straight down, then that knot is going to be 10 ft deep straight under the xducer....

Now, keeping the string taut, swing the knot away from vertical 37.5 degrees (half of the 75 degree DI sonar beam)...

That knot now is still exactly 10ft away from the xducer... BUT...that knot is now approximately 4ft away from its original position and only approximately 8ft under the water....

The DI technology cannot measure the water depth at this knot's new location...(8ft water depth).. It can only measure the distance from the xducer to the knot (10ft distance)..so it still displays this knot at 10ft depth... Even though it is now only 8ft water depth..

And the deeper the water, the more this difference in water depth to the fish is compounded...

Rickie

I'll take this string and knot analogy 1 step farther.....

We know the knot (fish) is 10ft away from the xducer...

We can swing that knot "anywhere" within the shape of that DI sonar beam and the DF has no way of knowing where that knot actually is... The only information the DF knows is "how far that knot is from the xducer"....so, therefore, the DF plots that 10ft linear distance onto the DI image the only way it can.... 10ft straight down....even tho the knot may not be "straight down"

Rickie

Originally Posted by rnvinc

I'll take this string and knot analogy 1 step farther.....

We know the knot (fish) is 10ft away from the xducer...

We can swing that knot "anywhere" within the shape of that DI sonar beam and the DF has no way of knowing where that knot actually is... The only information the DF knows is "how far that knot is from the xducer"....so, therefore, the DF plots that 10ft linear distance onto the DI image the only way it can.... 10ft straight down....even tho the knot may not be "straight down"

Rickie

thats a good way to show people what you saying.now rember if you do the same thing at 100ft.using 455khz the fish could be as far 75ft to either side,but would appear to be directly below the boat.if you fished virticully you would be close at all to that fish.(i dont think any of us crappie fishermen will be fishing that deep,but its still nice to no)

heres a picture of some fish . i think trhat this is below kentucky dam, and i think that im that i was moving really slow at the time the picture was taken. hope you enjoy!!!!!!!!!!!!!!! Doh i just seen the speed at the bottom of picture Doh

Question:: I am running 2D & DI on split screen sitting over a brushpile with a blob on the 2D screen & a tree outline showing on the DI.

I drop my jig down & see it falling at an angle until I stop it at a certain depth..Then the jig turns into a flat line because it is not moving...I then see an angled line coming out of the bob on the 2D and intersecting the flat line of my jig..I catch the fish..

What will I see on the DI screen that is running over the brushpile at the same time?

Probably won't see anything DonG. I know what your are talking about. Fish has to be in the di sweet spot to see it, then you would see it. Not too often like on 2d do I see Vertical (somewhat) movements that look like sticks in the ground..