Help Fish Finder Drop Off

[Maytham]

Wonder if you can help me I am new at using a fish finder I have a Lowrence HD 5 with down scan.Would like to know when you have located a drop off ,on what side of the boat is the drop off because as I understand the image on your sonar you are seeing is darived as the cone as the transducer goes over the drop off ,and how do you figure this out.

How do you mark the spot as you move away from the drop off.

[Wayne P.]

Do you really mean Down Scan or traditional 2D sonar??????

 

Traditional sonar has the inverted somewhat cone shaped sonar pulse. There is no "side". 

 

Everything within the coverage is recorded on the very first vertical row of screen pixels on the right side of the display no matter where in that coverage it is.

That is the only current data. All to the left of it is history.

When everything in a circle is recorded on a single vertical pixel row, there cannot be any "side".

 

If a drop, fish, stump, creek, etc is not constantly being recorded on that first row of pixels, it is not within the sonar pulse coverage.

 

If you want to save a GPS waypoint of a drop, you have to create it as soon as the depth change occurs. Or you move the position curser to the recorded depth change and create a waypoint.

 

This is a small creek channel profile navigating perpendicular to it. When I took the screen shot, I was already past the creek a couple of boat lengths.

 

 

You could create a waypoint on either side of the creek where the depth change occurs or at the bottom of it so you have the center marked.

 

If you had Side Scan, you could see which side of the boat it was on if you are parallel or perpendicular with it:

 

[Comfortably Numb]

Does this in simple terms answer what he is asking?

 

[Wayne P.]

Does this in simple terms answer what he is asking?

 

 

 

No, there are no "sides" with 2D sonar.

You just indicated the RTS window REAL TIME SONAR. That is a wide view of what is being recorded on that very first vertical row of screen pixels. Vertical Flasher.

[Wayne P.]

More stuff to ponder:

 

The coverage specifications are usually at a sound strength that is -10 DB of what is it directly under the transducer. Some brands use another sound strength to advertise their coverage from pulse center.

The actual coverage that the unit can detect is more at lesser strengths. Just need a common measurement to compare brand's coverage.

 

The spec coverage of most 2D sonar units using 200 kHz is 20 degrees. As the depth increases, so does the coverage.

20 degrees is equal to 1/3 the depth.

In 30' of water that coverage is 10' in diameter on the bottom. Everything within that 10' diameter and suspended higher in the water column in lesser coverage is combined into that very first vertical row of screen pixels.

 

Units with 83 kHz 2D have a spec coverage of 60 degrees. That is equal to the depth.

In 30' of water that coverage is 30' in diameter.

 

If you imagine a traffic cone turned upside down as the "cone of coverage" all inside that cone from the bottom to the top of the cone is what is recorded on the first vertical row of pixels and then that row is moved to the left, becomes history, and a new vertical row is recorded. There is no left, right, forward, rearward or any other direction, just distance from the transducer.

As the recorded vertical rows are moved to the left across the screen, they are older history until they scroll off the left side as the oldest history.